Liquid Crystal Display Power Supplies

Application Note 21
Micrel
Application Note 21
Liquid Crystal Display Power Supplies
This application note presents solutions for generating the
voltages required by Liquid Crystal Displays. Temperature
compensation of the display voltages and other functions are
also discussed.
of the crystals in the display. VLC is either positive or negative
and varies between 5V and 60V, depending on the display.
The backlight supply voltage is used to power the display’s
backlight. This voltage is dependent upon the type of lamp or
backlighting scheme used.
There are other factors to consider when supplying power to
a LCD display. They are:
• Efficiency
• Battery life
• Output disable
• Turn-on time
• Contrast adjustment
• LCD drive-voltage temperature compensation
Introduction
LCDs prevail as the display of choice in most electronic
devices such as notebook computers, pagers, phones and
hand-held games. LCDs are passive devices: instead of
generating light, they reflect ambient light. They consume
very little power, are compact, and are well suited for batterypowered applications. Other advantages of the LCD are a
relatively low driving voltage (5V to 60V), full-color display
capability, fast writing speed, and good availability from
different vendors.
The power requirements for LCDs vary widely and are
dependent on size, type, and manufacturer. The power
consumption of an LCD is proportional to the update frequency, capacitive load, supply voltage, and LCD driver
voltage swing.
Pdisplay ∝ fupdate·Cload·VLC(supply)·Vswing
The capacitive load (Cload) is proportional to the number of
rows and columns in the display.
Three voltages are required by a LCD display:
• VDD
• VLC
• Backlight supply (optional)
The VDD supply is required for the display’s driver and
controller circuits. This voltage is usually either +3.3V or +5V.
The liquid crystal drive voltage (VLC) controls the orientation
Power Topologies
Four switching converter topologies may be used to provide
bias for the LCD display are outlined in Table 1.
VDD Bias Generation
In many systems, especially those with off-line power sources,
a well regulated +5V or +3.3V voltage is available for the VDD
supply. LCD displays can usually be chosen to match the VDD
voltage of the system components. When the voltage requirements between system components and the LCD cannot be matched, a buck converter (+5V input to +3.3V output)
or a boost converter (+3.3V input to +5V output) is used.
The function of a buck converter is to step down the input
voltage. The MIC2179 IC shown in Figure 1 is a highefficiency 200kHz synchronous buck converter with an input
voltage range of 4.5V to 16.5V and a 1.5A maximum output
current capability.
Topology
Description
Comments
Recommended
Controller
Buck
Steps down input voltage
Simple, self-contained
switching regulator ICs
MIC2574 (52kHz)
MIC4754 (200kHz)
MIC2179 (200kHz)
Boost
Steps up input voltage
Good for positive and negative outputs
MIC3172 (100kHz)
MIC2570 (20kHz)
MIC2571 (20kHz)
Buck-Boost
Step up or step down,
depending on duty cycle
Inverting topology.
VIN may vary greater or less than VOUT
MIC2574 (52kHz)
MIC4754 (200kHz)
SEPIC
Step up or step down,
depending on duty cycle
Noninverting topology.
VIN may vary greater or less than VOUT
MIC3172 (100kHz)
MIC2570 (20kHz)
MIC2571 (20kHz)
Table 1 . Switching Converter Topology Summary
May 1998
219
Application Note 21
Application Note 21
5V
C1
100µF
10V
Micrel
L1
47µH
MIC2179-3.3SM
16,17
15
5
13
VIN
SW
EN
FB
PWM
3,4
SYNC
6
COMP SGND PGND
9–12
1,2,
19,20
R2
10k
D1
MBRSA130
14
BIAS
8
C2
100µF
10V
7
PWRGD
operation down to an input voltage of 3.0V. The switching
regulator uses a PWM control scheme with a 100kHz switching frequency.
Battery-powered applications require active voltage regulation to power LCDs because of the wide voltage difference
between a fully charged and a discharged battery. If the VDD
output voltage is always greater than the input voltage, a
boost regulator is used. If the VDD output voltage is always
less than the input voltage, then a buck regulator is used.
Consideration of the topology selected is important if the
output voltage is between the minimum and maximum input
voltage. A buck converter cannot be used because the input
voltage must always be greater than the output voltage. A
boost converter may be used with a linear post regulator as
shown in Figure 3. This is effective when the input voltage
rises above the desired output but suffers from the low
conversion efficiency of the linear regulator.
SEPIC (single-ended primary inductance converter) is a
topology which may be used when the output voltage falls
between the minimum and maximum input voltage (Fig-
VOUT
3.3V/200mA
C3
0.01µF
C4
6.8nF
U1
C1, 2
C3, 4
D1
L1
R1
Micrel
AVX
Motorola
Coilcraft
MIC2179-3.3SM, buck converter IC
TPSD107M010R0100, tantalum
X7R, ceramic
MBRA130LT3
DT3316P-473, 47µH
metal film
Figure 1. Buck Converter (MIC2179)
A boost converter steps up the input voltage. The MIC3172
shown in Figure 2 is a boost converter controller with a
maximum output voltage of 60V. It features guaranteed
VIN
3.3V
D1
L1
C1
100µF
10V
C2
100µF
10V
MIC3172
5
4
2
C3
0.22µF
R3
3.32k
VOUT
5V/40mA
IN
SW
EN
FB
7
R1
49.9k
3
COMP
R2
16.5k
SGND PGND1 PGND2
1
U1
C1
C2
C3
D1
L1
8
6
Micrel
AVX
AVX
 R1
VOUT = 1.24V 1 +

 R2 
MIC3172BM, boost converter IC
TAJD107M010R, tantalum
TPSD107M010R0100, tantalum
X7R, ceramic
10BQ040, 1A 40V Schottky
CTX25-1P, 25µH -orDT3316P-223, 22µH
metal film
IR
Coiltronics
Coilcraft
R1–3
Figure 2. Boost Converter (MIC3172)
Enable
Shutdown
VIN
3.6V to 6V
C1
100µF
10V
D1
2
IN
SW
EN
FB
7
VOUT
5V/100mA
OUT
EN
C2
100µF
10V
U1 MIC3172
4
C4
0.22µF
R3
3.32k
IN
L1
5
U2
MIC5205-5.0
BYP
GND
C5
470pF
R1
49.9k
3
COMP
C3
22µF
10V
R2
14.7k
SGND PGND1 PGND2
1
8
6
R1

VIN-linear = 1.24V 1 +


R2 
U1
U2
C1
C2
C3
C4, 5
D1
L1
R1–3
Micrel
Micrel
AVX
AVX
AVX
Motorola
Coilcraft
MIC3172BM, boost converter IC
MIC5205-5.0BM5, 150mA low-noise LDO regulator
TAJD107M010R, tantalum
TPSD107M010R0100, tantalum
TAJB226M010R, tantalum
X7R, ceramic
MBRA130LT3, 1A 30V Schottky
DT3316P-473, 47µH
metal film
Figure 3. Linear Post-Regulated Boost Converter (MIC3172, MIC5205)
Application Note 21
220
May 1998
Application Note 21
Micrel
VIN
3.6V to 6V
C1
10µF
16V
MIC3172
5
4
2
C4
0.22µF
R3
3.32k
C2
10µF 16V
L1a
D1
MBRA130LT3
C3
100µF
10V
L1b
IN
SW
EN
FB
VOUT
5V/100mA
R1
49.9k
R3
16.5k
7
3
COMP
 R1
VOUT = 1.24V 1 +

 R2 
SGND PGND1 PGND2
1
U1
C1, 2
C3
C4
D1
L1
R1–3
8
Micrel
Tokin
AVX
Motorola
Coiltronics
6
MIC3172, boost converter IC
C23Y5V1C106ZP, ceramic
TPSD107M010R0100, tantalum
X7R ceramic
MBRA130LT3, 1A 30V Schottky
CTX33-2P, 33µH 2-winding
metal film
Figure 4. SEPIC Converter (MIC3172)
ure 4). A SEPIC can increase or decrease the input voltage.
It has the advantage of isolating the input and output with a
capacitor, which allows the output voltage to fully discharge
when the converter is turned off. The two inductors, L1a and
L1b, may be coupled together in a two winding inductor. This
reduces the amount of board space used by the inductor. The
SEPIC converter shown in Figure 4 regulates a 5V output
over a 4-cell input voltage range.
VLC Bias Generation
LCDs operate by applying a bias voltage (VLC) across the
liquid crystal. This changes the crystal’s orientation and
therefore the contrast of the display. Some displays require
a positive voltage, but many displays—including the extended
temperature models—require a negative bias voltage.
There are three factors to consider when deciding on a
biasing topology for an LCD:
• Voltage polarity and magnitude
• Contrast adjustment
• Temperature compensation
The voltage polarity and magnitude depend on the display
chosen. A positive VLC bias is developed using the same
circuits that generated the VDD bias. A negative VLC bias is
created using either a buck-boost converter or a negative
output boost converter. Examples of these topologies are
shown in the next section.
Varying the voltage across the LCD provides contrast adjustment. This is done with a potentiometer connected to the
display or varying the VLC voltage generated by the power
supply. These circuits are presented in Figures 14 through 17.
The ambient temperature around the screen affects the
display screen contrast and viewing angle. A temperature
compensation circuit is used to provide a negative temperature coefficient to the VLC voltage to keep the contrast
constant. As with adjusting the contrast, temperature compensation may be performed by an external circuit connected
May 1998
to the display or adjusting the VLC voltage output of the power
supply. Circuits to perform these functions are discussed
later in this application note.
VLC Bias Topologies
Micrel Semiconductor switching converters, used for LCD
bias applications, have an input voltage range of 0.9V to 40V.
The topologies shown below are divided four categories.
• Low voltage input (VIN < 3.0V)
• Higher voltage input (VIN > 3.0V)
• An input voltage range which is both greater or less
than the output voltage
• High voltage output (> switch voltage rating)
Generating –VLC Bias From VIN < 3V
Use the MIC2570 or MIC2571 boost converter to develop a
negative VLC voltage from a 1- to 3-cell battery. Figure 5
shows the MIC2570 connected as a 2-cell to –24V switching
regulator. The maximum output voltage for a boost converter
using the MIC2570 or MIC2571 is 32V.
VIN
1.8V to 3V
C1
22µF
6.3V
D1
MBRA140
L1
MIC2570-2
8
7
IN
SW
SYNC
FB
GND
1
C4
22µF
35V
R2
C2
0.1µF 1M
50V
6
2
D3
MBRA140
VOUT
U1
C1
C2
C3, 4
D1–3
L1
R1–3
 R2 
= 0.22 1 +

 R1
Micrel
AVX
AVX
Motorola
Coiltronics
D2
MBRA140
R3
220k
R1
9.31k
C3
22µF
35V
VOUT
–24V/5mA
MIC2570-2BM, boost converter IC
TAJB226M063R, 22µF 6.3V tantalum
0.1µF 50V ceramic
TAJC226M035R, 22µF 35V tantalum
MBRA140T3, 1A 40V Schottky
CTX50-1P, 200µH series connected windings
metal film
Figure 5. Negative-Output Boost (MIC2570)
221
Application Note 21
Application Note 21
Micrel
VIN
5V
D1
MBRA140
L1
C1
68µF
16V
4
2
R4
4.99k
C5
0.1µF
IN
SW
EN
FB
C2
0.1µF R2
50V
100k
C4
33µF
25V
U1 MIC3172
5
7
3
COMP
SGND PGND1 PGND2
1
8
6
D3
MBRA140
R1
8.66k
D2
MBRA140
C3
33µF
25V
R3
220k
U1
C1
C2, 5
C3, 4
L1
D1–3
R1–4
Micrel
AVX
AVX
Coiltronics
Motorola
MIC3172BM, boost converter IC
TAJD686M016R, 68µF 16V tantalum
0.1µF 50V ceramic
TAJE336M025R, 33µF 25V tantalum
CTX50-1P, 200µH series connected windings
MBRA140T3, 1A 40V Schottky
metal film
VOUT
–15V/20mA
Figure 6. Negative Output Boost (MIC3172)
Generating –VEE Bias From VIN < VOUT or VIN > VOUT
The topology shown in Figure 7 is a buck-boost converter. It
will generate a negative output voltage from a positive input
voltage. The magnitude of the output voltage can be greater
or less than the input voltage. The MIC2574 switching regulator will convert a +10V to +14V input to a –12V output when
used in the buck-boost configuration. The algebraic sum of
VIN + VOUT must be less than 40V, the maximum rated
voltage across the MIC2574 internal switching transistor.
Generating High VLC Bias Voltages
The tapped-inductor boost converter circuit shown in Figure 8 is used when the output voltage is greater than the rated
switching voltage of the boost controller IC. The voltage on
the switching node is equal to the output voltage divided by
the turns ratio.
Generating –VLC Bias From VIN > 3V
Inputs from 3- or 4-cells (3V to 6V), or inputs of greater
voltage, should use the MIC3172 as the boost regulator IC
with the negative-output boost converter topology. The 100kHz
switching frequency of the MIC3172 allows the use of smaller
inductors at higher input voltages. The MIC3172 has an
enable input which will turn off the regulator IC. It draws only
5µA of current when it is disabled. Another advantage of the
MIC3172 is the 65V switching transistor rating. This allows
higher output voltages to be generated without voltage doublers or tapped inductors. C4 in Figure 6 provides dc
decoupling between the input and output, which prevents the
output from drawing current when the MIC3172 is not switching.
The circuit in Figure 6 uses a boost converter to generate a
negative output voltage from a positive input voltage. The
MIC3172 switching regulator is used to convert a +5V input
to a –15V output. The enable function may be used to turn off
the output and the IC. While disabled, the current drawn is
5µA into the MIC3172 and 45µA through R2 and R1.
VIN
10V to 14V
C1
33µF
25V
10
5
LM2574BWM
IN
SW
SHDN
SGND
4
FB
PGND
12
N2 

VSW = VIN + (VOUT + VD − VIN )
N1 + N2 

A center-tapped inductor will cut the output voltage stress on
the boost switch in half. It is important to closely couple the
L1 330µH
3
D1
MBRA140T3
6
R3
100k
C2
100µF
20V
R2
23.2k
R1
1k
U1
C1
C2
D1
L1
R1–2
Micrel
AVX
AVX
Motorola
Coilcraft
VOUT
–12V
20mA
LM2574BWM, buck converter IC
TAJE336M025R, 33µF 25V tantalum
TPSV107M020R0200, 100µF 20V tantalum
MBRA140T3, 1A 40V Schottky
DT3316P-334, 330µH
metal film
Figure 7. Negative-Output Buck-Boost (MIC2574)
Application Note 21
222
May 1998
Application Note 21
Micrel
Efficiency Calculations
With battery powered applications, efficiency is paramount.
Efficiency is measured as the ratio of output power to input
power.
two windings of the inductor to reduce the voltage spikes on
the switching pin of the IC. The negative output boost topology (shown in Figure 7), may be similarly configured to
generate a higher negative voltage.
Note that the output capacitors are connected in series to
meet the output voltage requirement since there are no
commonly available tantalum capacitors with a voltage rating
sufficient for a 60V output.
VIN
1.5V
C1
22µF
6.3V
L1
1
MIC2570-2
8
7
IN
SW
SYNC
FB
GND
2 4
N1 N2
C3
4.7µF
50V
1
6
2
VOUT
U1
C1
C2, 3
D1
L1
R1–3
Micrel
AVX
AVX
Motorola
IR
Coiltronics
C2
4.7µF
50V
 R2 
= 0.22 1 +

 R1
R2
1M
R1
3.65k
MIC2570-2BM, boost converter IC
TAJC226M063R, 22µF 6.3V tantalum
TAJD475M050R, 4.7µF 50V tantalum
MBRS1100T3, 1A 100V Schottky -or10BQ100, 1A 100V Schottky
CTX100-1P, 100µH series conneced windings
metal film
Figure 8. Positive-Output HV Boost (MIC2570)
Backlighting
The three commonly used backlighting technologies are
electroluminescent (EL), light emitting diode (LED) and cold
cathode fluorescent lamp (CCFL).
Electroluminescent lamps provide even lighting. They have
the advantage of being thin, lightweight, rugged, and have
low power consumption. These lamps must be driven from an
ac voltage. The optimum voltage and frequency depend on
the lamp design and are in the range of 200Hz to 1000Hz at
50Vac to 250Vac. The inverters that supply the ac voltage to
these lamps require an input voltage of 5Vdc to 24Vdc.
LEDs have the advantage of operating directly from a 5V
source and have the longest operating life of all the backlighting technologies. No separate power supply is required. Their
disadvantage is higher power consumption: therefore more
heat is generated. A current-limited power source is required.
Cold cathode fluorescent lamps (CCFLs) are more popular in
the larger LCDs. They provide a very bright white light. The
power source is an inverter which supplies the 200Vac to
1000Vac at a frequency of 50kHz to 250kHz. The voltage and
frequency is very specific to the type of bulb. Inverters for
CCFL tubes require an input of 5Vdc to 24Vdc.
LCD Power Circuit Considerations
Output Power
The maximum output power for a given circuit depends on the
inductor value, the input voltage, the maximum peak switch
current and the mode of operation (continuous or
discontinuous).
May 1998
POUT
PIN
=
POUT
PLOSS + POUT
An efficiency number of 1 means there is no power lost in the
switching converter. Although this is not realizable, the closer
to unity efficiency, the longer the battery life, and the less heat
dissipated in the unit.
Each of the components in a switching power supply contribute to power loss (PLOSS) and reduced efficiency. Power is
dissipated in the output diode, inductor, and switching regulator IC. The power dissipated in the regulator IC is the sum
of the power lost in the internal switching transistor and power
lost by the IC’s input bias supply.
Battery Life
Two modes of operation that must be considered when
calculating battery life: operating and standby. When the
power supply is operating, power drawn from the battery is
equal to the output power divided by the efficiency. While the
power supply is in standby mode (disabled), the power drawn
from the battery is equal to the battery voltage times IQ
(standby quiescent current). The standby quiescent current
is specified in the switching regulator data sheet.
Output Disable
Turning off the outputs is simpler in some regulator topologies
than in others. The positive output boost regulator has a dc
path from input to output. When the regulator IC is turned off,
input voltage is still present at the output. The buck and buckboost regulators have the switching transistor between the
input and output, which prevents the input voltage from
appearing at the output. The negative output boost and the
SEPIC converter topologies have a blocking capacitor between the input and output. As with the buck converter, this
prevents the input voltage from appearing at the output.
The buck and buck-boost converters using the MIC2179
regulator ICs may be shut down with a logic-low signal
applied to the enable input. The LM2574 and MIC4574 ICs
can be turned off with a logic-high signal applied to the on/off
input. Once the IC is turned off, the circuit will cease to
operate and the output voltage will discharge to zero volts.
The only current drawn from the input source is the IC’s
standby quiescent current. This is a maximum of 5µA for the
MIC2179 and 200µA for the MIC2574 or MIC4574.
With the exception of the positive boost topology, circuits
using the MIC3172 are turned off with a low signal on the
enable input of the IC. The IC will draw a maximum of 5µA
quiescent current in shutdown mode.
VOUT
60V/1mA
D1
3
Efficiency =
223
Application Note 21
Application Note 21
Micrel
MOSFET. The result is a more efficient converter. The
disadvantage of this configuration is the input voltage (minus
the boost-diode drop) appears at the output. For low input
voltages and higher output voltages this may not be a
problem. The 0.1µF bypass capacitor may be necessary to
reduce the voltage ripple on the input of the control chip due
to the on-resistance of the MOSFET.
The MIC2570 and MIC2571 switching regulator ICs do not
have a shutdown pin. Therefore, positive boost converter
circuits using the MIC3172, and any circuit using the MIC2570
or MIC2571, must use external methods of turning off the
output. The methods used to turn off the outputs are: increasing the voltage on the feedback pin and blocking the input
voltage with a MOSFET.
A high level may be applied to the feedback pin through a
switch and resistor (Figure 9). The voltage on the feedback
pin will increase over the 0.22V reference level and the IC will
stop switching. The output will be a diode drop less than the
input. The circuit will draw the 120µA quiescent current of the
MIC2570 or MIC2571 plus any current drawn by the output.
Resistor R3 is used to prevent excessive current draw from
the input through R1. This method of shutting down the
converter is good when the input voltage is low and a small
voltage on the output is acceptable.
L1
C1
22µF
6.3V
MIC2570-2
8
7
2
1
C2
47µF
25V
R3
100k
6
VIN
>1.5V
7
0.1µF
1
6
2
Turn-On Time
Some controllers automatically initialize the LCD if the bias
voltage rises within a certain period of time. The output rampup time of the regulator is determined by the peak current limit
of the control chip, the inductor value, output capacitance,
and the amount of current drawn by the load. A smaller
inductor value, capacitor value, and output current will allow
the output voltage to rise faster. A higher IC current limit will
also allow quicker output turn-on. This time is generally in the
millisecond range. If the output must turn on faster, a MOSFET switch may be placed between the output capacitor and
the load. Turn-on times are limited only by the gate drive to the
MOSFET. A preload resistor on the output of the converter,
before the MOSFET switch, may be necessary to reduce
turn-on overshoot. Refer to Figure 12 and 13 for examples
with positive and negative output converters.
R2
1M
If 120µA quiescent current is not acceptable during shutdown, a P-channel MOSFET may be placed in series with the
supply input of the MIC2570 or MIC2571, as shown in
Figure 10. Pulling the gate of the MOSFET high will remove
the input voltage to the IC and eliminate any voltage from
appearing on the output (through D1). The disadvantage of
this configuration is added cost and reduced operating efficiency due to the MOSFET. Make sure the threshold voltage
of the MOSFET is appropriate for the input voltage. A MOSFET with a 4V threshold voltage will not turn on with an input
voltage of 1.5V!
Temic
SI6459DQ, 60V
Converter
Output
COUT
RPRELOAD R1
100k
LBOOST
Q1
IRF7404
IN
SW
SYNC
FB
GND
Figure 11. Disabling the MIC2570
Figure 9. Disabling the MIC2570 (VOUT ≈ VIN)
VIN
>1.5V
8
ON
CBYPASS
R1
9.31k
Voltage at feedback node
must be > VREF (0.22V)
to inhibit switching
Q1
MMSF4P01HD
MIC2570
CIN
OFF
VOUT
12V/20mA
D1
IN
SW
SYNC
FB
GND
R1
1M
OFF
R2
20k
Load
VIN
1.5V
LBOOST
ON
MIC2570
R1
1M
OFF
8
ON
CIN
7
IN
SW
SYNC
FB
GND
1
Figure 12. Speeding Turn-On Time (Positive Output)
6
2
OFF
COUT
Figure 10. Disabling the MIC2570 (VOUT(off) = 0V)
If the losses of the P-channel MOSFET are unacceptable, the
switch can be placed in series with the input pin. Figure 11
shows the circuit connections. This configuration will eliminate the 120µA of current drawn into the switch with the
advantage of not passing all the input current through the
R1
20k
R2
100k
Load
ON
RPRELOAD
Temic
SI4480DY, 80V
Set R1/R2 voltage divider
so that VGS(on) < VGS < VGS(max).
Figure 13. Speeding Turn-On Time (Negative Output)
Application Note 21
224
May 1998
Application Note 21
Micrel
Contrast Adjustment
The liquid crystal operating voltage (VLC) affects the LCD
contrast and viewing angle. Several methods are used to
adjust VLC. Figure 14 shows a method that may be used with
low-power, low-cost displays.
R1 =
1.24 ⋅ R2
VOUT(min) − 1.24
R1a =
1.24 ⋅ R2
(V
VLC
VSS
VEE
For the example shown in Figure 17, the values of R1 and
R1a give an output adjustment range of VOUT(min) = –11.24V
and VOUT(max) = –16.0V. It is recommended that the adjustment potentiometer R1a be placed as shown in Figure 17. An
open circuit is the typical failure mode for potentiometers. If
R1a opens, the MIC2574 will stop switching and the output
voltage will discharge to zero volts. If the potentiometer is
placed in series with R2, and an open-circuit condition
occurred, the switching converter would operate at full duty
cycle and the output voltage would increase.
Figure 14. Simple Contrast Adjustment
For higher VLC currents, a buffer is placed between the supply
voltages and the LCD module. This circuit is shown in
Figure 15. Make sure the buffer can supply the necessary
current for the LCD module.
VIN
5V
Vcontrast(min) = VLC − 2.5V
VDD
+5V
C1
33µF
16V
(VDD ⋅ R3) − (VLC ⋅ R1)
VDD
R1
100k
VLC
VSS
R3
100k
U1
C1
C2
D1
L1
R1–2
R1a
(
)
R1 + R2 + R3
VDD
MIC6211
VLC
VSS
Micrel
AVX
AVX
Motorola
Coilcraft
MIC2574BWM, buck converter IC
TAJC336M016R, 33µF 16V tantalum
TPSC475M035R0600, 4.7µF 35V tantalum
MBRA140T3, 1A 40V Schottky
DT3316P-224, 220µH
metal film
potentiometer
Temperature Compensation Circuits
The ambient temperature around the screen affects the
display screen contrast and viewing angle. A temperature
compensation circuit is used provide a negative temperature
coefficient to the VLC voltage to keep the contrast constant.
Display manufacturers generally provide either a temperature coefficient for a given display or a list of VLC voltages vs.
temperature. From this information, a temperature is calculated for the operating temperature range.
The circuit shown in Figure 18 is a simple solution that uses
a negative temperature coefficient (NTC) thermistor to vary
the VLC voltage.
VDD ⋅ R2 − VLC (R1 + R3)
R1
100k
R3
100k
R2
20k
VOUT
–11.2 to –16V
20mA
Figure 17. Contrast Control
Directly From the Power Supply
VDD (R2 + R3) − VLC ⋅ R1
R1 + R2 + R3
R2
12.1k
R2 

VOUT = −1.23 1 +

 R1+ R1a 
LCD Module
VEE
–15V
PGND
R1a
500Ω
An op amp buffer can be used with LCD modules with
separate VLC and contrast inputs as shown in Figure 16.
VDD
+5V
R3
100k
C2
4.7µF
35V
D1
10BQ040
R1
1k
Figure 15. Buffered Contrast Control (MIC6211)
Vcontrast(min) =
FB
SGND
–5 < VCONTRAST < –12.5V
Vcontrast(max) =
SHDN
L1 220µH
R1 + R3
MIC6211
VEE
–15V
MIC4574BWM
IN
SW
LCD Module
Vcontrast(max) =
)
− 1.24 − R1
VEE
–4.1 < VEE < –13V
VDD
+5V
Figure 16. Contrast Control for Separate
Vcontrast and VLC Inputs
R1
11k
R3
20k
Instead of using an op amp to control the contrast voltage, the
power supply may be varied. Once the voltage adjustment
range is known, the value of R1 and R1a can be computed.
VEE
–15V
RT
Betatherm
10K3A1
R2
20k
Q1
2N4403
LCD Module
VDD
LCD Module
OUT(max)
VDD
Figure 18. Temperature Compensation
for Contrast Control
May 1998
225
Application Note 21
Application Note 21
Micrel
compensate for the non-linear temperature coefficient of the
LCD display. The temperature coefficient of the transistor
should also be taken into account. For the 2N4401 it is
–1.8mV/°C.
Over the temperature range shown, the graph in Figure 20
compares the operating voltage for a Seiko G1216 LCD
(circles) with the calculated results (line) for the circuit in
Figure 18. The measured results are also shown in the graph
(triangles).
As the temperature increases, the resistance of the thermistor decreases. This decrease in resistance causes the
voltage at the base of Q1 to increase. The emitter voltage of
Q1 increases, reducing the voltage across the LCD.
The collector to emitter voltage, VCE, is:
VCE(Q1) = (VCC − VEE )
R2
+ VBE
R1 + R2 + R T(equiv)
VOLTAGE ACROSS DISPLAY (V)
The voltage across the LCD module is:
VOPR = VCC − VCE(Q1) − VEE
where:
VCE(Q1) is the temperature dependent VCE voltage
across Q1 in Figure 18.
VBE is the temperature dependent base-to-emitter
voltage
RT(equiv) is the parallel combination of R3 and the
thermistor.
Resistor R3 is placed across the thermistor in a first-order
attempt to linearize its resistance vs. temperature characteristic. The graph in Figure 19 shows a resistance vs. temperature plot of a thermistor (Betatherm, 10K3A1, 10kΩ @ 25°C)
with and without a parallel resistor.
RT(equiv) (kΩ)
Rthermistor
R3 = 100kΩ
20
50kΩ
20kΩ
0
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
The temperature coefficient of an LCD is not linear with
temperature. It is usually larger at higher temperatures.
Some amount of nonlinearity is desired in the thermistor to
C1
33µF
16V
MIC2574BWM
IN
SW
SHDN
SGND
FB
VCALCULATED
H
J
VDISPLAY
12
HJ
H
VMEASURED
H H
J H
H
11
10
-20
VOUT = −1.23
Figure 19. Thermistor Linearization
VIN
5V
J
0
20
40
60
TEMPERATURE (°C)
80
Figure 21 shows how the NTC thermistor, R3, can be used to
control the output voltage of a switching converter. The
thermistor varies the resistance of the voltage feedback loop.
An increase in temperature will decrease the resistance of the
thermistor. This will cause the output voltage to decrease in
magnitude. The potentiometer, R1, is optional and can be
used to manually adjust the display contrast. The output
voltage is manually adjustable, using the potentiometer, from
–6.5V to –7.7V at 25°C. The thermistor will adjust the output
voltage over the temperature range of the LCD. Output
voltage is calculated as:
60
40
J
13
Figure 20. Temperature Compensated Voltage
Comparison
100
80
14
R3equiv =
(
1 + R4 + R3equiv
R2 + R1
)
R3 ⋅ R3a
R3 + R3a
L1 220µH
D1
PGND
R3
100k
C2
47µF
16V
R3a
37.4k
R4
20.5k
R2
9.09k
R1
2k
U1
C1
C2
D1
L1
R1
R2,3a,4
R3
VOUT
Micrel
AVX
AVX
Motorola
Coilcraft
MIC2574, buck converter IC
TAJC336M016, 33µF 16V tantalum
TPSD476M016R0150, 47µF 16V tantalum
MBRA140T3, 1A 40V Schottky
DT3316P-224, 220µH
potentiometer
metal film
Betatherm 100K6A1, 100k at 25°C thermistor
Figure 21. Temperature-Compensated Bias Supply
Application Note 21
226
May 1998
Application Note 21
Micrel
VIN
3V to 6V
C1
47µF
16V
D3
L1
C2
15µF
35V
MIC3172
5
4
2
C3
0.22µF
R3
3.32k
R1
43.2k
IN
SW
EN
FB
7
R3
2.74k
SGND PGND1 PGND2
U1
C1
C2
C3
D3
L1
8
Micrel
AVX
AVX
IR
Coiltronics
Coilcraft
R1, 2a, 3 ,4
R2
Betatherm
R2a
20k
R2
10k
3
COMP
1
VOUT
24V/40mA
6
MIC3172BM, boost converter IC
TAJD476M016R, tantalum
TPSD156M035R0300, tantalum
ceramic
10BQ040, 1A 40V Schottky
CTX25-1P, 25µH -orDT3316P-223, 22µH
metal film
10K3A1, 10K at 25°C thermistor
Figure 22. Temperature-Compensated Bias Supply
Table 2 shows output behavior for Figure 21 from 0°C to
70°C. The potentiometer set at 1kΩ.
Ambient
Temperature
Thermistor
Resistance
Output
Voltage
0°C
351,017Ω
–7.85V
10°C
207,807Ω
–7.59V
25°C
100,000Ω
–7.05V
40°C
51,058Ω
–6.36V
50°C
33,598Ω
–5.89V
70°C
15,502Ω
–5.07
Figure 23 shows an alternate temperature compensation
method without using a thermistor. It uses the negative
temperature coefficient of the base to emitter voltage of a
transistor to provide the temperature compensation to the
drive voltage of the LCD display. The variation in output
voltage using this topology is more linear than the thermistor
method. If the temperature coefficient of the display is linear
this method may be preferable.
VCC
5V
R6 301k
R1
49.9k
R4
49.9k
Table 2. Temperature-Compensated Output
VOUT = 1.24
(
1 + R1 + R2equiv
C1
4.7nF
R3
20k
R5
10k
Q1
MPSA05
Figure 22 uses an NTC thermistor, R2, to control the output
voltage of a positive output boost converter.
An increase in temperature decreases the thermistor resistance. This causes the feedback voltage to increase and the
output voltage to decrease. Output voltage is:
R2
20k
VOUT
U1 –7.5V at 25°C
MIC6211
R7
165k
VEE
–15V
)
Figure 23. VBE Temperature Compensation
The temperature coefficient of the transistor is –1.8mV/°C. If
the desired temperature coefficient of the display is –33mV/°C,
a gain of 18.3 is required from the amplifier. If R2 is chosen
as 20k, then R6 must be 365k. The VBE of the transistor will
be approximately 0.6V with R1 at 49.9k. The output voltage
of the op amp will be:
R3
where:
R2 ⋅ R2a
R2 + R2a
Table 3 shows output behavior for Figure 22 from 0°C to
70°C.
R2equiv =
Ambient
Temperature
Thermistor
Resistance
Output
Voltage
0°C
32,651Ω
26.4V
10°C
19,904Ω
25.3V
25°C
10,000Ω
23.8V
40°C
5,325Ω
22.7V
50°C
3,601Ω
22.2V
70°C
1,752Ω
21.5V
(
VOUT = − gain VIN( − ) − VIN( + )
)
If the output is desired to be –7.5V at 25°C, the voltage at the
positive input must be:
VIN( + ) =
VOUT
+ VIN( − )
gain
VIN( + ) =
−7.5V
+ 0.6
18.3
VIN( + ) = 0.19V
Table 3. Temperature-Compensated Output
May 1998
227
Application Note 21
Application Note 21
Micrel
Backlighting
LED Backlight Supply
LEDs are used in some displays as backlighting. In a typical
circuit, the LED is connected to VCC (+5V) through a current
limiting resistor. This circuit is sufficient for displays with a
narrow temperature range. Extended temperature range
LED backlighted displays must have the forward LED current
reduced at higher temperatures to prevent damage to the
LEDs.
The potentiometer is adjusted for proper contrast and the
variation in VBE will compensate for the negative temperature
coefficient of the display.
Figure 24 increases the current drive capability of the temperature compensating circuit by placing a transistor, Q2, at
the output. The feedback circuit eliminates any VBE temperature dependence of Q2.
VCC
5V
VCC 5V
R6 365k
R1
49.9k
Q1
MPSA05
R4
49.9k
R2
20k
C1
VOUT
4.7nF –7.5V at 25°C
Q2
2N4403
R3
20k
R5
10k
R7
165k
LED
Backlight
R2
MPSA05
U1
MIC6211
R3
RT
R1
R1
VEE
–15V
Figure 24. High-Power VBE Temperature
Compensation
Figure 25. LED Backlight Temperature Compensation
VIN
3V to 6V
D1
D2
MIC3172BM
4
2
C1
0.1µF
R4
4.99k
U1
CIN
T1
D1
D2–4
R1–3
IN
SW
EN
FB
7
D4
3
COMP
SGND PGND1 PGND2
1
8
Micrel
AVX
COUT2 R3
22µF 4.12k
COUT1 R1
47µF 49.9k
N2 = 2
5
N3 = 6
CIN
22µF
N1 = 1
D3
6
R3
16.5k
MIC3172BM, boost converter IC
VOUT
TPS-series, tantalum
1:2:6 flyback transformer, LPRI = 10µH
1SMB5927BT3, 12V Zener
MBRS140T3, 1A 40V
metal film
Motorola
Motorola
VOUT2
–15V
VOUT1
+5V
1.24V 1 +
R1
R2
Figure 26. Multiple-Output Flyback Topology (MIC3172)
VIN
5V ±5%
D1
MBRA140
L1
N1
C1
100µF
10V
N2
7
IN
SW
SYNC
FB
GND
C2
68µF
25V
C4
150µF
16V
MIC2571-2
8
VOUT1
15V/10mA
1
R2
1M
 R1
VOUT 1 = 0.22 1 +

 R2 
6
2
D3
MBRA140
D2
MBRA140
R3
220k
Inductor Connection
Coiltronics VP1-0190
R1
14.7k
C3
150µF
16V
VOUT2
–8.5V/10mA
1,2
Input
Output
6
3,5
2,4
8.5V
tap
9,10,11
7,8
Figure 27. Multiple-Output Boost Topology (MIC2571)
Application Note 21
228
May 1998
Application Note 21
Micrel
The forward voltage drop of the LED has a negative temperature coefficient of between –1.8mV/°C and –2.3mV/°C. In a
series connected resistor/LED circuit, as the LED voltage
decreases, the current through the LED increases. Temperature compensation has the following advantages:
• Prevents the current from exceeding the
maximum rating of the LED
• Maintains a constant brightness
• Reduces current at higher temperature
to extend battery life
The circuits shown in Figure 25 can be used to regulate the
LED current. Set up the thermistor and the voltage divider
resistors so the required current flows through the LED at
25°C. The current through the LED is equal to the emitter
voltage of Q1 divided by the resistance of R1. As the temperature increases, the required current through the LEDs will
decrease. The thermistor resistance will decrease which will
decrease the emitter voltage of Q1. The resistor divider (R2
and R4) and the thermistor circuit (RT and R3) should be set
up to adjust the current flow as specified by the LED backlight
requirements.
Multiple-Output Power Topologies
It is advantageous in some applications to generate several
voltages from the same converter. If both the VDD and VLC
voltages need to be generated, a two-output converter has
the advantages of fewer parts, lower cost, and less board
space than two single-output converters. The advantage of a
single-output power supply is better output regulation and no
interaction between outputs (cross regulation).
A two-output flyback topology is shown in Figure 26. It uses
the MIC3172 as the switching regulator IC and has a VDD
output (+5V) and a VLC output (–15V). The output voltage,
VDD, is set by the voltage divider and is calculated by the
equation:
 R1
VDD = 1.24 1 +

 R2 
Output 2, VLC, is set by the transformer turns ratio N2/N3.
If the input voltage does not vary by more than a few percent
and the output current is constant, the circuit shown in
Figure 27 may be used. It is a boost converter with a tapped
inductor. The tap location (ratio of N1/N2) determines the
voltage of the negative output. The output voltage is calculated using the equation below:
 R2 
VOUT1 = 0.22 1 +

 R1
VOUT 2 = VIN +
(VOUT + VD − VIN ) N2 − 2V
D
N1 + N2
Resistor R3 is used as a preload resistor to prevent the output
voltage from varying at light load.
Conclusion
Different ICs and topologies have been shown in this application note, which provide the optimum circuitry for powering
liquid crystal displays. Temperature compensation circuits
are used to keep the display contrast constant under varying
temperature conditions. Output disable and turn-on time
features are discussed. Multiple-output power topologies are
presented and should be used where they are effective.
A summary of the Micrel power supply control ICs is shown
in Table 4. It will help to review and select the optimum
controller for a given application.
Controller
Input
Voltage
Range
Maximum
Output
Votlage
Operating
Frequency
Operating
Modes
Package
MIC2570
1.3V to 15V
32V
20kHz
boost,
inverting boost
SOIC-8
• Higher efficiency at low power
• Skip-mode control
• Higher output ripple
• Positive and negative output
MIC2571
0.9V to 15V
32V
20kHz
boost,
inverting boost
MSOP-8
• Higher efficiency at low power
• Skip-mode control
• Higher output ripple
• Positive or negative output voltage
MIC3172
3V to 40V
65V
100kHz
boost,
inverting boost
SOIC-8
DIP-8
• Higher power
• PWM-mode control
• Lower output ripple
• Positive or negative output voltage
MIC4574
4.5V to 24V
1.3V to 18V
200kHz
buck,
inverting boost
SOIC-14
DIP-8
• Positive or negative output voltages
• Lower output ripple
SOIC-24
TO-263-5
DIP-16
TO-220-5
• Positive or negative output voltage
• Lower output ripple
SSOP-24
• High-efficiency buck
• Lower output ripple
MIC2574
4V to 40V
1.3V to 37V
52kHz
buck,
buck-boost
MIC2179
4.5V to 16.5V
1.3V to 16V
200kHz
synchronous
buck
Notes
Table 4. Micrel Switching Converter Summary
May 1998
229
Application Note 21