MAX17127 Six-String WLED Driver with Integrated Step
EVALUATION KIT AVAILABLE
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
General Description
Features
The MAX17127 can implement brightness control through
the PWM signal input, and LED current is directly
controlled by the external dimming signal’s frequency and
duty cycle.
●●
●●
●●
●●
●●
●●
The MAX17127 is a high-efficiency driver for white lightemitting diodes (LEDs). It is designed for large liquidcrystal displays (LCDs) that employ an array of LEDs as
the light source. An internal switch current-mode step-up
converter drives the LED array, which can be configured
for up to six strings in parallel and 13 LEDs per string.
Each string is terminated with ballast that achieves ±2%
current-regulation accuracy, ensuring even LED brightness. The MAX17127 has a wide input voltage range
from 5V to 26V, and provides adjustable 10mA to 30mA
full-scale LED current.
The MAX17127 has multiple features to protect the
controller from fault conditions. Once an open/short string
is detected, the fault string is disabled while other strings
can still operate normally. The controller features cycleby-cycle current limit to provide constant operation and
soft-start capability. If the MAX17127 is in current-limit
condition, the step-up converter is latched off after an
internal timer expires. A thermal-shutdown circuit provides another level of protection. When thermal shutdown
happens, the MAX17127 is latched off.
The MAX17127’s step-up controller features an internal
0.12Ω (typ), 48V (max) power MOSFET with local currentsense amplifier for accurate cycle-by-cycle current limit.
This architecture greatly simplifies the external circuitry
and saves PCB space. Low-feedback voltage at each
LED string helps reduce power loss and improve efficiency.
The MAX17127 features resistor-adjustable switching
frequency from 250kHz to 1MHz, which enables a wide
variety of applications that can trade off component size
for operating frequency.
●● 5V to 26V Input Supply Voltage
●● Up to Six Parallel Strings Multiple Series-Connected
LEDs
●● 250kHz to 1MHz Adjustable Switching Frequency
●● 0.12Ω Internal HV Power MOSFET (48V max)
●● Low String Feedback Voltage: 480mV at 20mA LED
Current
●● Full-Scale LED Current Adjustable from 10mA to
30mA
±2% Current-Regulation Accuracy Between Strings
400ns Minimum String On-Time
100Hz to 25kHz PWM Input Range
Open and Short LED Protection
Output Overvoltage Protection
Thermal Shutdown
●● Small 20-Pin, 4mm x 4mm Thin QFN Package
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX17127ETP+
-40°C to +85°C
20 TQFN-EP*
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
Applications
●● Notebook, Subnotebook, and Tablet Computer
Displays
●● Handy Terminals
The MAX17127 is available in a thermally enhanced,
lead-free, 20-pin, 4mm x 4mm thin QFN package.
Simplified Operating Circuit appears at end of data sheet.
19-5164; Rev 1; 11/14
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Absolute Maximum Ratings
VIN to AGND .........................................................-0.3V to +30V
FB_, SW to PGND.................................................-0.3V to +52V
PGND to AGND.....................................................-0.3V to +0.3V
VDDIO, PWM, EN, FPO, I.C. to AGND.....................-0.3V to +6V
COMP, ISET, R_FPWM, OVP, FSLCT
to AGND............................................... -0.3V to VDDIO + 0.3V
SW Switch Maximum Continuous RMS Current...................1.6A
Continuous Power Dissipation (TA = +70°C)
TQFN (derate 16.9mW/NC above +70°C).................1349mW
Operating Temperature Range............................ -40°C to +85°C
Junction Temperature.......................................................+150°C
Storage Temperature Range............................. -60°C to +150°C
ESD
HBM....................................................................................2kV
MM....................................................................................200V
Lead Temperature (soldering, 10s).................................. +300°C
Soldering Temperature (reflow)........................................+260°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation
of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum
rating conditions for extended periods may affect device reliability.
Electrical Characteristics
(Circuit of Figure 1. VIN = 12V, CCOMP = 0.51nF, CCOUT = 4.7µF, RCOMP = 82.5kΩ, RISET = 180kΩ, RFSLCT = 100kΩ, L = 10µH,
TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
VIN Input Voltage Range
VIN Quiescent Current
VDDIO Output Voltage
MIN
TYP
MAX
UNITS
26
V
2.7
3.2
mA
5
10
µA
5
5.15
5
MAX17127 is enabled, VEN = 3.3V, VIN = 26V
MAX17127 is disabled, EN = AGND
MAX17127 is enabled, VEN = 3.3V,
5.4V < VIN < 26V, 0A < IVDDIO < 10mA
4.85
MAX17127 is enabled, VEN = 3.3V, VIN = 5V, IVDDIO = 10mA,
dropout condition
4.6
4.75
V
MAX17127 is disabled, EN = AGND, 0A < IVDDIO < 50µA
3.1
3.7
4.1
V
VDDIO Current Limit
VDDIO is forced to 4.2V
25
45
70
mA
VDDIO UVLO Threshold
Rising edge, typical hysteresis = 250mV
3.90
4.00
4.10
V
Falling edge
4.3
4.5
4.7
Rising edge
4.55
4.75
4.95
0.12
0.25
W
1
µA
0.95
1.0
1.05
RFSLCT = 400kW
0.225
0.25
0.275
RFSLCT Range
Operating range
90
Maximum Duty Cycle
At fSW = 1MHz
91
Minimum On-Time
(Note 1)
SW Current Limit
Duty cycle = 75%
VIN UVLO Threshold
V
BOOST CONVERTER
SW On-Resistance
20mA from SW to PGND
SW Leakage Current
40V on SW, TA = +25°C
Operating Frequency
RFSLCT = 100kW
3.12
MHz
500
kW
50
80
ns
3.9
4.7
A
95
%
CONTROL INPUT
PWM, EN Logic-Input
High Level
2.1
V
PWM, EN Logic-Input Low
Level
EN Pulldown Resistor
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120
200
0.8
V
280
kW
Maxim Integrated │ 2
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Electrical Characteristics (continued)
(Circuit of Figure 1. VIN = 12V, CCOMP = 0.51nF, CCOUT = 4.7µF, RCOMP = 82.5kΩ, RISET = 180kΩ, RFSLCT = 100kΩ, L = 10µH,
TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
FPO OUTPUT
FPO Off-Leakage Current
Fault inactive, TA = +25°C
100
nA
FPO On Output-Voltage
Low
ISINK = 1mA, fault active
0.4
V
INPUT LEAKAGE
PWM Leakage Current
TA = +25NC, VPWM = 0V, VPWM = 5V
-1
+1
µA
OVP Leakage Current
TA = +25NC, VOVP = 0V, VOVP = 5V
-0.1
+0.1
µA
RISET = 120kW
29.1
30
30.9
19.6
20
20.4
RISET = 360kW
9.7
10
10.3
0.2
0.3
Operating range
100
400
Accuracy = 3%
120
360
10mA < IFB_< 30mA
-2.0
+2.0
IFB_ = 30mA
400
LED CURRENT
Full-Scale FB_ Output
Current
RISET = 180kW
VISET < 0.7V
RISET Range
Current Regulation
Between Strings
mA
0.4
kW
%
555
770
IFB_ = 20mA
460
670
IFB_ = 10mA
350
630
FB_ On-Resistance
VFB_ = 50mV (includes 10W sense resistor)
17.5
28.4
W
FB_ Bias Current
VFB_ = 40V, TA = +25°C
Minimum FB_ Regulation
Voltage
FB_ Minimum On-Time
mV
0.1
1
µA
400
580
700
ns
1.23
1.25
1.27
V
7
8
9
V
FAULT PROTECTION
OVP Threshold Voltage
Rising edge, typical hysteresis = 90mV
FB_ Overvoltage
Threshold
FB_ Enable Threshold
Voltage
1.2
V
FB_ Open Threshold
Voltage
130
280
mV
FB_ Check LED Source
Current
0.4
1.3
mA
FB_ Check LED Time
0.7
1.3
ms
1.0
Thermal-Shutdown
Threshold
(Note 1)
+150
°C
Overcurrent Fault Timer
Latchoff timer
128
µs
PWM CONTROL
PWM Input On-Time
400
PWM Input Frequency
Range
0.1
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ns
25
kHz
Maxim Integrated │ 3
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Electrical Characteristics
(Circuit of Figure 1. VIN = 12V, CCOMP = 0.51nF, CCOUT = 4.7µF, RCOMP = 82.5kΩ, RISET = 180kΩ, RFSLCT = 100kΩ, L = 10µH,
TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
VIN Input Voltage Range
VIN Quiescent Current
MIN
5
TYP
MAX
UNITS
26
V
MAX17127 is enabled, VEN = 3.3V, VIN = 26V
3.2
mA
MAX17127 is disabled, EN = AGND
15
μA
MAX17127 is enabled, VEN = 3.3V,
5.4V < VIN < 26V, 0A < IVDDIO < 10mA
4.85
MAX17127 is enabled , VEN = 3.3V, VIN = 5V,
IVDDIO = 10mA, dropout condition
4.6
EN = AGND, 0A < IVDDIO < 50µA
3.1
4.1
VDDIO Current Limit
VDDIO is forced to 4.2V
25
70
mA
VDDIO UVLO Threshold
Rising edge, typical hysteresis = 250mV
3.90
4.10
V
Falling edge
4.3
4.7
Rising edge
4.55
4.95
VDDIO Output Voltage
VIN UVLO Threshold
5.15
V
V
BOOST CONVERTER
SW On-Resistance
20mA from SW to PGND
SW Leakage Current
40V on SW, TA = +25°C
Operating Frequency
0.25
W
1
μA
RFSLCT = 100kW
0.95
1.05
RFSLCT = 400kW
0.225
0.28
90
500
RFSLCT Operative Range
92
MHz
kW
Maximum Duty Cycle
At fSW = 1MHz
%
Boost Output Voltage
With suitable OVP network
45
V
Minimum On-Time
(Note 1)
80
ns
CONTROL INPUT
PWM, EN Logic-Input High Level
2.1
PWM, EN Logic-Input Low Level
EN Pulldown Resistor
110
V
0.8
V
290
kW
0.4
V
FPO OUTPUT
FPO On Output-Voltage Low
ISINK = 1mA, fault active
LED CURRENT
Full-Scale FB_ Output Current
RISET Range
Current Regulation Between
Strings
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RISET = 120kW
29.1
30.9
RISET = 180kW
19.4
20.6
RISET = 360kW
9.7
10.3
VISET < 0.7V
0.2
0.4
Operating range
100
400
Accuracy = 3%
120
360
10mA < IFB_< 30mA
-2.0
+2.0
mA
kW
%
Maxim Integrated │ 4
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Electrical Characteristics (continued)
(Circuit of Figure 1. VIN = 12V, CCOMP = 0.51nF, CCOUT = 4.7µF, RCOMP = 82.5kΩ, RISET = 180kΩ, RFSLCT = 100kΩ, L = 10µH,
TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
IFB_ = 30mA
TYP
MAX
400
UNITS
770
Minimum FB_ Regulation Voltage IFB_= 20mA
670
mV
IFB_= 10mA
630
FB_ On-Resistance
VFB_= 50mV (includes 10W sense resistor)
28.4
W
FB_ Bias Current
VFB_ = 40V, TA = +25°C
1
μA
400
700
ns
1.23
1.27
V
7
9
V
FB_ Open Threshold Voltage
130
280
mV
FB_ Check LED Source Current
0.4
1.3
mA
FB_ Check LED Time
0.7
1.3
ms
88
168
μs
FB_ Minimum On-Time
FAULT PROTECTION
OVP Threshold Voltage
Rising edge, typical hysteresis = 90mV
FB_ Overvoltage Threshold
Overcurrent Fault Timer
Latchoff timer
PWM CONTROL
PWM Input On-Time
400
PWM Input Frequency Range
0.1
ns
25
kHz
Note 1: Specifications are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1. VIN = 12V, TA = +25°C, unless otherwise noted.)
90
88
80
EFFICIENCY (%)
EFFICIENCY (%)
90
MAX17127 toc01
92
BOOST CONVERTER EFFICIENCY vs. BRIGHTNESS
(VS = 2V, VOUT = 32V, IOUT = 120mA AT 100%)
86
84
82
MAX17127 toc02
BOOST CONVERTER EFFICIENCY vs. INPUT VOLTAGE (VS)
(VOUT = 32V, IOUT = 120mA, BRIGHTNESS = 100%)
70
60
80
78
5
8
11
14
17
20
INPUT VOLTAGE (V)
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23
26
50
0
20
40
60
80
100
BRIGHTNESS (%)
Maxim Integrated │ 5
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VIN = 12V, TA = +25°C, unless otherwise noted.)
15
20.15
20.10
10
20.05
5
0
20.20
MAX17127 toc04
MAX17127 toc03
fPWM = 200Hz
LED CURRENT (ILED = 20mA AT 100% BRIGHTNESS)
vs. INPUT VOLTAGE (VS)
LED CURRENT (mA)
LED CURRENT (mA)
20
LED CURRENT vs. BRIGHTNESS SETTING
20
0
40
60
80
100
20.00
5
8
11
14
17
20
23
PWM DUTY CYCLE (%)
INPUT VOLTAGE (V)
LED CURRENT (ILED = 20mA AT 10% BRIGHTNESS)
vs. INPUT VOLTAGE (VS)
IN QUIESCENT CURRENT
vs. IN VOLTAGE
2.00
1.98
1.96
5
4
100% BRIGHTNESS
3
2
200Hz/1% BRIGHTNESS
1
0
5
8
11
14
17
20
23
26
INPUT VOLTAGE (V)
7
11
14
17
EN = LOW
8
23
26
VOUT = 32V, IOUT = 120mA
VLX
20V/div
0V
6
INDUCTOR
CURRENT
500mA/div
4
2
0
20
IN VOLTAGE (V)
MAX17127 toc08
MAX17127 toc07
10
5
SWITCHING WAVEFORMS
(VS = 5V, BRIGHTNESS = 100%)
IN SHUTDOWN CURRENT
vs. IN VOLTAGE
SHUTDOWN CURRENT (µA)
MAX17127 toc06
MAX17127 toc05
2.02
6
QUIESCENT CURRENT (mA)
LED CURRENT (mA)
2.04
26
0mA
5
8
11
14
17
20
23
26
1µs/div
IN VOLTAGE (V)
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Maxim Integrated │ 6
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VIN = 12V, TA = +25°C, unless otherwise noted.)
SWITCHING WAVEFORMS
(VS = 26V, BRIGHTNESS = 100%)
STARTUP WAVEFORMS
(BRIGHTNESS = 100%)
MAX17127 toc09
MAX17127 toc10
VOUT = 32V, IOUT = 120mA
VLX
20V/div
0V
VLX
20V/div
INDUCTOR
CURRENT
200mA/div
0V
0mA
VOUT
20V/div
0V
12V
1µs/div
1ms/div
STARTUP WAVEFORMS
(BRIGHTNESS = 20%)
LED CURRENT WAVEFORMS
(BRIGHTNESS = 50%)
MAX17127 toc11
VEN
5V/div
0V
INDUCTOR
CURRENT
500mA/div
0A
MAX17127 toc12
VFB1
10V/div
0V
ILED
20mA/div
0mA
VLX
20V/div
0V
INDUCTOR
CURRENT
500mA/div
0mA
VOUT
20V/div
0V
12V
VEN
5V/div
0V
INDUCTOR
CURRENT
500mA/div
0A
2ms/div
1ms/div
LED CURRENT WAVEFORMS
(BRIGHTNESS = 1%)
LED-OPEN FAULT PROTECTION
(BRIGHTNESS = 100%, LED OPEN ON FB1)
MAX17127 toc13
MAX17127 toc14
VFB1
10V/div
0V
VFB1
1V/div
0V
IFB1
20mA/div
0mA
VFB2
10V/div
0V
INDUCTOR
CURRENT
500mA/div
0mA
1ms/div
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0mA
20mA
200ms/div
32V
VOUT
10V/div
10V
IFB2
10mA/div
Maxim Integrated │ 7
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VIN = 12V, TA = +25°C, unless otherwise noted.)
LINE-TRANSIENT RESPONSE
(VS = 9V  21V, BRIGHTNESS = 100%)
LED-SHORT FAULT PROTECTION
(BRIGHTNESS = 100%, 3 LEDs SHORT ON FB1)
MAX17127 toc16
MAX17127 toc15
VFB1
10V/div
0V
9V
VOUT
(AC-COUPLED)
2V/div
0V
VS
10V/div
21V
0V
IFB1
50mA/div
0A
INDUCTOR
CURRENT
1A/div
IFB1
10mA/div
0mA
0mA
20mA
10µs/div
200µs/div
LINE-TRANSIENT RESPONSE
(VS = 21V  9V, BRIGHTNESS = 100%)
MAXIMUM UNBALANCE RATE BETWEEN STRING
vs. BRIGHTNESS (VS = 12V, ILED = 20mA)
MAX17127 toc17
21V
MAXIMUM UNBALANCE RATE (%)
9V
0A
INDUCTOR
CURRENT
1A/div
IFB1
10mA/div
20mA
0mA
200µs/div
MAX17127 toc18
1.0
0V
VOUT
(AC-COUPLED)
1V/div
VS
10V/div
0V
0.8
0.6
0.4
0.2
0
MAXIMUM = IFB_ − I FB(AVG) MAX%
UNBALANCE RATE (%)
IFB(AVG)
10
20
30
40
50
60
70
80
90 100
BRIGHTNESS (%)
MAXIMUM UNBALANCE RATE BETWEEN STRINGS
(ILED = 20mA) vs. INPUT VOLTAGE (VS)
MAX17127 toc19
MAXIMUM UNBALANCE RATE (%)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
MAXIMUM = IFB_− I FB(AVG) MAX %
UNBALANCE RATE (%)
IFB(AVG)
5
8
11
14
17
20
23
26
INPUT VOLTAGE (V)
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Maxim Integrated │ 8
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
OVP
R_FPWM
FB1
FB2
TOP VIEW
PGND
Pin Configuration
15
14
13
12
11
SW 16
10
FB3
I.C. 17
9
AGND
8
FB4
7
FB5
6
FB6
MAX17127
COMP 18
VIN 19
3
4
ISET
5
FPO
2
FSLCT
VDDIO
1
EN
PWM 20
EP
+
THIN QFN
(4mm × 4mm)
Pin Description
PIN
NAME
1
VDDIO
2
EN
3
FSLCT
FUNCTION
5V Linear Regulator Output. VDDIO provides power to the MAX17127. Bypass VDDIO to AGND with
a ceramic capacitor of 1µF or greater.
Enable Pin. EN = high enables the MAX17127. An internal 200kI (typ) pulldown resistor keeps the
MAX17127 in disabled mode if the EN pin is high impedance.
Oscillator Frequency-Adjustment Pin. The resistance from FSLCT to AGND sets the step-up
converter’s oscillator frequency:
fSW = 1MHz × 100kW/RFSLCT
The acceptable resistance range is 100kW < RFSLCT < 400kW, which corresponds to the switching
frequency of 1MHz > fSW > 250kHz.
Full-Scale LED Current-Adjustment Pin. The resistance from ISET to AGND controls the full-scale
current in each LED string:
ILEDMAX = 20mA × 180kW/RISET
4
ISET
5
FPO
Fault-Diagnostic Output. Open drain, active low. The FPO output is asserted low when the following
faults occur: overcurrent fault, thermal fault, output-voltage short condition, or output overvoltage.
6
FB6
LED String 6 Cathode Connection. FB6 is the open-drain output of an internal regulator, which
controls current through FB6. FB6 can sink up to 30mA. If unused, connect FB6 to AGND.
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The acceptable resistance range is 120kW < RISET < 360kW, which corresponds to a full-scale LED
current of 30mA > ILEDMAX > 10mA. Connecting ISET to AGND sets the test mode for 0.3mA (typ)
full-scale LED current.
Maxim Integrated │ 9
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Pin Description (continued)
PIN
NAME
7
FB5
LED String 5 Cathode Connection. FB5 is the open-drain output of an internal regulator, which
controls current through FB5. FB5 can sink up to 30mA. If unused, connect FB5 to AGND.
8
FB4
LED String 4 Cathode Connection. FB4 is the open-drain output of an internal regulator, which
controls current through FB4. FB4 can sink up to 30mA. If unused, connect FB4 to AGND.
9
AGND
10
FB3
LED String 3 Cathode Connection. FB3 is the open-drain output of an internal regulator, which
controls current through FB3. FB3 can sink up to 30mA. If unused, connect FB3 to AGND.
11
FB2
LED String 2 Cathode Connection. FB2 is the open-drain output of an internal regulator, which
controls current through FB2. FB2 can sink up to 30mA. If unused, connect FB2 to AGND.
12
FB1
LED String 1 Cathode Connection. FB1 is the open-drain output of an internal regulator, which
controls current through FB1. FB1 can sink up to 30mA. If unused, connect FB1 to AGND.
13
R_FPWM
14
OVP
15
PGND
16
SW
Boost Regulator Power Switch Node
17
I.C.
Internal Connection. Not connected externally.
18
COMP
19
VIN
20
PWM
—
EP
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FUNCTION
Analog Ground
Connect R_FPWM to AGND
Overvoltage Sense. Connect OVP to the boost converter output through a resistor:
VOVP = 1.25V × (1 + R1/R2)
Boost Regulator Power Ground
Step-Up Converter Compensation Pin. Connect a ceramic capacitor in series with a resistor from
COMP to AGND.
Supply Input. VIN biases the internal 5V linear regulator that powers the device. Bypass VIN to AGND
directly at the pin with a 0.1Fµ or greater ceramic capacitor.
PWM Signal Input. This signal is used for brightness control. The brightness is proportional to the
PWM duty cycle, and the PWM signal directly controls the LED turning on/off.
Exposed Backside Pad. Solder to the circuit board ground plane with sufficient copper connection to
ensure low thermal resistance. See the PCB Layout Guidelines section.
Maxim Integrated │ 10
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Typical Operating Circuit
The MAX17127 typical operating circuit is shown as
Figure 1. Table 1 lists some recommended components,
L1
10µH
VS
5V TO 26V
and Table 2 lists the contact information for component
suppliers.
D1
COUT
4.7µF
CIN
4.7µF
VIN
VIN
0.1µF
R1
2.21MΩ
SW
VDDIO
PGND
1µ F
OVP
COMP
ISET
RISET
180kΩ
AGND
FSLCT
RFSLCT
100kΩ
RCOMP
82.5kΩ
CCOMP
510pF
R2
71.5kΩ
MAX17127
I.C.
EN
FB1
PWM
FB2
R_FPWM
FB3
3.3V
FB4
FB5
10kΩ
FAULT INDICATOR
FPO
FB6
EP
Figure 1. Typical Operating Circuit
Table 1. Component List
DESIGNATION
DESCRIPTION
CIN
4.7μF ±10%, 25V X5R ceramic capacitor
(1206)
Murata GRM319R61E475KA12D
C1, C2
2.2μF ±20%, 50V X7R ceramic capacitors
(1206)
Murata GRM31CR71H225K
D1
DESIGNATION
DESCRIPTION
L1
10μH, 1.2A power inductor
Sumida CR6D09HPNP-100MC
TDK VLP6810T-100M1R2
White LED
3.2V (typ), 3.5V (max) at 20mA
Nichia NSSW008C
2A, 40V Schottky diode (M-flat)
Toshiba CMS11
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Maxim Integrated │ 11
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Table 2. Component Suppliers
SUPPLIER
PHONE
WEBSITE
Murata Electronics North America, Inc.
770-436-1300
www.murata.com
Nichia Corp.
248-352-6575
www.nichia.com
Sumida Corp.
847-545-6700
www.sumida.com
Toshiba America Electronic Components, Inc.
949-455-2000
www.toshiba.com/taec
Vishay
203-268-6261
www.vishay.com
EN
VIN
1.25V
FAULT
CONTROL
5V LINEAR REGULATOR
OVP
ERROR
COMPARATOR
VDDIO
CONTROL
AND
DRIVER
LOGIC
VDDIO
SLOPE
COMPENSATION
OSCILLATOR
FSLCT
TO FAULT
CONTROL
ERROR
AMPLIFIER
COMP
OVP
Gm
PGND
8V
OVERVOLTAGE
COMPARATOR
HVC
S&H
LVC
Gm
N
CURRENT
SENSE
Σ
ERROR
AMPLIFIER
1.25V
CLAMP
SW
FB6
FB5
FB4
FB3
FB2
VSAT
FB1
ISET
ISET
EN
EN
PWM
N
MAX17127
PWM CONTROL
AGND
R_FPWM
I.C.
VDDIO
FPO
FAULT
CONTROL
CURRENT SOURCE
FB2
CURRENT SOURCE
FB3
CURRENT SOURCE
FB4
CURRENT SOURCE
FB5
CURRENT SOURCE
FB6
Figure 2. Functional Diagram
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Maxim Integrated │ 12
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Detailed Description
Fixed-Frequency Step-Up Controller
The MAX17127 is a high-efficiency driver for arrays
of white LEDs. It contains a fixed-frequency currentmode PWM step-up controller, a 5V linear regulator, a
dimming control circuit, an internal power MOSFET,
and six regulated current sources. Figure 2 shows
the MAX17127 functional diagram. When enabled, the
step-up controller boosts the output voltage to provide
sufficient headroom for the current sources to regulate their respective string currents. The MAX17127
features resistor-adjustable switching frequency (250kHz
to 1MHz), which allows trade-offs between external
component size and operating efficiency.
The MAX17127’s fixed-frequency, current-mode, step-up
controller automatically chooses the lowest active FB_
voltage to regulate the feedback voltage. Specifically,
the difference between the lowest FB_ voltage and the
current source control signal plus an offset is integrated at
the COMP output. The resulting error signal is compared
to the internal switch current plus slope compensation
to determine the switch on-time. As the load changes,
the error amplifier sources or sinks current to the COMP
output to deliver the required peak inductor current. The
slope-compensation signal is added to the current-sense
signal in order to improve stability at high duty cycles.
The MAX17127 can implement brightness control through
the PWM signal input. The LED current is directly
controlled by the external dimming signal's frequency and
duty cycle.
Internal 5V Linear Regulator and UVLO
The MAX17127 has multiple features to protect the
controller from fault conditions. Separate feedback
loops limit the output voltage in all circumstances. The
MAX17127 checks each FB_ voltage during operation.
If one or more strings are open, the corresponding FB_
voltages are pulled below 180mV (max), and an opencircuit fault is detected. As a result, the respective current
sources are disabled.
When one or more LEDs are shorted and the related
FB_ voltage exceeds 8V, short fault is detected and the
respective current source is disabled if at least one FB_
voltage is lower than the minimum FB_ regulation voltage
+460mV (typ).
The MAX17127 includes an internal low-dropout linear
regulator (VDDIO). When VIN is higher than 5.0V, this
linear regulator generates a 5V supply to power the
internal PWM controller, control logic, and MOSFET
driver. The VDDIO voltage drops to 3.3V in shutdown. If 5V
< VIN < 5.5V, VDDIO and VIN can be connected together
and powered from an external 5V supply. There is a body
diode from VDDIO to VIN, so VIN must be greater than
VDDIO (see Figure 2).
The MAX17127 is disabled until VDDIO exceeds the
UVLO threshold. The hysteresis on UVLO is approximately 250mV. In standby mode, the internal LDO is
in low-power mode with 10µA (max) input current and
approximately regulated at 3.3V (typ). When EN = high,
the internal LDO is enabled and regulated accurately at
5V (typ).
When in LED open or short conditions, the fault string is
disabled while other strings can still operate normally.
The VDDIO pin should be bypassed to AGND with a
minimum 1µF ceramic capacitor.
The MAX17127 also includes other kinds of fault
protections, which are overcurrent, thermal shutdown,
and output overvoltage. The MAX17127 features cycleby-cycle current limit to provide consistent operation and
soft-start protection. In an overcurrent condition, the IC
latches off if the fault still exists after a 128µs overcurrent
fault timer expires. The output overvoltage is a nonlatched
operation, and the step-up converter stops switching
during the fault. A thermal-shutdown circuit provides
another level of protection. The MAX17127 is latched off
once thermal shutdown occurs.
Startup
The MAX17127 includes a 5V linear regulator that
provides the internal bias and gate driver for the step-up
controller.
www.maximintegrated.com
At startup, the MAX17127 performs a diagnostic test of
the LED array. In the test phase, all FB_ pins are pulled up
by a given current source (0.4mA min) during 1ms (typ). If
some FB_ voltage is lower than 1.2V (max), the string is
considered to be unused. Therefore, when a string is not
in use, it should be connected to AGND. All other strings
with FB_ higher than 1.2V (max) are detected as in use.
After the LED string diagnostic phases are finished, the
boost converter starts. An additional 1ms after boost
soft-start end is used as minimum FB_ control. The total
startup time is less than 10ms, including 2ms (typ) softstart. Figure 3 shows the sequence.
Maxim Integrated │ 13
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Shutdown
The MAX17127 can be placed into shutdown by pulling the EN pin low. When a critical failure is detected,
the IC also enters shutdown mode. In shutdown mode,
all functions of the IC are turned off, including the 5V
linear regulator. Only a crude linear regulator remains on,
providing a 3.3V (typ) output voltage to VDDIO with 1µA
current-sourcing capability.
Frequency Selection
The boost converter switching frequency can be adjusted
by the external resistor on the FSLCT pin. The switching-frequency adjustable range is 250kHz to 1MHz.
High-frequency (1MHz) operation optimizes the regulator for the smallest component size at the expense of
efficiency due to increased switching losses. Lowfrequency (250kHz) operation offers the best overall
efficiency, but requires larger components and PCB area.
VIN
0V
VOUT
ILED
0V
VEN
0V
VDDIO
0V
CHECK LED
STEP-UP REGULATOR
SOFT-START
MIN FB_ CONTROL
(1ms)
Figure 3. Startup Sequence
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Maxim Integrated │ 14
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Overvoltage Protection
To protect the step-up regulator when the load is open,
or if the output voltage becomes excessive for any
reason, the MAX17127 features a dedicated overvoltagefeedback input (OVP). The OVP pin is connected to the
center tap of a resistive voltage-divider from the highvoltage output. When the OVP pin voltage (VOVP)
exceeds 1.25V (typ), a comparator turns off the
internal power MOSFET. This step-up regulator switch is
reenabled after the VOVP drops 90mV (typ hysteresis) below the protection threshold. This overvoltageprotection feature ensures the step-up regulator fail-safe
operation when the LED strings are disconnected from
the output.
LED Current Sources
Maintaining uniform LED brightness and dimming
capability is critical for backlight applications. The
MAX17127 is equipped with a bank of six matched
current sources. These specialized current sources are
accurate within ≤ 3% and match each other within 2%.
They can be switched on and off at PWM frequencies
of up to 25kHz. LED full-scale current is set through the
ISET pin (10mA < ILED < 30mA).
The minimum voltage drop across each current source
is 480mV (typ) when the LED current is 20mA. The lowvoltage drop helps reduce dissipation while maintaining
sufficient compliance to control the LED current within the
required tolerances.
The LED current sources can be disabled by connecting
the respective FB_ pin to AGND at startup. When the
IC is enabled, the controller scans settings for all FB_
pins. If an FB_ pin is not connected to AGND, an internal
circuit pulls this pin high, and the controller enables the
corresponding current source to regulate the string
current. If the FB_ pin is connected to AGND, the
controller disables the corresponding current regulator.
The current regulator cannot be disabled by connecting
the respective FB_ pin to AGND after the IC is enabled.
Current-Source Fault Protection
LED fault open/short is detected after startup. When
one or more strings fail after startup, the corresponding
current source is disabled. The remaining LED strings are
still operated normally. The LED open/short detection is
not executed when LED on-time is less than 2µs.
The MAX17127 can tolerate a slight mismatch between
LED strings. When severe mismatches or WLED
shorts occur, the FB_ voltages are uneven because of
mismatched voltage drops across strings. At each LED
turn-on, the FB_ voltage is brought down to the regulation
voltage quickly. When FB_ voltage is higher than 8V (typ)
after LED turn-on, an LED short is detected if at least one
FB_ voltage is lower than the minimum FB_ regulation
voltage, +460mV (typ). The remaining LED strings can
still operate normally. The LED short protection is disabled during the soft-start phase of the step-up regulator.
Open Current-Source Protection
The MAX17127 step-up regulator output voltage is
regulated according to the minimum FB_ voltages on all
the strings in use. If one or more strings are open, the
respective FB_ pins are pulled to ground. For any FB_
lower than 180mV, the corresponding current source is
disabled. The remaining LED strings can still operate
normally. If all strings in use are open, the MAX17127
shuts the step-up regulator down.
FPO Function
The fault conditions trigger FPO function and pull the
FPO pin low. Table 3 shows the state of the FPO pin with
different fault conditions.
Dimming Control
The MAX17127 performs brightness control with a PWM
input signal. Dimming duty cycle and frequency of current
sources follow the signal at the PWM pin directly.
All FB_ pins in use are combined to extract a lowest FB_
voltage (LVC) (see Figure 2). LVC is fed into the step-up
regulator’s error amplifier and is used to set the output
voltage.
Table 3. FPO Function Table
FAULT CONDITION
THERMAL FAULT
OUTPUT OVERVOLTAGE
INPUT OVERCURRENT
LATCHED
Yes
No (stop switching)
Yes (after time expires)
FPO PIN STATE
Low
Low
Low
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Maxim Integrated │ 15
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Full-Scale and Low-Level LED Current
The full-scale LED current is set by:
ILED_MAX =
20mA × 180kΩ
R ISET
The acceptable resistance range for ISET is 120kΩ <
RISET < 360kΩ, which corresponds to full-scale LED
current of 30mA > ILED_MAX > 10mA.
The MAX17127 includes a thermal-protection circuit.
When the local IC temperature exceeds +150°C (typ),
the controller and current sources shut down. When the
thermal shutdown happens, the FPO output pin is
asserted low. The controller and current sources do not
restart until the next enable signal is sent or input supply
is recycled.
Design Procedure
All MAX17127 designs should be prototyped and tested
prior to production.
External component-value choice is primarily dictated
by the output voltage and the maximum load current, as
well as maximum and minimum input voltages. Begin by
selecting an inductor value. Once the inductor is known,
choose the diode and capacitors.
Step-Up Converter Current Calculation
To ensure stable operation, the MAX17127 includes
slope compensation, which sets the minimum inductor
value. In continuous-conduction mode (CCM), the minimum
inductor value is calculated with the following equation:
(VOUT(MAX) + VDIODE − 2 × VIN(MIN)) × R S
2 × SF × f SW(MIN)
where:
=
SF 72mV, when VIN < 12.5V


72mV
SF
, when VIN > 12.5V
V
− 12.5V

1 + IN

10.6V
SF is a scale factor from the slope compensation
depending on input voltage (this allows a higher current
capability), the LCCM(MIN) is the minimum inductor value
for stable operation in CCM, and RS = 15mΩ (typ) is
the equivalent sensing scale factor from the controller’s
internal current-sense circuit.
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

VIN(MIN)

L DCM(MAX)= 1 −
 VOUT(MAX) + VDIODE 


×
Thermal Shutdown
L CCM(MIN) =
The controller can also operate in discontinuousconduction mode (DCM). In this mode, the inductor
value can be lower, but the peak inductor current is
higher than in CCM. In DCM, the maximum inductor
value is calculated with the following equation:
VIN(MIN) 2 × η
2 × f SW(MAX) × VOUT(MAX) × I OUT(MAX)
where the LDCM(MAX) is the maximum inductor value for
DCM, η is the nominal regulator efficiency (85%), and
IOUT(MAX) is the maximum output current.
The output current capability of the step-up regulator
is a function of current limit, input voltage, operating
frequency, and inductor value. Because the slope
compensation is used to stabilize the feedback loop,
the inductor current limit depends on the duty cycle,
and is determined with the following equation:
SF

I
=
× 0.97, when D < 30%
 LIM RS

SF
I
=
× (1.27-D), when D > 30%
 LIM RS
where SF is the scale factor from the slope compensation,
2.5A is the current limit specified at 75% duty cycle, and
D is the duty cycle.
The output current capability depends on the current-limit
value and operating mode. The maximum output current
in CCM is governed by the following equation:

0.5 × D × VIN 
VIN
I OUT_CCM(MAX)
= ILIM −
×η
×
f
L
V
×
SW
OUT


where ILIM is the current limit calculated above, η is the
nominal regulator efficiency (85%), and D is the duty
cycle. The corresponding duty cycle for this current is:
D=
VOUT − VIN + VDIODE
VOUT − ILIM × R ON + VDIODE
where VDIODE is the forward voltage of the rectifier
diode and RON is the internal MOSFET’s on-resistance
(0.2Ω typ).
Maxim Integrated │ 16
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
The maximum output current in DCM is governed by the
following equation:
I OUT_DCM(MAX) =
L × ILIM 2 × f SW × η × (VOUT + VDIODE )
2 × VOUT × (VOUT + VDIODE − VIN )
Inductor Selection
The inductance, peak current rating, series resistance,
and physical size should all be considered when
selecting an inductor. These factors affect the converter’s
operating mode, efficiency, maximum output load capability,
transient response time, output voltage ripple, and cost.
The maximum output current, input voltage, output
voltage, and switching frequency determine the
inductor value. Very high inductance minimizes the
current ripple, and therefore reduces the peak current,
which decreases core losses in the inductor and I2R losses
in the entire power path. However, large inductor values
also require more energy storage and more turns of wire,
which increase physical size and I2R copper losses. Low
inductor values decrease the physical size but increase
the current ripple and peak current. Finding the best
inductor involves compromises among circuit efficiency,
inductor size, and cost.
In choosing an inductor, the first step is to determine the
operating mode: continuous-conduction mode (CCM) or
discontinuous-conduction mode (DCM). The MAX17127
has a fixed internal-slope compensation, which requires
a minimum inductor value. When CCM is chosen, the
ripple current and the peak current of the inductor can
be minimized. If a small-size inductor is required, DCM
can be chosen. In DCM, the inductor value and size can
be minimized, but the inductor ripple current and peak
current are higher than those in CCM. The controller can
be stable, independent of the internal slope-compensation
mode, but there is a maximum inductor-value requirement
to ensure the DCM operating mode.
The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current to
the average DC inductor current at the full-load current.
The controller operates in DCM mode when LIR is higher
than 2.0, and it works in CCM mode when LIR is lower
than 2.0. The best trade-off between inductor size and
converter efficiency for step-up regulators generally has
an LIR between 0.3 and 0.5. However, depending on the
AC characteristics of the inductor core material and ratio
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of inductor resistance to other power-path resistances,
the best LIR can shift up or down. If the inductor
resistance is relatively high, more ripples can be accepted
to reduce the number of required turns and increase
the wire diameter. If the inductor resistance is relatively
low, increasing inductance to lower the peak current can
reduce losses throughout the power path. If extremely
thin high-resistance inductors are used, as is common
for LCD panel applications, LIR higher than 2.0 can be
chosen for DCM operating mode.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency
improvements in typical operating regions. The detailed
design procedure for CCM can be described as follows:
Calculate the approximate inductor value using the
typical input voltage (VIN), the maximum output
current (IOUT(MAX)), the expected efficiency (ηTYP) taken
from an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
 VIN(MIN) 
L = 

 VOUT 
2
 VOUT − VIN(MIN)  η


 TYP
 I OUT(MAX) × f SW  LIR 


The MAX17127 has a minimum inductor-value limitation
for stable operation in CCM operating mode at low input
voltage because of the internal fixed-slope compensation.
The minimum inductor value for stability is calculated with
the following equation:
L CCM(MIN) =
(VOUT(MAX) + VDIODE − 2 × VIN(MIN)) × R S
2 × SF × f SW(MIN)
where SF is a scale factor from slope compensation,
and RS is the equivalent current-sensing scale factor
(15mΩ typ).
Choose an available inductor value from an appropriate inductor family. Calculate the maximum DC input
current at the minimum input voltage VIN(MIN), using
conservation of energy and the expected efficiency at that
operating point (ηMIN) taken from an appropriate curve in
the Typical Operating Characteristics:
IIN(DC,MAX) =
I OUT(MAX) × VOUT
VIN(MIN) × η MIN
Maxim Integrated │ 17
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Calculate the ripple current at that operating point and the
peak current required for the inductor:
IRIPPLE =
(
VIN(MIN) × VOUT(MAX) − VIN(MIN)
The peak inductor current at minimum input voltage is
calculated as follows:
)
L × VOUT(MAX) × f SW
7V × (32V − 7V )
120mA × 32V
+
=
IPEAK =
0.95A
7V × 0.85
2 × 10µH × 32V × 0.9MHz
I
IPEAK IIN(DC,MAX) + RIPPLE
=
2
When DCM operating mode is chosen to minimize
the inductor value, the calculations are different from
those above in CCM operating mode. The maximum
inductor value for DCM mode is calculated with the
following equation:


VIN(MIN)

L DCM(MAX)= 1 −
 VOUT(MAX) + VDIODE 


×
A 10µH inductor is chosen, which is higher than the
minimum L that guarantees stability in CCM.
Alternatively, choose a DCM operating mode by using
lower inductance and estimating efficiency of 85% at this
operating point. Since DCM has higher peak inductor
current at lower input, it causes current limit when the
parameters are not chosen properly. Considering the
case with six 10-LED strings and 20mA LED full-scale
current to prevent excessive switch current from causing
current limit:
7V


L DCM(MAX)= 1 −

 32V + 0.4V 
VIN(MIN) 2 × η
2 × f SW(MAX) × VOUT(MAX) × I OUT(MAX)
The peak inductor current in DCM is calculated with the
following equation:
I OUT(MAX) × 2 × VOUT(MAX)
IPEAK =
(
)
×
A 3.3µH inductor is chosen. The peak inductor current at
minimum input voltage is calculated as follows:
IPEAK
× VOUT(MAX) + =
VDIODE − VIN(MIN)
(
L × f SW(MIN) × η × VOUT(MAX) + VDIODE
)
The inductor’s saturation current rating should exceed
IPEAK, and the inductor’s DC current rating should exceed
IIN(DC,MAX). For good efficiency, choose an inductor with
less than 0.1Ω series resistance.
Considering the circuit with six 10-LED strings and
20mA LED full-scale current, the maximum load current
(IOUT(MAX)) is 120mA with a 32V output and a minimal
input voltage of 7V.
Choosing a CCM operating mode with LIR = 0.7 at 1MHz
and estimating efficiency of 85% at this operating point:
2
 7V   32V − 7V  0.85 
=
L 
 
 =
 12.1µH
 32V   120mA × 1MHz  0.7 
In CCM, the inductor has to be higher than LCCM(MIN):
L CCM(MIN)
=
(32V + 0.4V − 2 × 7V) × 13.7mΩ
=
2 × 25.5mV × 0.9MHz
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5.5µH
(7V) 2 × 0.85
3.9µH
=
2 × 1.1MHz × 32V × 120mA
120mA × 2 × 32V × (32V + 0.4V − 7V )
= 1.40A
3.3µH × 1.1MHz × 0.85 × (32V + 0.4V )
Output Capacitor Selection
The total output-voltage ripple has two components: the
capacitive ripple caused by the charging and discharging
on the output capacitor, and the ohmic ripple due to the
capacitor’s equivalent series resistance (ESR):
=
VRIPPLE VRIPPLE(C) + VRIPPLE(ESR)
VRIPPLE(C) ≈
I OUT(MAX)  VOUT(MAX) − VIN(MIN) 


C OUT  VOUT(MAX) × f SW 
and:
VRIPPLE(ESR) ≈ IPEAKR ESR(COUT)
where IPEAK is the peak inductor current (see the Inductor
Selection section).
The output-voltage ripple should be low enough for the
FB_ current-source regulation. The ripple voltage should
be less than 200mVP-P. For ceramic capacitors, the
output-voltage ripple is typically dominated by VRIPPLE(C).
Maxim Integrated │ 18
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
The voltage rating and temperature characteristics of the
output capacitor must also be considered.
Rectifier Diode Selection
The MAX17127’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. The diode should be rated to
handle the output voltage and the peak switch current.
Make sure that the diode’s peak current rating is at least
IPEAK calculated in the Inductor Selection section and
that its breakdown voltage exceeds the output voltage.
Overvoltage-Protection Determination
The overvoltage-protection circuit ensures the circuit
safe operation; therefore, the controller should limit the
output voltage within the ratings of all MOSFET, diode,
and output capacitor components, while providing
sufficient output voltage for LED current regulation. The
OVP pin is connected to the center tap of a resistive
voltage-divider (R1 and R2 in Figure 1) from the highvoltage output. When the controller detects the OVP pin
voltage reaching the threshold VOVP_TH, typically 1.25V,
overvoltage protection is activated. Hence, the step-up
converter output overvoltage-protection point is:
R1
VOUT(OVP)
= VOVP_TH × (1 +
)
R2
VOUT(OVP) depends on how many LEDs are used for
each string and VOUT(OVP) = 1.25V x VOUT, generally
and where VOUT is the LED’s operating voltage for each
string.
In Figure 1, the output OVP voltage is set to:
VOUT(OVP)
= 1.25V × (1 +
Input Capacitor Selection
2.21MΩ
=
) 39.71V
71.5kΩ
The input capacitor (CIN) filters the current peaks
drawn from the input supply and reduces noise
injection into the IC. A 4.7µF ceramic capacitor is used in the
typical operating circuit (Figure 1) because of the high
source impedance seen in typical lab setups. Actual
applications usually have much lower source impedance since the step-up regulator often runs directly
from the output of another regulated supply. In some
applications, CIN can be reduced below the values used in the
typical operating circuit. Ensure a low-noise supply at
IN by using adequate CIN. Alternatively, greater voltage
variation can be tolerated on CIN if IN is decoupled from
CIN using an RC lowpass filter.
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LED Selection and Bias
The series/parallel configuration of the LED load and the
full-scale bias current have a significant effect on regulator
performance. LED characteristics vary significantly from
manufacturer to manufacturer. Consult the respective
LED data sheets to determine the range of output voltages for a given brightness and LED current. In general,
brightness increases as a function of bias current. This
suggests that the number of LEDs could be decreased
if higher bias current is chosen; however, high current
increases LED temperature and reduces operating life.
Improvements in LED technology are resulting in devices
with lower forward voltage while increasing the bias
current and light output.
LED manufacturers specify LED color at a given LED
current. With lower LED current, the color of the emitted
light tends to shift toward the blue range of the spectrum.
A blue bias is often acceptable for business applications,
but not for high-image-quality applications such as DVD
players. Direct-DPWM dimming is a viable solution for
reducing power dissipation while maintaining LED color
integrity. Careful attention should be paid to switching
noise to avoid other display-quality problems.
Using fewer LEDs in a string improves step-up converter
efficiency, and lowers breakdown voltage requirements of
the external MOSFET and diode. The minimum number of
LEDs in series should always be greater than maximum
input voltage. If the diode voltage drop is lower than maximum input voltage, the voltage drop cross the currentsense inputs (FB_) increases and causes excess heating
in the IC. Between 8 and 12 LEDs in series are ideal for
input voltages up to 20V.
Applications Information
LED VFB_Variation
The forward voltage of each white LED may vary up
to 25% from part to part and the accumulated voltage
difference in each string equates to additional power
loss within the IC. For the best efficiency, the voltage
difference between strings should be minimized. The
difference between lowest voltage string and highest
voltage string should be less than 8V (typ). Otherwise,
the internal LED short-protection circuit disables the high
FB_ voltage string.
Maxim Integrated │ 19
MAX17127
FB Pin Maximum Voltage
The current through each FB_ pin is controlled only during
the step-up converter’s on-time. During the converter offtime, the current sources are turned off. The output voltage does not discharge and stays high. The MAX17127
disables the FB_ current source, which the string is
shorted. In this case, the step-up converter’s output voltage is always applied to the disabled FB_ pin. The FB_
pin can withstand 45V.
PCB Layout Guidelines
Careful PCB layout is important for proper operation. Use
the following guidelines for good PCB layout:
1)Minimize the area of high-current switching loop of
rectifier diode, internal MOSFET, and output capacitor
to avoid excessive switching noise.
2)Connect high-current input and output components
with short and wide connections. The high-current
input loop goes from the positive terminal of the input
capacitor to the inductor, to the internal MOSFET,
and then to the input capacitor’s negative terminal.
The high-current output loop is from the positive
terminal of the input capacitor to the inductor, to the
rectifier diode, and to the positive terminal of the output
capacitors, reconnecting between the output
capacitor and input capacitor ground terminals. Avoid
using vias in the high-current paths. If vias are
unavoidable, use multiple vias in parallel to reduce
resistance and inductance.
www.maximintegrated.com
Six-String WLED Driver with
Integrated Step-Up Converter
3)Create a ground island (PGND) consisting of the
input and output capacitor ground. Connect all these
together with short, wide traces or a small ground
plane. Maximizing the width of the power ground
traces improves efficiency and reduces outputvoltage ripple and noise spikes. Create an analog
ground island (AGND) consisting of the overvoltagedetection divider (R1 and R2) ground connection; the
ISET, FSLCT, COMP resistor connections; and the
device’s exposed backside pad. Connect the AGND
and PGND islands by connecting the AGND pins
directly to the exposed backside pad. Make no other
connections between these separate ground planes.
4)Place the overvoltage-detection divider resistors as
close as possible to the OVP pin. The divider’s center
trace should be kept short. Placing the resistors far
away causes the sensing trace to become antennae
that can pick up switching noise. Avoid running the
sensing traces near SW.
5) Place the VIN pin and VDDIO pin bypass capacitors as
close to the device as possible. The ground connection
of the bypass capacitors should be connected directly
to AGND pins with a wide trace.
6) Minimize the size of the SW node while keeping it wide
and short. Keep the SW node away from the feedback
node and ground. If possible, avoid running the SW
node from one side of the PCB to the other. Use DC
traces as a shield if necessary.
Refer to the MAX17127 Evaluation Kit data sheet for an
example of proper board layout.
Maxim Integrated │ 20
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Simplified Operating Circuit (Direct-PWM Mode)
VIN
VDDIO
SW
PGND
ISET
OVP
COMP
FSLCT
AGND
MAX17127
I.C.
EN
FB1
PWM
FB2
R_FPWM
FB3
3.3V
FB4
FB5
FPO
FB6
EP
Chip Information
PROCESS: BiCMOS
www.maximintegrated.com
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status
only. Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
PACKAGE TYPE
PACKAGE CODE
DOCUMENT NO.
20 TQFN
T2044+3
21-0139
Maxim Integrated │ 21
MAX17127
Six-String WLED Driver with
Integrated Step-Up Converter
Revision History
REVISION
NUMBER
REVISION
DATE
0
3/10
Initial release
—
11/14
No /V OPN in Ordering Information; deleted automotive reference from Applications
section
1
1
PAGES
CHANGED
DESCRIPTION
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
© 2014 Maxim Integrated Products, Inc. │ 22
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