MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step

MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step
EVALUATION KIT AVAILABLE
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
General Description
The MAX1836/MAX1837 high-efficiency step-down converters provide a preset 3.3V or 5V output voltage from
supply voltages as high as 24V. Using external feedback
resistors, the output voltage can be adjusted from 1.25V
to VIN. An internal current-limited switching MOSFET
delivers load currents up to 125mA (MAX1836) or 250mA
(MAX1837).
The unique current-limited control scheme, operating
with duty cycles up to 100%, minimizes the dropout voltage (120mV at 100mA). Additionally, this control scheme
reduces supply current under light loads to 12μA. High
switching frequencies allow the use of tiny surface-mount
inductors and output capacitors.
The MAX1836/MAX1837 step-down converters with internal switching MOSFETs are available in 6-pin SOT23
and 3mm x 3mm TDFN packages, making them ideal
for low-cost, low-power, space-sensitive applications.
For increased output drive capability, use the MAX1776
step-down converter that uses an internal 24V switch to
deliver up to 500mA. For even higher currents, use the
MAX1626/MAX1627 step-down controllers that drive an
external P-channel MOSFET to deliver up to 20W.
Applications
●●
●●
●●
●●
●●
●●
●●
9V Battery Systems
Notebook Computers
Distributed Power Systems
Backup Supplies
4mA to 20mA Loop Power Supplies
Industrial Control Supplies
Handheld Devices
IN
SHDN
LX
MAX1836
MAX1837
●● 4.5V to 24V Input Voltage Range
●● Preset 3.3V or 5V Output
●● Adjustable Output from 1.25V to VIN
●● Output Currents Up to 125mA (MAX1836) or
250mA (MAX1837)
●● Efficiency Over 90%
●● 12μA Quiescent Current
●● 3μA Shutdown Current
●● 100% Maximum Duty Cycle for Low Dropout
●● Small 6-Pin SOT23 and TDFN Packages
Ordering Information
PART
TEMP RANGE
PINPACKAGE
MAX1836ETT33-T
-40°C to +85°C
6 TDFN-EP*
AJG
MAX1836ETT50-T
-40°C to +85°C
6 TDFN-EP*
AJE
MAX1836EUT33-T
-40°C to +85°C
6 SOT23
AANY
MAX1836EUT50-T
-40°C to +85°C
6 SOT23
AANW
MAX1837ETT33-T
-40°C to +85°C
6 TDFN-EP*
MAX1837ETT50-T
-40°C to +85°C
6 TDFN-EP*
MAX1837EUT33-T
-40°C to +85°C
6 SOT23
AANZ
MAX1837EUT50-T
-40°C to +85°C
6 SOT23
AANX
FB
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
19-1919; Rev 3; 7/06
AJF
Selector Guide appears at end of data sheet.
Pin Configurations
OUTPUT
3.3V OR 5V
TOP VIEW
GND 2
MAX1836
MAX1837
6
OUT
5
SHDN
4
LX
FB 1
GND 2
MAX1836
MAX1837
IN 3
GND
AJH
T = Tape and reel.
FB 1
OUT
TOP
MARK
*EP = Exposed pad.
Typical Operating Circuit
INPUT
4.5V TO 24V
Features
IN 3
SOT23
TDFN
6
OUT
5
SHDN
4
LX
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Absolute Maximum Ratings
IN, SHDN to GND...................................................-0.3V to +25V
LX to GND.......................................................-2V to (VIN + 0.3V)
OUT, FB to GND.......................................................-0.3V to +6V
Continuous Power Dissipation (TA = +70°C) (Note 1)
6-Pin SOT23 (derate 8.7mW/°C above +70°C)...........696mW
6-Pin TDFN (derate 24.4mW/°C above +70°C).........1951mW
Operating Temperature Range..............................-40°C to +85°C
Junction Temperature.......................................................+150°C
Storage Temperature Range...............................-65°C to +150°C
Lead Temperature (soldering, 10s)...................................+300°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.
Note 1: Thermal properties are specified with product mounted on PC board with 1in2 of copper area and still air.
Electrical Characteristics
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = 0°C to +85°C. Typical values are at TA = +25°C,
unless otherwise noted.)
PARAMETER
Input Supply Range
Input Undervoltage Lockout
Threshold
Input Supply Current
Input Supply Current in Dropout
SYMBOL
CONDITIONS
VIN
VUVLO
MIN
MAX
UNITS
24
V
VIN rising
3.55
4.0
4.4
VIN falling
3.45
3.9
4.3
12
25
IIN
IIN(DROP)
TYP
4.5
VIN = 5V
18
Input Shutdown Current
SHDN = GND
3
7
Output Voltage (Preset Mode)
VOUT
FB = GND,
ILOAD = 0 to 125mA
(MAX1836) or
250mA (MAX1837)
Output Voltage Range
(Adjustable Mode)
VOUT
(Note 2)
Feedback Set Voltage
(Adjustable Mode)
VFB
OUT Bias Current
FB Bias Current
FB Dual Mode
TM
Threshold
LX Switch Minimum Off-Time
tOFF(MIN)
LX Switch Maximum On-Time
tON(MAX)
LX Switch On-Resistance
LX Current Limit
RLX
ILIM
µA
µA
MAX183_EUT50,
MAX183_ETT50
4.80
5.00
5.20
MAX183_EUT33,
MAX183_ETT33
3.168
3.30
3.432
µA
V
1.25
1.200
VOUT = 5V
IFB
V
1.25
2.5
VIN
V
1.300
V
7.4
µA
+25
nA
100
150
mV
0.2
0.4
0.6
µs
7
10
13
µs
1.1
2
Ω
MAX1836
250
312
450
MAX1837
500
625
850
VFB = 0 or 1.25V, TA = +25°C
-25
VFB rising or falling
50
VFB = 1.3V
VIN = 6V
LX Zero-Crossing Threshold
-75
+75
mA
mV
Dual Mode is a trademark of Maxim Integrated Products, Inc.
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Maxim Integrated │ 2
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Electrical Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = 0°C to +85°C. Typical values are at TA = +25°C,
unless otherwise noted.)
PARAMETER
SYMBOL
Zero-Crossing Timeout
LX Switch Leakage Current
Dropout Voltage
CONDITIONS
MIN
LX does not rise above the threshold
TYP
VIN = 18V, LX = GND, TA = +25°C
VDROPOUT
MAX
30
UNITS
µs
1
µA
IOUT = 100mA, VIN = 5V
120
mV
Line Regulation
VIN = 5V to 24V
0.05
%
Load Regulation
IOUT = 0 to 125mA (MAX1836) or 250mA
(MAX1837)
0.3
%
Shutdown Input Threshold
VSHDN
VIN = 4.5V to 24V (Note 3)
0.8
Shutdown Leakage Current
ISHDN
VSHDN = 0 or 24V
-1
Thermal Shutdown
10°C hysteresis (typ)
2.4
V
+1
µA
160
°C
Electrical Characteristics
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = -40°C to +85°C, unless otherwise noted.) (Note 4)
PARAMETER
Input Supply Range
Input Undervoltage Lockout
Threshold
Input Supply Current
SYMBOL
CONDITIONS
MAX
UNITS
4.5
24
V
VIN rising
3.55
4.4
VIN falling
3.45
4.3
VIN
VUVLO
IIN
Input Shutdown Current
SHDN = GND
Output Voltage (Preset Mode)
VOUT
FB = GND,
ILOAD = 0 to 125mA
(MAX1836) or
250mA (MAX1837)
Output Voltage Range
(Adjustable Mode)
VOUT
(Note 2)
Feedback Set Voltage
(Adjustable Mode)
VFB
OUT Bias Current
VOUT = 5V
FB Dual Mode Threshold
VFB rising or falling
LX Switch Minimum Off-Time
tOFF(MIN)
LX Switch Maximum On-Time
tON(MAX)
LX Switch On-Resistance
RLX
LX Current Limit
ILIM
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MIN
VFB = 1.3V
TYP
V
25
µA
7
µA
MAX183_EUT50,
MAX183_ETT50
4.80
5.20
MAX183_EUT33,
MAX183_ETT33
3.168
3.432
1.25
VIN
V
1.200
1.300
V
7.4
µA
50
150
mV
0.2
0.6
µs
7
13
µs
2
Ω
V
VIN = 6V
MAX1836
250
450
MAX1837
500
900
mA
Maxim Integrated │ 3
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Electrical Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = -40°C to +85°C, unless otherwise noted.) (Note 4)
PARAMETER
SYMBOL
CONDITIONS
MIN
LX Zero-Crossing Threshold
TYP
MAX
UNITS
-75
+75
mV
Shutdown Input Threshold
VSHDN
VIN = 4.5V to 24V (Note 3)
0.8
2.4
V
Shutdown Leakage Current
ISHDN
VSHDN = 0 or 24V
-1
+1
µA
Note 2: When using the shutdown input, the maximum output voltage allowed with external feedback is 5.5V. If the output voltage is
set above 5.5V, connect shutdown to the input.
Note 3: Shutdown input minimum slew rate (rising or falling) is 10V/ms.
Note 4: Specifications to -40°C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25°C.)
3.30
VIN = 9V to 12V
3.29
90
85
80
100
150
70
200
0.1
1
VIN = 5V
VIN = 9V
85
80
FIGURE 2
VOUT = 3.3V
160
1
10
100
LOAD CURRENT (mA)
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0
50
140
VIN = 12V
120
1000
100
80
60
0
50
100
150
200
250
LOAD CURRENT (mA)
MAX1836/7 toc03
150
200
250
300
350
MAX1837EUT33
OUTPUT VOLTAGE vs. INPUT VOLTAGE
3.33
300
IOUT = 10mA
3.32
3.31
IOUT = 200mA
3.30
3.29
FIGURE 2
VOUT = 3.3V
L1 = 47µH
3.28
VIN = 5V
0
100
LOAD CURRENT (mA)
VIN = 9V
20
VIN = 12V
0.1
3.27
1000
40
75
70
180
MAX1836/7 toc04
FIGURE 2
VOUT = 3.3V
VIN = 12V
3.29
MAX1837EUT33
SWITCHING FREQUENCY vs. LOAD CURRENT
MAX1837EUT33
EFFICIENCY vs. LOAD CURRENT
90
100
VIN = 5V
VIN = 9V
3.30
LOAD CURRENT (mA)
FREQUENCY (kHz)
EFFICIENCY (%)
95
10
OUTPUT VOLTAGE (V)
50
3.31
3.28
MAX1836/7 toc05
0
FIGURE 2
3.32
75
LOAD CURRENT (mA)
100
VIN = 9V
VIN = 12V
3.28
3.27
VIN = 5V
3.33
350
MAX1836/7 toc06
3.31
FIGURE 1
VOUT = 3.3V
95
EFFICIENCY (%)
OUTPUT VOLTAGE (V)
VIN = 5V
MAX1837EUT33
OUTPUT VOLTAGE vs. LOAD CURRENT
OUTPUT VOLTAGE (V)
FIGURE 1
3.32
100
MAX1836/7 toc01
3.33
MAX1836EUT33
EFFICIENCY vs. LOAD CURRENT
MAX1836/7 toc02
MAX1836EUT33
OUTPUT VOLTAGE vs. LOAD CURRENT
3.27
0
4
8
12
16
20
24
INPUT VOLTAGE (V)
Maxim Integrated │ 4
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Typical Operating Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25°C.)
85
IOUT = 200mA
80
10
IOUT = 10mA
75
70
0
4
8
IOUT = 10mA
12
16
20
1
24
0
8
12
16
20
5.00
VIN = 7V
4.98
400
200
LIMITED BY
tON(MIN)
0
0
95
FIGURE 6
0
50
100
150
200
250
VIN = 9V
VIN = 12V
90
250
200
150
100
20
24
VIN = 7V
VIN = 18V
0.1
1
10
VIN = 24V
100
1000
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
15
MAX1836/7 toc13
300
16
LOAD CURRENT (mA)
SUPPLY CURRENT (µA)
MAX1836/7 toc12
DROPOUT VOLTAGE (mV)
350
12
80
70
300
MAX1837EUT50
DROPOUT VOLTAGE vs. LOAD CURRENT
FIGURE 6
VOUT = 5V
8
85
LOAD CURRENT (mA)
400
LIMITED BY
ILIM
4
FIGURE 6
VOUT = 5V
75
4.96
MAX1836/7 toc09
IOUT = 10mA
MAX1837EUT50
EFFICIENCY vs. LOAD CURRENT
100
MAX1836/7 toc10
VIN = 12V TO 24V
VIN = 9V
IOUT = 200mA
600
24
MAX1837EUT50
OUTPUT VOLTAGE vs. LOAD CURRENT
5.02
800
INPUT VOLTAGE (V)
EFFICIENCY (%)
OUTPUT VOLTAGE (V)
4
FIGURE 2
VOUT = 3.3V
L1 = 47µH
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
5.04
MAX1836/7 toc08
FIGURE 2
VOUT = 3.3V
L1 = 47µH
1000
MAX1836/7 toc11
90
IOUT = 200mA
PEAK INDUCTOR CURRENT (mA)
EFFICIENCY (%)
95
100
FREQUENCY (kHz)
FIGURE 2
VOUT = 3.3V
L1 = 47µH
MAX1836/7 toc07
100
MAX1837EUT33
PEAK INDUCTOR CURRENT vs. INPUT VOLTAGE
MAX1837EUT33
SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX1837EUT33
EFFICIENCY vs. INPUT VOLTAGE
14
13
12
11
50
0
0
100
200
LOAD CURRENT (mA)
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300
10
0
4
8
12
16
20
24
INPUT VOLTAGE (V)
Maxim Integrated │ 5
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Typical Operating Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25°C.)
MAX1837EUT50
LOAD TRANSIENT
400mA
MAX1837EUT50
LINE TRANSIENT
MAX1836/7 toc14
200mA
20V
A
0
5.02V
5.1V
B
4.9V
750mA
500mA
C
C
0
100µs/div
400µs/div
A: IOUT = 10mA to 250mA, 200mA/div
B: VOUT = 5V, 20mV/div
C: IL, 500mA/div
VIN = 12V, FIGURE 6
A: VIN = 9V to 18V, 10V/div
B: VOUT = 5V, ROUT = 100Ω, 100mV/div
C: IL, 500mA/div
FIGURE 6
MAX1837EUT50
STARTUP WAVEFORM
MAX1837EUT50
LINE TRANSIENT NEAR DROPOUT
MAX1836/7 toc17
MAX1836/7 toc16
15V
B
5.0V
4.98V
250mA
0
A
10V
0
5.00V
MAX1836/7 toc15
10V
A
2V
A
0
5V
5.1V
B
5.0V
4V
2V
B
0
4.9V
500mA
C
0
400µs/div
A: VIN = 5V to 12V, 5V/div
B: VOUT = 5V, ROUT = 100Ω, 100mV/div
C: IL, 500mA/div
FIGURE 6
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500mA
C
0
200µs/div
A: VSHDN = 0 to 2V, 2V/div
B: VOUT = 5V, ROUT = 100Ω, 2V/div
C: IL, 500mA/div
VIN = 12V, FIGURE 6
Maxim Integrated │ 6
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Pin Description
PIN
NAME
FUNCTION
1
FB
Dual-Mode Feedback Input. Connect to GND for the preset 3.3V (MAX183_EUT33) or 5.0V (MAX183_
EUT50) output. Connect to a resistive divider between the output and FB to adjust the output voltage between
1.25V and VIN, and connect the OUT pin to GND. When setting output voltages above 5.5V, permanently
connect SHDN to IN.
2
GND
3
IN
Input Voltage. 4.5V to 24V input range. Connected to the internal p-channel power MOSFET’s source.
4
LX
Inductor Connection. Connected to the internal p-channel power MOSFET’s drain.
5
SHDN
6
OUT
—
EP
INPUT
4.5V OR 12V
CIN
10µF
25V
Ground
Shutdown Input. A logic-low shuts down the MAX1836/MAX1837 and reduces supply current to 3µA. LX is
high impedance in shutdown. Connect to IN for normal operation. When setting output voltages above 5.5V,
permanently connect SHDN to IN.
Regulated Output Voltage High-Impedance Sense Input. Internally connected to a resistive divider. Connect to
the output when using the preset output voltage. Connect to GND when using an external resistive divider to
adjust the output voltage.
Exposed Metal Pad. Connect to GND. This pad is internally connected to GND through a soft connect. For
proper grounding and good thermal dissipation. Connect the exposed pad to GND.
IN
L1
47µH
LX
D1
SHDN
MAX1836
GND
OUT
OUTPUT
3.3V OR 5V
COUT
100µF
6.3V
FB
INPUT
4.5V OR 12V
CIN
10µF
25V
IN
LX
D1
SHDN
MAX1837
GND
L1
22µH
OUT
OUTPUT
3.3V OR 5V
COUT
150µF
6.3V
FB
CIN = TAIYO YUDEN TMK432BJ106KM
L1 = SUMIDA CDRH5D28-470
COUT = SANYO POSCAP 6TPC100M (SMALLER CAPACITORS CAN BE USED FOR 5V)
D1 = NIHON EP05Q03L
CIN = TAIYO YUDEN TMK432BJ106KM
L1 = SUMIDA CDRH5D28-220
COUT = SANYO OS-CON 6SA150M (SMALLER CAPACITORS CAN BE USED FOR 5V)
D1 = NIHON ED05Q03L
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Figure 1. Typical MAX1836 Application Circuit
Figure 2. Typical MAX1837 Application Circuit
Detailed Description
high switching frequency minimize PC board space and
component cost.
The MAX1836/MAX1837 step-down converters are
designed primarily for battery-powered devices, notebook
computers, and industrial control applications. A unique
current-limited control scheme provides high efficiency
over a wide load range. Operation up to 100% duty cycle
allows the lowest possible dropout voltage, increasing
the useable supply voltage range. Under no-load, the
MAX1836/MAX1837 draw only 12μA, and in shutdown
mode, they draw only 3μA to further reduce power consumption and extend battery life. Additionally, an internal
24V switching MOSFET, internal current sensing, and a
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Current-Limited Control Architecture
The MAX1836/MAX1837 use a proprietary current-limited
control scheme that operates with duty cycles up to 100%.
These DC-DC converters pulse as needed to maintain
regulation, resulting in a variable switching frequency that
increases with the load. This eliminates the high supply
currents associated with conventional constant-frequency
pulse-width-modulation (PWM) controllers that switch the
MOSFET unnecessarily.
Maxim Integrated │ 7
MAX1836/MAX1837
INPUT
4.5V OR 24V
CIN
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
L1
LX
IN
SHDN
OUTPUT
3.3V OR 5V
D1
COUT
VSENSE
OUT
R
Q
FB
MAXIMUM
OFF-TIME
DELAY
S
Q
TRIG
100mV
Q
MAX1836
MAX1837
TRIG
MAXIMUM
ON-TIME
DELAY
VSET
1.25V
GND
Figure 3. Functional Diagram
When the output voltage is too low, an error comparator sets a flip-flop, which turns on the internal p-channel
MOSFET and begins a switching cycle (Figure 3). As
shown in Figure 4, the inductor current ramps up linearly,
charging the output capacitor and servicing the load. The
MOSFET turns off when the current limit is reached, or
when the maximum on-time is exceeded while the output
voltage is in regulation. Otherwise, the MOSFET remains
on, allowing a duty cycle up to 100% to ensure the lowest
possible dropout voltage. Once the MOSFET turns off, the
flip-flop resets, diode D1 turns on, and the current through
the inductor ramps back down, transferring the stored
energy to the output capacitor and load. The MOSFET
remains off until the 0.5μs minimum off-time expires and
the inductor current ramps down to zero, and the output
voltage drops back below the set point.
10V
A
0
B
3.3V
500mA
C
0
4µs/div
CIRCUIT OF FIGURE 2, VIN = 12V
A. VLX, 5V/div
B. VOUT = 3.3V, 20mV/div, 200mA LOAD
C. INDUCTOR CURRENT, 500mA/div
Figure 4. Discontinuous-Conduction Operation
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Maxim Integrated │ 8
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Input-Output (Dropout) Voltage
the Selector Guide. For example, the MAX1836EUT33
has a preset 3.3V output voltage.
A step-down converter’s minimum input-to-output voltage differential (dropout voltage) determines the lowest
useable input supply voltage. In battery-powered systems, this limits the useful end-of-life battery voltage. To
maximize battery life, the MAX1836/MAX1837 operate
with duty cycles up to 100%, which minimizes the inputto-output voltage differential. When the supply voltage
approaches the output voltage, the P-channel MOSFET
remains on continuously to supply the load.
Dropout voltage is defined as the difference between the
input and output voltages when the input is low enough for
the output to drop out of regulation. For a step-down converter with 100% duty cycle, the dropout voltage depends
on the MOSFET drain-to-source on-resistance (RDS(ON))
and inductor series resistance; therefore, it is proportional
to the load current:
(
VDROPOUT =
I OUT × R DS(ON) + R INDUCTOR
)
Shutdown (SHDN)
A logic-level low voltage on SHDN shuts down the
MAX1836/MAX1837. When shut down, the supply current drops to 3μA to maximize battery life, and the internal
P-channel MOSFET turns off to isolate the output from the
input. The output capacitance and load current determine
the rate at which the output voltage decays. A logic-level
high voltage on SHDN activates the MAX1836/MAX1837.
Do not leave SHDN floating. If unused, connect SHDN to
IN. When setting output voltages above 5.5V, the shutdown feature cannot be used, so SHDN must be permanently connected to IN. The SHDN input voltage slew rate
must be greater than 10V/ms.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation in the MAX1836/MAX1837. When the junction temperature exceeds TJ = +160°C, a thermal sensor turns off
the pass transistor, allowing the IC to cool. The thermal
sensor turns the pass transistor on again after the IC’s
junction temperature cools by 10°C, resulting in a pulsed
output during continuous thermal-overload conditions.
The MAX1836/MAX1837 output voltage may be adjusted
by connecting a voltage divider from the output to FB
(Figure 5). When externally adjusting the output voltage,
connect OUT to GND. Select R2 in the 10kΩ to 100kΩ
range. Calculate R1 with the following equation:
 V
 
=
R1 R2  OUT  − 1
V
 FB  
where VFB = 1.25V, and VOUT may range from 1.25V to
VIN. When setting output voltages above 5.5V, the shutdown feature cannot be used, so SHDN must be permanently connected to IN.
Inductor Selection
When selecting the inductor, consider these four parameters: inductance value, saturation current rating, series
resistance, and size. The MAX1836/MAX1837 operate
with a wide range of inductance values. For most applications, values between 10μH and 100μH work best with
the controller’s switching frequency. Calculate the minimum inductance value as follows:
L (MIN) =
The feedback input features dual-mode operation.
Connect the output to OUT and FB to GND for the preset
output voltage. The MAX1836/MAX1837 are supplied
with factory-set output voltages of 3.3V or 5V. The twodigit part number suffix identifies the output voltage. See
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ILIM
where tON(MIN) = 1.0μs. Inductor values up to six times
L(MIN) are acceptable. Low-value inductors may be smaller in physical size and less expensive, but they result in
higher peak-current overshoot due to current-sense comparator propagation delay (300ns). Peak-current overshoot reduces efficiency and could exceed the current
ratings of the internal switching MOSFET and external
components.
INPUT
4.5V OR 24V
CIN
IN
OUTPUT
1.25V TO VIN
L1
LX
D1
SHDN
R1
COUT
FB
Design Information
Output Voltage Selection
(VIN(MAX) − VOUT ) t ON(MIN)
MAX1836
MAX1837
GND
R2
OUT
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Figure 5. Adjustable Output Voltage
Maxim Integrated │ 9
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
The inductor’s saturation current rating must be greater
than the peak switching current, which is determined
by the switch current limit plus the overshoot due to the
300ns current-sense comparator propagation delay:
capacitor selection, but final values should be set by testing a prototype or evaluation circuit. As a general rule, a
smaller amount of charge delivered in each pulse results
in less output ripple. Since the amount of charge delivered in each oscillator pulse is determined by the inductor
value and input voltage, the voltage ripple increases with
larger inductance but decreases with lower input voltages.
IPEAK
= ILIM +
(VIN − VOUT ) 300ns
L
where the switch current-limit (ILIM) is typically 312mA
(MAX1836) or 625mA (MAX1837). Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support, and the inductance
starts to fall.
Inductor series resistance affects both efficiency and
dropout voltage. See the Input-Output (Dropout) Voltage
section. High series resistance limits the maximum current
available at lower input voltages and increases the dropout voltage. For optimum performance, select an inductor
with the lowest possible DC resistance that fits in the
allotted dimensions. Typically, the inductor’s series resistance should be significantly less than that of the internal
P-channel MOSFET’s on-resistance (1.1Ω typ). Inductors
with a ferrite core, or equivalent, are recommended.
The maximum output current of the MAX1836/MAX1837
current-limited converter is limited by the peak inductor
current. For the typical application, the maximum output
current is approximately:
I OUT(MAX)
IPEAK
Output Capacitor
Choose the output capacitor to supply the maximum load
current with acceptable voltage ripple. The output ripple
has two components: variations in the charge stored in
the output capacitor with each LX pulse, and the voltage
drop across the capacitor’s equivalent series resistance
(ESR) caused by the current into and out of the capacitor:
VRIPPLE ≈ VRIPPLE(ESR) + VRIPPLE(C)
The output voltage ripple as a consequence of the ESR
and output capacitance is:
VRIPPLE(ESR) = IPEAKESR
VRIPPLE(C) =
2

L(IPEAK − I OUT ) 
VIN


2C OUT VOUT  VIN − VOUT 
With low-cost aluminum electrolytic capacitors, the ESRinduced ripple can be larger than that caused by the
current into and out of the capacitor. Consequently, highquality low-ESR aluminum-electrolytic, tantalum, polymer,
or ceramic filter capacitors are required to minimize output ripple. Best results at reasonable cost are typically
achieved with an aluminum-electrolytic capacitor in the
100μF range, in parallel with a 0.1μF ceramic capacitor.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching. The
input capacitor must meet the ripple-current requirement
(IRMS) imposed by the switching currents defined by the
following equation:
IRMS = ILOAD
VOUT (VIN − VOUT )
VIN
For most applications, nontantalum chemistries (ceramic,
aluminum, polymer, or OS-CON) are preferred due to
their robustness with high inrush currents typical of systems with low-impedance battery inputs. Alternatively,
two (or more) smaller-value low-ESR capacitors can be
connected in parallel for lower cost. Choose an input
capacitor that exhibits < +10°C temperature rise at the
RMS input current for optimal circuit longevity.
Diode Selection
The current in the external diode (D1) changes abruptly
from zero to its peak value each time the LX switch turns
off. To avoid excessive losses, the diode must have a
fast turn-on time and a low forward voltage. Use a diode
with an RMS current rating of 0.5A or greater, and with a
breakdown voltage > VIN. Schottky diodes are preferred.
For high-temperature applications, Schottky diodes may
be inadequate due to their high leakage currents. In
such cases, ultra-high-speed silicon rectifiers are recommended, although a Schottky diode with a higher reverse
voltage rating can often provide acceptable performance.
where IPEAK is the peak inductor current. See the Inductor
Selection section. These equations are suitable for initial
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Maxim Integrated │ 10
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Table 1. Component Suppliers
SUPPLIER
PHONE
FAX
WEBSITE
Coilcraft
847-639-6400
847-639-1469
www.coilcraft.com
Coiltronics
561-241-7876
561-241-9339
www.coiltronics.com
Sumida USA
847-956-0666
847-956-0702
www.sumida.com
Toko
847-297-0070
847-699-1194
www.tokoam.com
AVX
803-946-0690
803-626-3123
www.avxcorp.com
Kemet
408-986-0424
408-986-1442
www.kemet.com
Panasonic
847-468-5624
847-468-5815
www.panasonic.com
Sanyo
619-661-6835
619-661-1055
www.secc.co.jp
Taiyo Yuden
408-573-4150
408-573-4159
www.t-yuden.com
Central Semiconductor
516-435-1110
516-435-1824
www.centralsemi.com
International Rectifier
310-322-3331
310-322-3332
www.irf.com
Nihon
847-843-7500
847-843-2798
www.niec.co.jp
On Semiconductor
602-303-5454
602-994-6430
www.onsemi.com
Zetex
516-543-7100
516-864-7630
www.zetex.com
INDUCTORS
CAPACITORS
DIODES
MAX1836/MAX1837 Stability
Commonly, instability is caused by excessive noise on the
feedback signal or ground due to poor layout or improper
component selection. When seen, instability typically
manifests itself as “motorboating,” which is characterized
by grouped switching pulses with large gaps and excessive low-frequency output ripple during no-load or lightload conditions.
PC Board Layout and Grounding
High switching frequencies and large peak currents make
PC board layout an important part of the design. Poor layout may introduce switching noise into the feedback path,
resulting in jitter, instability, or degraded performance.
High-power traces, bolded in the typical application circuits (Figure 1 and Figure 2), should be as short and wide
as possible. Additionally, the current loops formed by the
power components (CIN, COUT, L1, and D1) should be
as tight as possible to avoid radiated noise. Connect the
ground pins of these power components at a common
node in a star-ground configuration. Separate the noisy
traces, such as the LX node, from the feedback network
with grounded copper. Furthermore, keep the extra copper on the board, and integrate it into a pseudoground
plane. When using external feedback, place the resistors
as close to the feedback pin as possible to minimize noise
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coupling. The MAX1837 evaluation kit shows the recommended layout.
Applications Information
High-Voltage Step-Down Converter
The typical application circuits’ (Figure 1 and Figure 2)
components were selected for 9V battery applications.
However, the MAX1836/MAX1837 input voltage range
allows supply voltages up to 24V. Figure 6 shows a modified application circuit for high-voltage applications. When
using higher input voltages, verify that the input capacitor’s voltage rating exceeds VIN(MAX) and that the inductor value exceeds the minimum inductance recommended
in the Inductor Selection section.
Inverter Configuration
Figure 7 shows the MAX1836/MAX1837 in a floating
ground configuration. By connecting what would normally be the output to the supply-voltage ground, the IC’s
ground pin is forced to regulate to -5V (MAX183_EUT50)
or -3.3V (MAX183_EUT33). Avoid exceeding the maximum ratings of 24V between IN and GND, and 5.5V
between OUT and GND. Other negative voltages may be
generated by placing a resistive divider across the output
capacitor and connecting the tap to FB in the same manner as the normal step-down configuration.
Maxim Integrated │ 11
MAX1836/MAX1837
INPUT
4.5V TO 24V
CIN
10µF
25V
IN
SHDN
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
L1
47µH
LX
OUTPUT
5V
COUT
68µF
10V
D1
MAX1837
OUT
INPUT
3.6V TO 18V
CIN
10µF
IN
L1
47µH
LX
OUT
SHDN
OUTPUT
-3.3V OR -5V
GND
FB
GND
COUT
100µF
D1
MAX1836
MAX1837
FB
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
CIN = TAIYO YUDEN TMK432BJ106KM
L1 = SUMIDA CDRH5D28-470
COUT = SANYO POSCAP 10TPC68M
D1 = NIHON EP05Q03L
Figure 7. MAX1836/MAX1837 Inverter Configuration
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Chip Information
Figure 6. High-Voltage Application
TRANSISTOR COUNT: 731
Selector Guide
PRESET OUTPUT
VOLTAGE (V)
LOAD CURRENT
(mA)
MAX1836ETT33
3.3
125
MAX1836ETT50
5
125
MAX1836EUT33
3.3
125
MAX1836EUT50
5
125
MAX1837ETT33
3.3
250
MAX1837ETT50
5
250
MAX1837EUT33
3.3
250
MAX1837EUT50
5
250
PART
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PROCESS: BiCMOS
Maxim Integrated │ 12
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
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.
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Maxim Integrated │ 13
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Package Information (continued)
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.
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Maxim Integrated │ 14
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Package Information (continued)
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.
Revision History
Pages changed at Rev 3: 1, 7, 8, 12
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.
© 2006 Maxim Integrated Products, Inc. │ 15
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