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19-1406; Rev 1; 1/99
KIT ATION EET EVALU ATA SH WS D FOLLO
ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation
General Description
The MAX1729 micropower step-up/step-down DC-DC converter is ideally suited for electrically controlled birefringence (ECB) and liquid-crystal-display (LCD) biassupply generation. It provides step-up/step-down voltage conversion and reduces output ripple by using a step-up DC-DC converter followed by a linear regulator. This architecture permits a physically smaller inductor than those used in competing SEPIC and flyback topologies. This device features low quiescent current (67A typical). A logic-controlled shutdown mode further reduces quiescent current to 0.4A typical. The MAX1729 features an input that dynamically adjusts the output voltage to control display color or contrast. It offers two feedback modes: internal and external. Internal feedback mode allows output voltages between 2.5V and 16V, and is specifically designed to hold temperature drift to 11ppm/C. External feedback mode allows the MAX1729 output voltage range to be tailored for various displays. An on-chip temperature sensor with a positive temperature coefficient provides compensation for LCD/ECB display temperature characteristics. In internal feedback mode, the buffered temperature sensor output is read and used to adjust the output voltage via a digital control signal. External feedback mode features an additional compensation method in which the temperature output is summed directly into the feedback network to provide first-order negative temperature compensation of the output voltage. The MAX1729 is available in the space-saving 10-pin MAX package. o 11ppm/C Output Voltage Drift o On-Chip Temperature Sensor Output o Accurate Voltage and Temperature Provide: Consistent ECB Colors Consistent LCD Gray-Scale Contrast o +2.7V to +5.5V Input Voltage Range o Output Voltage Range +2.5V to +16V in Internal Feedback Mode Programmable in External Feedback Mode o Dynamic Control of the Output Voltage o 67A Supply Current o 0.4A Shutdown Current o 10-Pin MAX Package (1.09mm max height) o Evaluation Kit Available (MAX1729EVKIT)
Features
o High-Accuracy Reference Voltage (1%)
MAX1729
Ordering Information
PART MAX1729EUB TEMP. RANGE -40C to +85C PIN-PACKAGE 10 MAX
Applications
ECB Display Bias & Color Adjustment LCD Display Bias & Contrast Adjustment Cellular Phones Personal Digital Assistants
VIN 2.7V to 5.5V
Typical Operating Circuit
IN GND
LX PS
Pin Configuration
TOP VIEW
IN 1 TC REF COMP FB 2 3 4 5 10 GND 9 LX PS OUT CTLIN DIGITAL PWM CONTROLLER ADC
REF
MAX1729
OUT
VOUT +2.5V to +16V
CTLIN
FB
MAX1729
8 7 6
TC
COMP
MAX ________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769.
ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729
ABSOLUTE MAXIMUM RATINGS
IN to GND .................................................................-0.3V to +6V LX, PS, OUT to GND...............................................-0.3V to +20V CTLIN, FB, REF, COMP, TC to GND ...........-0.3V to (VIN + 0.3V) LX to PS ..................................................................-20V to +1.0V LX, PS, OUT Current ...........................................................60mA Continuous Power Dissipation (TA = +70C) 10-pin MAX (derate 5.6mW/C above +70C) ..........444mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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
(VIN = +3V, CTLIN = IN, FB = GND, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1) PARAMETER Operating Voltage Range Undervoltage Lockout Threshold (Note 2) IN Supply Current PS Supply Current Shutdown Supply Current Reference Output Voltage SYMBOL VIN VLO IIN IPS ISHDN VREF VOUT
(MIN)
CONDITIONS
MIN 2.7 2.0
TYP
MAX 5.5 2.6
UNITS V V A A A V
37 30 CTLIN = GND, ISHDN = IIN + IPS IREF = 0 FB = GND, CTLIN = 0.1% duty cycle, IOUT = 0 to 0.5mA IOUT = 0 to 0.5mA FB = GND, CTLIN = 0.1% to 100% duty cycle, IOUT = 0 TA = 0C to +85C TA = -40C to +85C TA = 0C to +85C TA = -40C to +85C 0.5 TA = +25C TA = 0C to +85C TA = -40C to +85C TA = +25C TA = -40C to +85C VFB = +1.25V VIN = +5.5V VIN = +2.7V 2 1.3 1.178 15.5 14.5 50 1.215 1.200 90 122 5 1.228 TA = +25C TA = -40C to +85C TA = 0C to +85C TA = -40C to +85C 1.215 1.200 2.35 2.35 16 13.90 13.60 11 18 2.5 1.228 16.5 16.5 16.40 13.95 2.45 0.4 1.228
50 40 2 1.241 1.256 2.5
Minimum Output Voltage
V 2.52 V 14.00 V/100% 14.20 30 65 1.278 17.5 18.5 1.241 1.256 150 50 ppm/C mA V mV/C A V mV nA V
Maximum Output Voltage
VOUT (MAX)
CTLIN to VOUT Gain Output Voltage Temperature Coefficient Maximum Output Current TC Output Voltage TC Output Temperature Coefficient (Note 3) TC Output Current Feedback Set Voltage (FB) FB Mode Threshold FB Bias Current CTLIN High Voltage
TCOUT IOUT VTC TCTC ITC VFB VMODE IFB VIH
VPS = +18V (Note 3)
2
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ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729
ELECTRICAL CHARACTERISTICS
(VIN = +3V, CTLIN = IN, FB = GND, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1) PARAMETER CTLIN Low Voltage CTLIN Bias Current COMP Impedance CTLIN Minimum Pulse Width for Shutdown CTLIN Minimum Pulse Width for VOUT Control COMP Rise/Fall Time Switch On-Resistance Switch Off-Leakage Current LX to PS Diode Forward Voltage PFM On-Time Constant PS to OUT Voltage (Note 5) Note 1: Note 2: Note 3: Note 4: Note 5: SYMBOL VIL IIHL RCOMP tOFF tCTLIN tR/tF RON ILX(OFF) VLX-PS K VFB = +1.25V ILX = 30mA VLX = 18V IDIODE = 30mA TA = +25C TA = -40C to +85C 6 5 0.4 VIN = +2.7V VIN = +5.5V CONDITIONS VIN = +2.7V to +5.5V VIN = +5.5V, CTLIN = GND or IN VFB = 0, Internal Feedback Mode VFB = +1.25V, External Feedback Mode (Note 4) 700 33 60 1250 25 20 2.5 1.5 0.1 700 8 0.6 1 970 10 11 1.0 5.0 150 2400 MIN TYP MAX 0.4 1 UNITS V A k s ns ns A mV V-s V
Specifications to -40C are guaranteed by design, not production tested. When VIN is below this level, the boost and LDO outputs are disabled. Guaranteed by design. Minimum time to hold CTLIN low to invoke shutdown. If CTLIN is held low for less than tOFF, device does not enter shutdown. Switching regulator regulates this voltage to keep LDO from dropping out.
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ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729
Typical Operating Characteristics
(Circuit of Figure 2, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT VOUT = 9.4 (CTLIN AT 50% DUTY CYCLE)
MAX1729 toc 01
EFFICIENCY vs. OUTPUT CURRENT VOUT = 16.4 (CTLIN = IN)
MAX1729 toc 02
NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX1729 toc 03
60
60
0.40 0.35 SUPPLY CURRENT (mA) 0.30 0.25 0.20 0.15 0.10 0.05 VOUT = 9.4V VOUT = 2.5V 2.5 3.0 3.5 4.0 4.5 5.0 5.5 VOUT = 16.4V
50 VIN = 2.7V EFFICIENCY (%) 40 30 20 10 0 0.01 0.1 1
50 EFFICIENCY (%) 40 30 20 10 0 VIN = 2.7V VIN = 5.5V
VIN = 5.5V
10
0.01
0.1
1
10
6.0
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
SUPPLY VOLTAGE (V)
MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE
MAX1729 toc 04
PS TO OUT (LDO) POWER-SUPPLY REJECTION RATIO
MAX1729 toc 05
SHUTDOWN SUPPLY CURRENT
MAX1729 toc 06
7 MAXIMUM OUTPUT CURRENT (mA) 6 5 4 3 2 1 0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 VOUT = 16.4V VOUT = +2.5V
0
3.0 2.5 SHUTDOWN CURRENT (A) 2.0 1.5 1.0 0.5 0
-20
PSRR (dB) 6.0
VOUT = 9.4V
-40
-60
-80
-100 1 10 100 1k 10k 100k SUPPLY VOLTAGE (V) FREQUENCY (Hz)
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
INPUT VOLTAGE (V)
OUTPUT VOLTAGE vs. DUTY CYCLE
MAX1729 toc 07
START-UP DELAY FROM SHUTDOWN
MAX1729 toc 08
DELAY TO SHUTDOWN
MAX1729 toc 09a
18 16 14 OUTPUT VOLTAGE (V) 12 10 8 6 4 2 0 0
VCTLIN 5V/div VREF 1V/div
VCTLIN 5V/div VREF 1V/div
VTC 1V/div VOUT 10V/div 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 20ms/div VIN = 5V IO = 0.5mA
VTC 1V/div VOUT 10V/div
VIN = 5V IO = 0.5mA 500ms/div
4
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ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729
Typical Operating Characteristics (continued)
(Circuit of Figure 2, TA = +25C, unless otherwise noted.)
DELAY TO SHUTDOWN
MAX1729 toc 09b
SWITCHING WAVEFORMS HEAVY LOAD
MAX1729 toc 10
SWITCHING WAVEFORMS MEDIUM LOAD
IL 20mA/div
MAX1729 toc 11
VCTLIN 5V/div VREF 1V/div
IL 20mA/div
VTC 1V/div VOUT 10V/div 5ms/div VIN = 5V IO = 0.5mA
OUTPUT RIPPLE 10mV/div
OUTPUT RIPPLE 10mV/div
VLX 5V/div
VLX 5V/div
2s/div
2s/div
LOAD-TRANSIENT RESPONSE
MAX1729 toc 13
LINE-TRANSIENT RESPONSE
CTLIN = IN IOUT = 0.5mA VOUT 20mV/div 16.400 5V 4V
MAX1729 toc 14
16.404
VOUT 20mV/div
500A IOUT 250A/div 20A 1ms/div
VIN 1V/div
2ms/div
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ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729
Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 NAME IN TC REF COMP FB CTLIN OUT PS LX GND FUNCTION Supply Input. Bypass with 0.1F capacitor to ground. Connect to supply side of inductor (L1). Temperature-Sensor Output. Bypass to GND with a 1000pF capacitor. Reference Voltage Output. Bypass to GND with a 0.1F capacitor. Compensation Pin. In internal feedback mode (Figure 2), bypass with a 1F capacitor. In external feedback mode, COMP is a buffered inverse version of CTLIN (Figure 3). Feedback and Mode Control Input. Connect to GND for internal feedback mode operation. Control Input. Drive low for more than 1.2ms to put the device into shutdown. Bypass to GND with a 1.0F capacitor. Output of boost converter and input to LDO. Bypass to GND with a 0.068F capacitor. Drain of the internal MOSFET Switch Ground
Detailed Description
The MAX1729 is designed to provide bias voltage for ECB or LCD displays. It is composed of a step-up DC-DC converter followed by a linear regulator (Figure 1), a combination that provides step-up/stepdown voltage conversion while minimizing output ripple. The device allows you to adjust a display's color or contrast by dynamically adjusting the MAX1729's output voltage using a PWM control signal. In internal feedback mode, the output voltage is adjustable between +2.5V and +16V. In external feedback mode, the output voltage is adjustable, and its range is set by a resistor network that is programmed to match the output voltage range of LCD/ECB displays needing a maximum output up to +18V.
boost converter's input and output voltage values, as follows: t ON = t OFF K VIN K VPS - VIN
where K is typically 8V-s. This timing maintains discontinuous conduction and sets the peak inductor current (IPEAK) to: K IPEAK = L where L is the inductance of L1 (Figures 2, 3, and 4). When the error comparator detects that the drop across the linear regulator (VPS - VOUT) is less than approximately 0.6V, the internal switch is turned on (tON initiates) and current through the inductor ramps to IPEAK. At the end of tON, the switch is turned off for at least tOFF, allowing the
Boost Converter
The MAX1729's DC-DC boost converter is implemented with an on-chip N-channel MOSFET, a diode, and an error comparator. The IC's unique PFM control system varies the on-time and off-time of the switch based on the
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ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation
IN TC TEMPERATURE SENSOR BOOST CONVERTER PS LDO LINEAR REGULATOR OUT FB LX
equal to the reference voltage at room temperature. TC is capable of sinking or sourcing 50A. This output is used to compensate for ECB color or LCD contrast variations caused by changes in temperature. It may be read with an ADC and used to modify an external PWM control signal or, in external feedback mode, summed directly into the feedback-resistor network.
MAX1729
REF
VOLTAGE REFERENCE
Control Signal
An externally generated PWM control signal on CTLIN controls VOUT in internal feedback mode and influences VOUT in external feedback mode. In either mode, if CTLIN is held low for longer than 1.24ms, the MAX1729 enters shutdown mode, decreasing the supply current below 2A. Shutdown mode limits the minimum duty cycle and frequency that may be used to keep the device active. CTLIN frequencies between 2kHz and 12kHz are recommended.
MAX1729
FEEDBACK CONTROL
122mV
Internal Feedback Mode
SHUTDOWN CONTROL VREF CTLIN GND GND COMP
In internal feedback mode, the signal at CTLIN is inversely buffered, level-shifted, and output at COMP through a resistor. Internal resistance (33k typical) and C6 then filter the signal before it is used by the internal feedback network to set VOUT. If temperature compensation is used, the temperature sensor output voltage is read by an ADC and used to adjust the duty cycle of the PWM control signal. See the Designing for Internal Feedback Mode section for more information.
NOTE: SWITCH STATES SHOWN FOR INTERNAL FEEDBACK MODE.
External Feedback Mode
In external feedback mode, the output voltage of the MAX1729 is controlled by the duty cycle of the PWM control signal and an external resistor network, as shown in Figure 3. In this mode, the signal at CTLIN is inverted, level-shifted, and presented directly to COMP. R3, R4, and C6 filter the signal, before it is summed into the feedback node.
Figure 1. Internal Block Diagram
inductor current to ramp down and VPS to increase. If, at the end of tOFF, VPS - VOUT is still too low, then another tON is initiated immediately. Otherwise, the boost converter remains idle in a low-quiescent-current state until VPS - VOUT drops again and the error comparator initiates another cycle.
Design Procedure
Designing for Internal Feedback Mode
For a 3kHz PWM control signal use a 1F low-leakage ceramic capacitor for C6. For applications requiring a higher-frequency PWM control signal, reduce the value of C6 to between 1F and 0.22F for frequencies between 3kHz and 12kHz. Higher C6 values reduce output ripple. In Figure 2, VOUT is governed by the following equation: VOUT = VOUT(MIN) + Duty Cycle Gain where V OUT(MIN) is 2.45V and Gain is nominally 13.95V/100%, as listed in the Electrical Characteristics.
7
Linear Regulator
The PNP low-dropout linear regulator of the MAX1729 regulates the boost-converter output to the desired output voltage. The boost converter's regulation circuitry holds the linear regulator's input voltage (VPS) approximately 0.6V above the output voltage to keep the regulator out of dropout, thereby enhancing ripple rejection. The linear regulator incorporates short-circuit protection, which limits the output current to approximately 6mA.
Temperature Sensor Output
The MAX1729 generates a temperature sensor voltage (VTC) that varies at 16.5mV/C (typ) and is nominally
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ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729
To use a DC control signal to adjust the output voltage, use the circuit shown in Figure 4. In this configuration, VOUT is governed by the following equation: VOUT 24.67VFB - 22.71VCOMP The impedance looking into COMP is nominally 33k. A source output impedance of less than 500 is recommended. Also, ensure V OUT 18V by keeping VCOMP above 0.6V.
External Component Value Formulas 1) Given the maximum output voltage needed (VMAX), choose the maximum feedback current and solve for R1 (10A to 30A is recommended for maximum feedback current) as follows: V - VFB R1 = MAX IFB
Designing for External Feedback Mode
To solve for VOUT in external feedback mode, assume the current into the FB pin is zero and the voltage at FB is 1.228V. Then take the sum of the currents into FB and solve for VOUT: 1 1 1 1 VOUT = R1 + + + V R1 R2 R 3 + R4 R5 FB R1 R1 - VCOMP - V R 3 + R4 R5 TC
C3 0.1F PWM GENERATOR VIN IN GND
L1 220H LX PS
C1 0.1F
C2 0.068F VOUT R1 C4 1F
REF
MAX1729
OUT
CTLIN
FB R5 R2
Using the following formulas, calculate the external component values required for MAX1729 operation in external feedback mode, as shown in Figure 3. An example follows the formulas.
COMP R4
TC R3 C6 1F C5 1000pF
Figure 3. External Feedback Mode
L1 VIN IN GND 220H LX PS VIN IN GND
L1 220H C1 0.1F LX PS
C1 0.1F
C2 0.068F VOUT
C2 0.068F VOUT
C3 0.1F DIGITAL PWM CONTROLLER ADC C5 1000pF
REF
MAX1729
OUT
C4 1F
C3 0.1F
REF
MAX1729
OUT
C4 1F
CTLIN
FB
CTLIN
FB DC CONTROL INPUT VCOMP
TC
COMP C6 1F
C5 1000pF
TC
COMP
Figure 2. Internal Feedback Mode
8
Figure 4. Using a DC Control Signal
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ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation
2) Given the maximum output voltage (VMAX) and minimum output voltage (VMIN), calculate values for R3 and R4 as follows: R1 R3 = 1/2 VFB VMAX - VMIN R4 = R3 3) For first-order temperature compensation, calculate R5 as shown below. (If temperature compensation is not used, leave R5 open.) R1 R5 = 16.5mV/ C Tempco where Tempco is the negative temperature coefficient needed to compensate the ECB or LCD display for changes in temperature. 4) Solve for VCOMP. The duty cycle used here corresponds to the duty cycle that yields the maximum output voltage, not including first-order temperature compensation. R4 VCOMP =VFB 1 - Duty Cycle R3 + R4 where a 90% duty cycle corresponds to Duty Cycle = 0.9. 5) Use the results from the above calculations to solve for R2. (For applications not utilizing temperature compensation, use 1 / R5 = 0.)
1 1 VOUT VCOMP VFB = + + R2 VFB R1 R3 R5 1 1 1 - + + R1 R3 R5
2) VMAX = 10V and VMIN = 5V, therefore: 300k R3 = 1/2 1.228 = 36,840 5V with R3 = 36.7k, then VMIN = 5.019V. Let R4 = R3 = 36.7k. 3) Tempco = 33mV/C, therefore: 300k R5 = 16.5mV / C = 150k 33mV/ C 4) If external circuitry limits the duty cycle to 90%, the following equation is true: 0.9 VCOMP = 1.228 1 - = 0.6754V 2 5) Solving for R2: V V1 V 1 = OUT + COMP + FB R2 R3 R5 V R1 FB 1 1 1 1 - + + = R1 R3 R5 56560 With R2 = 56k, a duty cycle of 87.4% generates a VOUT of 10V.
MAX1729
Component Selection
Inductors Use a 220H inductor to maximize output current (2.5mA typical). Use an inductor with DC resistance less than 10 and a saturation current exceeding 35mA. For lower peak inductor current, use a 470H inductor with DC resistance less than 20 and a saturation current over 18mA. This limits output current to typically less than 1mA. See Table 1 for a list of recommended inductors. The inductor should be connected from the battery to the LX pin, as close to the IC as possible. Capacitors The equivalent series resistance (ESR) of output capacitor C2 directly affects output ripple. To minimize output ripple, use a low-ESR capacitor. A physically smaller capacitor, such as a common ceramic capacitor, minimizes board space and cost while creating an output ripple that's acceptable in most applications. Refer to Table 2 for recommended capacitor values.
External Component Value Example The example application requires the output voltage to adjust between 5V and 10V, using the circuit shown in Figure 3. The device in our example needs a temperature coefficient of 33mV/C, which yields the following results.
1) VMAX = 10V and IFB = 29.24A is within the limits and yields a reasonable resistor value, therefore: R1 = 10V - 1.228V = 300k 29.24A
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ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729
Table 1. Recommended Inductors
SUPPLIER Murata Panasonic PART LQH3C221K04M00 ELT3KN115B INDUCTANCE (H) 220 470 DC RESISTANCE () 8.4 19 SATURATION CURRENT (mA) 70 40 MAX HEIGHT (mm) 2.2 1.6
Table 2. Recommended Capacitor Values
CAPACITOR C1 C2 C3 C4 C5 C6* *Use a low-leakage capacitor. CAPACITANCE (F) 0.1 0.068 0.1 1 1000pF 1
Applications Information
PC Board Layout Considerations
Proper PC board layout minimizes output ripple and increases efficiency. For best results, use a ground plane, minimize the space between C1, C2, and GND of the MAX1729, and place the inductor as close to LX and IN as possible. For an example of proper PC board layout, refer to the MAX1729 Evaluation Kit.
Chip Information
TRANSISTOR COUNT: 1154
10
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ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation
Package Information
10LUMAXB.EPS
MAX1729
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11
ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729
NOTES
12
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