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19-1795; Rev 0; 11/00
Low-Power Triple-Output TFT LCD DC-DC Converter
General Description
The MAX1779 triple-output DC-DC converter provides highly efficient regulated voltages required by small active matrix, thin-film transistor (TFT) liquid-crystal displays (LCDs). One high-power DC-DC converter and two low-power charge pumps convert the +2.7V to +5.5V input supply voltage into three independent output voltages. The primary high-power DC-DC converter generates a boosted output voltage (VMAIN) up to 13V that is regulated within 1%. The low-power BiCMOS control circuitry and the low on-resistance (1) of the integrated power MOSFET allows efficiency up to 91%. The 250kHz current-mode pulse-width modulation (PWM) architecture provides fast transient response and allows the use of ultra-small inductors and ceramic capacitors. The dual charge pumps independently regulate one positive output (VPOS) and one negative output (VNEG). These low-power outputs use external diode and capacitor stages (as many stages as required) to regulate output voltages up to +40V and down to -40V. A proprietary regulation algorithm minimizes output ripple, as well as capacitor sizes for both charge pumps. The MAX1779 is available in the ultra-thin TSSOP package (1.1mm max height).
Features
o Three Integrated DC-DC Converters o 250kHz Current-Mode PWM Boost Regulator Up to +13V Main High-Power Output 1% Accuracy High Efficiency (91%) o Dual Charge-Pump Outputs Up to +40V Positive Charge-Pump Output Down to -40V Negative Charge-Pump Output o Internal Supply Sequencing o Internal Power MOSFETs o +2.7V to +5.5V Input Supply o 0.1A Shutdown Current o 0.5mA Quiescent Current o Internal Soft-Start o Power-Ready Output o Ultra-Small External Components o Thin TSSOP Package (1.1mm max)
MAX1779
Ordering Information
PART MAX1779EUE TEMP. RANGE -40C to +85C PIN-PACKAGE 16 TSSOP
Pin Configuration ________________________Applications
TFT Active-Matrix LCD Displays Passive-Matrix LCD Displays PDAs Digital-Still Cameras Camcorders
TOP VIEW
RDY 1 FB 2 INTG 3 IN 4 GND 5 REF 6 FBP 7 FBN 8 16 TGND 15 LX 14 PGND
MAX1779
13 SUPP 12 DRVP 11 SUPN 10 DRVN 9 SHDN
TSSOP Typical Operating Circuit appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products
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For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
ABSOLUTE MAXIMUM RATINGS
IN, SHDN, TGND to GND .........................................-0.3V to +6V DRVN to GND .........................................-0.3V to (VSUPN + 0.3V) DRVP to GND..........................................-0.3V to (VSUPP + 0.3V) PGND to GND.....................................................................0.3V RDY to GND ...........................................................-0.3V to +14V LX, SUPP, SUPN to PGND .....................................-0.3V to +14V INTG, REF, FB, FBN, FBP to GND ...............-0.3V to (VIN + 0.3V) Continuous Power Dissipation (TA = +70C) 16-Pin TSSOP (derate 9.4mW/C above +70C) ..........755mW Operating Temperature Range MAX1779EUE ..................................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+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 = +3.0V, SHDN = IN, VSUPP = VSUPN = +10V, TGND = PGND = GND, CREF = 0.22F, CINTG = 2200pF, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Input Supply Range Input Undervoltage Threshold IN Quiescent Supply Current SUPP Quiescent Current SUPN Quiescent Current IN Shutdown Current SUPP Shutdown Current SUPN Shutdown Current MAIN BOOST CONVERTER Output Voltage Range FB Regulation Voltage FB Input Bias Current Operating Frequency Oscillator Maximum Duty Cycle Load Regulation Line Regulation Integrator Gm LX Switch On-Resistance LX Leakage Current LX Current Limit Maximum RMS LX Current FB Fault Trip Level POSITIVE CHARGE PUMP VSUPP Input Supply Range VSUPP Falling edge 1.07 2.7 RLX(ON) ILX ILIM ILX = 100mA VLX = +13V 350 IMAIN = 0 to 50mA, VMAIN = +5V VMAIN VFB IFB fOSC VFB = +1.25V, INTG = GND VIN 1.235 -50 212 79 250 85 0.1 0.1 320 1.0 0.01 450 250 1.1 1.14 13 2.0 20 650 1.248 13 1.261 50 288 92 V V nA kHz % % %/V s A mA mA V V SYMBOL VIN VUVLO IIN ISUPP ISUPN VIN rising, 40mV hysteresis (typ) VFB = VFBP = +1.5V, VFBN = -0.2V VFBP = +1.5V VFBN = -0.1V V SHDN = 0, VIN = +5V V SHDN = 0, VSUPP = +13V V SHDN = 0, VSUPN = +13V CONDITIONS MIN 2.7 2.2 2.4 0.5 0.25 0.25 0.1 0.1 0.1 TYP MAX 5.5 2.6 1 0.55 0.55 10 10 10 UNITS V V mA mA mA A A A
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Low-Power Triple-Output TFT LCD DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = +10V, TGND = PGND = GND, CREF = 0.22F, CINTG = 2200pF, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Operating Frequency FBP Regulation Voltage FBP Input Bias Current DRVP PCH On-Resistance DRVP NCH On-Resistance FBP Power-Ready Trip Level FBP Fault Trip Level Maximum RMS DRVP Current NEGATIVE CHARGE PUMP VSUPN Input Supply Range Operating Frequency FBN Regulation Voltage FBN Input Bias Current DRVN PCH On-Resistance DRVN NCH On-Resistance FBN Power-Ready Trip Level FBN Fault Trip Level Maximum RMS DRVN Current REFERENCE Reference Voltage Reference Undervoltage Threshold LOGIC SIGNALS SHDN Input Low Voltage SHDN Input High Voltage SHDN Input Current RDY Output Low Voltage RDY Output High Voltage I SHDN ISINK = 2mA V RDY = +13V VREF -2A < IREF < 50A VREF rising 1.231 0.9 1.25 1.05 1.269 1.2 V V VFBN IFBN -50 -50 VSUPN 2.7 0.5 x fOSC 0 3 1.5 20 80 120 140 0.1 165 VFBN = -0.05V VFBN = +0.050V VFBN = -0.050V Falling edge Rising edge 50 50 10 5 13 V Hz mV nA k mV mV A VFBP IFBP 1.20 -50 SYMBOL CONDITIONS MIN TYP 0.5 x fOSC 1.25 3 VFBP = +1.200V VFBP = +1.300V Rising edge Falling edge 20 1.09 1.13 1.11 0.1 1.16 1.5 VFBP = +1.5V 1.30 50 10 5 MAX UNITS Hz V nA k V V A
MAX1779
0.25V hysteresis (typ) 2.1 0.01 0.25 0.01
0.9 1 0.5 1
V V A V A
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Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
ELECTRICAL CHARACTERISTICS
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = +10V, TGND = PGND = GND, CREF = 0.22F, CINTG = 2200pF, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input Supply Range Input Undervoltage Threshold IN Quiescent Supply Current SUPP Quiescent Current SUPN Quiescent Current IN Shutdown Current SUPP Shutdown Current SUPN Shutdown Current MAIN BOOST CONVERTER Output Voltage Range FB Regulation Voltage FB Input Bias Current Operating Frequency Oscillator Maximum Duty Cycle LX Switch On-Resistance LX Leakage Current LX Current Limit FB Fault Trip Level POSITIVE CHARGE PUMP SUPP Input Supply Range FBP Regulation Voltage FBP Input Bias Current DRVP PCH On-Resistance DRVP NCH On-Resistance FBP Power-Ready Trip Level NEGATIVE CHARGE PUMP SUPN Input Supply Range FBN Regulation Voltage FBN Input Bias Current DRVN PCH On-Resistance DRVN NCH On-Resistance FBN Power-Ready Trip Level REFERENCE Reference Voltage Reference Undervoltage VREF -2A < IREF < 50A VREF rising 1.223 0.9 1.269 1.2 V V VSUPN VFBN IFBN VFBN = -0.05V VFBN = +0.050V VFBN = -0.050V Falling edge 20 80 165 2.7 -50 -50 13 50 50 10 5 V mV nA k mV VFBP = +1.200V VFBP = +1.300V Rising edge 20 1.09 1.16 VSUPP VFBP IFBP VFBP = +1.5V 2.7 1.20 -50 13 1.30 50 10 5 V V nA k V RLX(ON) ILX ILIM Falling edge ILX = 100mA VLX = +13V 350 1.07 VMAIN VFB IFB fOSC VFB = +1.25V, INTG = GND VIN 1.225 -50 195 79 13 1.271 50 305 92 2.0 20 700 1.14 V V nA kHz % A mA V SYMBOL VIN VUVLO IIN ISUPP ISUPN VIN rising, 40mV hysteresis (typ) VFB = VFBP = +1.5V, VFBN = -0.2V VFBP = +1.5V VFBN = -0.1V V SHDN = 0, VIN = +5V V SHDN = 0, VSUPP = +13V V SHDN = 0, VSUPN = +13V CONDITIONS MIN 2.7 2.2 MAX 5.5 2.6 1 0.55 0.55 10 10 10 UNITS V V mA mA mA A A A
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Low-Power Triple-Output TFT LCD DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = +10V, TGND = PGND = GND, CREF = 0.22F, CINTG = 2200pF, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER LOGIC SIGNALS SHDN Input Low Voltage SHDN Input High Voltage SHDN Input Current RDY Output Low Voltage RDY Output High Leakage I SHDN ISINK = 2mA V RDY = +13V 0.25V hysteresis (typ) 2.1 1 0.5 1 0.9 V V A V A SYMBOL CONDITIONS MIN MAX UNITS
MAX1779
Note 1: Specifications to -40C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 5, VIN = +3.3V, TA = +25C, unless otherwise noted.)
MAIN STEP-UP CONVERTER EFFICIENCY vs. LOAD CURRENT (L = 10H, 5V OUTPUT)
MAX1779-02
MAIN OUTPUT VOLTAGE vs. LOAD CURRENT (L = 10H, 5V OUTPUT)
MAX1779-01
MAIN OUTPUT VOLTAGE vs. LOAD CURRENT (L = 33H, 5V OUTPUT)
MAX1779-03
5.02
100 VIN = +4.2V VIN = +3.0V
5.02
5.01 EFFICIENCY (%) VIN = +3.0V VMAIN (V) 5.00 VIN = +4.2V
90
5.01 VIN = +3.0V VMAIN (V) 5.00 VIN = +4.2V 80
70
4.99
60 FIGURE 6 FIGURE 6
4.99
4.98 0 50 100 IMAIN (mA) 150 200
50 0 50 100 IMAIN (mA) 150
4.98 200 0 50 100 150 IMAIN (mA) 200
FIGURE 5 250 300
MAIN STEP-UP CONVERTER EFFICIENCY vs. LOAD CURRENT (L = 33H, 5V OUTPUT)
MAX1779-04
MAIN OUTPUT VOLTAGE vs. LOAD CURRENT (L = 33H, 10V OUTPUT)
MAX1779-05
MAIN STEP-UP CONVERTER EFFICIENCY vs. LOAD CURRENT (L = 33H, 10V OUTPUT)
MAX1779-06
100 VIN = +4.2V VIN = +3.0V
10.04
100 VIN = +5.5V VIN = +3.3V 80
90 EFFICIENCY (%)
90 EFFICIENCY (%)
10.02 VMAIN (V)
VIN = +3.3V VIN = +5.0V
80
10.00
70 9.98 60 FIGURE 5 0 50 100 150 IMAIN (mA) 200 250 300 FIGURE 5 0 50 IMAIN (mA) 100 150
70
60 FIGURE 5 0 50 IMAIN (mA) 100 150
50
9.96
50
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Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
Typical Operating Characteristics (continued)
(Circuit of Figure 5, VIN = +3.3V, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (BOOST CONVERTER AND CHARGE PUMPS)
MAX1779-07
NEGATIVE CHARGE-PUMP OUTPUT VOLTAGE vs. LOAD CURRENT
VSUPN = +5V -7.80 -7.84 VNEG (V) -7.88 -7.92 -7.96 -8.00 VSUPN = +7V EFFICIENCY (%) VSUPN = +6V
MAX1779-08
NEGATIVE CHARGE-PUMP EFFICIENCY vs. LOAD CURRENT
MAX1779-09
100 VMAIN = +5V TWO-STAGE CHARGE PUMPS
-7.76
100 90 80 70 60 50 40 VNEG = -8V 30 VSUPN = +7V VSUPN = +5V
VSUPN = +6V
90
EFFICIENCY (%)
80
70
VMAIN = +10V SINGLE-STAGE CHARGE PUMPS
60 VNEG = -8V, INEG = 1mA VPOS = +12V, IPOS = 1mA 50 0 50 100 150 200 250 IMAIN (mA)
-8.04 -8.08 0 5 10 INEG (mA) 15 20
0
5
10 INEG (mA)
15
20
POSITIVE CHARGE-PUMP OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1779-10
POSITIVE CHARGE-PUMP EFFICIENCY vs. LOAD CURRENT
MAX1779-11
SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX1779-12
12.24
100 90 80 VSUPP = +5V VSUPP = +6V
300
12.12
SWITCHING FREQUENCY (kHz)
280
VPOS (V)
VPOS (V)
12.00
VSUPP = +7V
70 60 50
260
11.88
240
VSUPP = +7V
11.76 VSUPP = +5V 11.64 0 5 10 15 IPOS (mA)
VSUPP = +6V
220
40 30 VPOS = +12V 0 5 10 15 IPOS (mA) 20 25 30 200 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V)
20
25
30
REFERENCE VOLTAGE vs. REFERENCE LOAD CURRENT
MAX1779-13
RIPPLE WAVEFORMS
MAX1779-14
LOAD TRANSIENT (L = 10H, 500s PULSE)
5.1V
MAX1779-15
1.256 1.254 1.252 VREF (V) 1.250 1.248
5V
A
5.0V
A
-8V
B
4.9V 50mA
12V 1.246 1.244 0 10 20 30 40 50 IREF (A) 4.0s/div A. VMAIN = 5V, IMAIN = 100mA, 10mV/div B. VNEG = -8V, INEG = 1mA, 5mV/div C. VPOS = 12V, IPOS = 1mA, 5mV/div, FIGURE 5
C 0
B
100s/div A. VMAIN = 5V, 50mV/div B. VMAIN = 5mA to 50mA, 25mA/div FIGURE 6
6
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Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
Typical Operating Characteristics (continued)
(Circuit of Figure 5, VIN = +3.3V, TA = +25C, unless otherwise noted.)
LOAD TRANSIENT WITHOUT INTEGRATOR (L = 10H, 500s PULSE)
MAX1779-16
LOAD TRANSIENT WITHOUT INTEGRATOR (L = 10H, 5s PULSE)
MAX1779-17
LOAD TRANSIENT (L = 33H, 500s PULSE)
5.1V
MAX1779-18
5.0V A 4.9V
5.0V A 5V 400mA 200mA B 4.9V 100mA B C 0 10s/div A. VMAIN = 5V, 100mV/div B. IL, 200mA/div C. IMAIN = 10mA to 100mA, 100mA/div INTG = REF, FIGURE 6 A
50mA B 0
0 100mA 0
100s/div A. VMAIN = 5V, 50mV/div B. VMAIN = 5mA to 50mA, 25mA/div INTG = REF, FIGURE 6
100s/div A. VMAIN = 5V, 50mV/div B. IMAIN = 10mA to 100mA, 50mA/div FIGURE 5
LOAD TRANSIENT WITHOUT INTEGRATOR (L = 33H, 500s PULSE)
5.1V
MAX1779-19
LOAD TRANSIENT (L = 33H, 5s PULSE)
5.1V
MAX1779-20
STARTUP WAVEFORM (L = 10H)
MAX1779-21
2V 5.0V A 5.0V A 0 5V 4.9V 100mA B 0 0 4.9V 200mA B 3V 500mA 0 10s/div A. VMAIN = 5V, 50mV/div B. IMAIN = 20mA to 200mA, 100mA/div FIGURE 5
A
B
C
100s/div A. VMAIN = 5V, 50mV/div B. IMAIN = 10mA to 100mA, 50mA/div INTG = REF, FIGURE 5
200s/div A. VSHDN = 0 to 2V, 2V/div B. VMAIN = 5V, 1V/div C. IL, 500 mA/div FIGURE 6, RMAIN = 100
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Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
Typical Operating Characteristics (continued)
(Circuit of Figure 5, VIN = +3.3V, TA = +25C, unless otherwise noted.)
STARTUP WAVEFORM (L = 33H)
MAX1779-22
POWER-UP SEQUENCING
MAX1779-23
2V 0 5V
A
2V 0 5V
A
B B 3V 500mA 0 C 0 -10V 10V 0 200s/div A. VSHDN = 0 to 2V, 2V/div B. VMAIN = 5V, 1V/div C. IL, 500mA/div RMAIN = 50 4ms/div A. VSHDN = 0 to 2V, 2V/div B. VMAIN = 5V, RMAIN = 50, 2.5V/div C. VNEG = -8V, RNEG = 8k, 10V/div D. VPOS = +12V, RPOS = 12k, 10V/div D C
Pin Description
PIN 1 2 3 4 5 6 7 8 9 NAME RDY FB INTG IN GND REF FBP FBN SHDN FUNCTION Active-Low Open-Drain Output. Indicates all outputs are ready. The on-resistance is 125 (typ). Main Boost Regulator Feedback Input. Regulates to 1.25V nominal. Connect feedback resistive divider to analog ground (GND). Main Boost Integrator Output. If used, connect 2200pF to analog ground (GND). To disable integrator, connect to REF. Supply Input. +2.7V to +5.5V input range. Bypass with a 0.1F capacitor between IN and GND, as close to the pins as possible. Analog Ground. Connect to power ground (PGND) underneath the IC. Internal Reference Bypass Terminal. Connect a 0.22F capacitor from this terminal to analog ground (GND). External load capability to 50A. Positive Charge-Pump Regulator Feedback Input. Regulates to 1.25V nominal. Connect feedback resistive divider to analog ground (GND). Negative Charge-Pump Regulator Feedback Input. Regulates to 0V nominal. Active-Low Logic-Level Shutdown Input. Connect SHDN to IN for normal operation.
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Low-Power Triple-Output TFT LCD DC-DC Converter
Pin Description (continued)
PIN 10 11 12 13 14 15 16 NAME DRVN SUPN DRVP SUPP PGND LX TGND FUNCTION Negative Charge-Pump Driver Output. Output high level is VSUPN, and low level is PGND. Negative Charge-Pump Driver Supply Voltage. Bypass to PGND with a 0.1F capacitor. Positive Charge-Pump Driver Output. Output high level is VSUPP, and low level is PGND. Positive Charge-Pump Driver Supply Voltage. Bypass to PGND with a 0.1F capacitor. Power Ground. Connect to GND underneath the IC. Main Boost Regulator Power MOSFET N-Channel Drain. Connect output diode and output capacitor as close to PGND as possible. Must be connected to ground.
MAX1779
Detailed Description
The MAX1779 is a highly efficient triple-output power supply for TFT LCD applications. The device contains one high-power step-up converter and two low-power charge pumps. The primary boost converter uses an internal N-channel MOSFET to provide maximum efficiency and to minimize the number of external components. The output voltage of the main boost converter (VMAIN) can be set from VIN to 13V with external resistors. The dual charge pumps independently regulate a positive output (VPOS) and a negative output (VNEG). These low-power outputs use external diode and capacitor stages (as many stages as required) to regulate output voltages up to +40V and down to -40V. A proprietary regulation algorithm minimizes output ripple as well as capacitor sizes for both charge pumps. Also included in the MAX1779 are a precision 1.25V reference that sources up to 50A, logic shutdown, soft-start, power-up sequencing, fault detection, and an active-low open-drain ready output.
the output voltage error signal shift the switch current trip level, consequently modulating the MOSFET duty cycle.
Dual Charge-Pump Regulator
The MAX1779 contains two individual low-power charge pumps. One charge pump inverts the supply voltage (SUPN) and provides a regulated negative output voltage. The second charge pump doubles the supply voltage (SUPP) and provides a regulated positive output voltage. The MAX1779 contains internal P-channel and N-channel MOSFETs to control the power transfer. The internal MOSFETs switch at a constant 125kHz (0.5 fOSC). Negative Charge Pump During the first half-cycle, the P-channel MOSFET turns on and the flying capacitor C5 charges to VSUPN minus a diode drop (Figure 2). During the second half-cycle, the P-channel MOSFET turns off, and the N-channel MOSFET turns on, level shifting C5. This connects C5 in parallel with the reservoir capacitor C6. If the voltage across C6 minus a diode drop is lower than the voltage across C5, charge flows from C5 to C6 until the diode (D5) turns off. The amount of charge transferred to the output is controlled by the variable N-channel on-resistance. Positive Charge Pump During the first half-cycle, the N-channel MOSFET turns on and charges the flying capacitor C3 (Figure 3). This initial charge is controlled by the variable N-channel on-resistance. During the second half-cycle, the Nchannel MOSFET turns off and the P-channel MOSFET turns on, level shifting C3 by VSUPP volts. This connects C3 in parallel with the reservoir capacitor C4. If the voltage across C4 plus a diode drop (VPOS + VDIODE) is smaller than the level-shifted flying capacitor voltage
9
Main Boost Converter
The MAX1779 main step-up converter switches at a constant 250kHz internal oscillator frequency to allow the use of small inductors and output capacitors. The MOSFET switch pulse width is modulated to control the power transferred on each switching cycle and to regulate the output voltage. During PWM operation, the internal clock's rising edge sets a flip-flop, which turns on the N-channel MOSFET (Figure 1). The switch turns off when the voltage-error, slope-compensation, and current-feedback signals trip the comparators and reset the flip-flop. The switch remains off for the rest of the clock cycle. Changes in
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Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
L1 VIN = 2.7V TO 5.5V VOUT = [1 + (R1 / R2)] x VREF VREF = 1.25V
IN OSC S D1 VMAIN (UP TO 13V)
LX
R
Q R1
+
-
PGND ILIM
C1
+
+
INTG CINTG
Figure 1. PWM Boost Converter Block Diagram
(VC3 + VSUPP), charge flows from C3 to C4 until the diode (D3) turns off.
The main boost regulator does not have soft-start. For the charge pumps, soft-start is achieved by controlling the rise rate of the output voltage. The output voltage regulates within 16ms, regardless of output capacitance and load, limited only by the regulator's output impedance (see the Startup Waveforms in the Typical Operating Characteristics).
A logic-low level on SHDN disables all three MAX1779 converters and the reference. When shut down, the supply current drops to 0.1A to maximize battery life and the reference is pulled to ground. The output
10
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Gm
+
MAX1779
+ +
SLOPE COMP FB
RCOMP
REF
R2 C2 CCOMP
+1.25V
GND
Soft-Start
capacitance and load current determine the rate at which each output voltage will decay. A logic-level high on SHDN activates the MAX1779 (see Power-Up Sequencing). Do not leave SHDN floating. If unused, connect SHDN to IN.
Power-Up Sequencing
Upon power-up or exiting shutdown, the MAX1779 starts a power-up sequence. First, the reference powers up. Then the main DC-DC step-up converter powers up. Once the main boost converter reaches regulation, the negative charge pump turns on. When the negative output voltage reaches approximately 90% of its nominal value (V FBN < 120mV), the positive charge pump starts up. Finally, when the positive output voltage reaches 90% of its nominal value (VFBP >
Shutdown
Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
SUPN OSC D4 DRVN C5 D5 VSUPN = 2.7V TO 13V
+ +
MAX1779
GND PGND VREF 1.25V
FBN
R5 C6 R6
VNEG
REF CREF 0.22F VNEG = - R5 VREF R6 VREF = 1.25V
()
Figure 2. Negative Charge-Pump Block Diagram
SUPP OSC D2 C3 DRVP D3
VSUPP = 2.7V TO 13V
+ + MAX1779
GND PGND VREF 1.25V
FBP
R3 C4 R4
VPOS
VPOS = 1 + R3 R4 VREF = 1.25V
( )V
REF
Figure 3. Positive Charge-Pump Block Diagram
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11
Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
1.125V), the active-low ready signal (RDY) is pulled low (see Power Ready section).
Power Ready
Power ready is an open-drain output. When the powerup sequence is properly completed, the MOSFET turns on and pulls RDY low with a typical 125 on-resistance. If a fault is detected, the internal open-drain MOSFET appears as a high impedance. Connect a 100k pullup resistor between RDY and IN for a logiclevel output.
output voltage. With high inductor values, the MAX1779 sources higher output currents, has less output ripple, and enters continuous-conduction operation with lighter loads; however, the circuit's transient response time is slower. On the other hand, low-value inductors respond faster to transients, remain in discontinuous-conduction operation, and typically offer smaller physical size. The maximum output current an inductor value will support may be calculated by the following equations: A. Continuous-conduction: if IMAIN(MAX) then
VMAIN - VIN(MIN) 2 VIN(MIN) VIN(MIN) 1 1 L I -I 2 VMAIN VMAIN LIM(MIN) MAIN(MAX)
Fault Detection
Once RDY is low, if any output falls below its faultdetection threshold, then RDY becomes high impedance. For the reference, the fault threshold is 1.05V. For the main boost converter, the fault threshold is 88% of its nominal value (VFB < 1.1V). For the negative charge pump, the fault threshold is approximately 88% of its nominal value (VFBN < 140mV). For the positive charge pump, the fault threshold is 88% of its nominal value (VFBP < 1.11V). Once an output faults, all outputs later in the power sequence shut down until the faulted output rises above its power-up threshold. For example, if the negative charge-pump output voltage falls below the fault detection threshold, the main boost converter remains active while the positive charge pump stops switching and its output voltage decays, depending on output capacitance and load. The positive charge-pump output will not power up until the negative charge-pump output voltage rises above its power-up threshold (see the Power-Up Sequencing section).
1 VIN(MIN) ILIM(MIN) 2 VMAIN
B. Discontinuous-conduction: if IMAIN(MAX) < then 1 IMAIN(MAX) VMAIN - VIN(MIN) L 2 ILIM(MIN)2 1 VIN(MIN) ILIM(MIN) 2 VMAIN
(
)

Voltage Reference
The voltage at REF is nominally 1.25V. The reference can source up to 50A with good load regulation (see Typical Operating Characteristics). Connect a 0.22F bypass capacitor between REF and GND.
where I LIM(MIN) = 350mA and = 250kHz (see the Electrical Characteristics). The inductor's saturation current rating should exceed peak inductor current throughout the normal operating range. Under fault conditions, the inductor current may reach up to 600mA (I LIM(MAX) , see the Electrical Characteristics). However, the MAX1779's fast currentlimit circuitry allows the use of soft-saturation inductors while still protecting the IC. The inductor's DC resistance significantly affects efficiency due to the power loss in the inductor. The power loss due to the inductor's series resistance (PLR) may be approximated by the following equation: I x VMAIN PLR MAIN x RL VIN
2
Design Procedure
Main Boost Converter
Inductor Selection Inductor selection depends upon the minimum required inductance value, saturation rating, series resistance, and size. These factors influence the converter's efficiency, maximum output load capability, transient response time, and output voltage ripple. For most applications, values between 10H and 33H work best with the controller's switching frequency. The inductor value depends on the maximum output load the application must support, input voltage, and
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Low-Power Triple-Output TFT LCD DC-DC Converter
where RL is the inductor's series resistance. For best performance, select inductors with resistance less than the internal N-channel MOSFET on-resistance (1 typ). Output Capacitor The output capacitor selection depends on circuit stability and output voltage ripple. In order to deliver the maximum output current capability of the MAX1779, the inductor must run in continuous-conduction mode (see Inductor Selection). The minimum recommended output capacitance is: 60 x L x IMAIN(MAX) VMAIN x VIN(MIN) Feedback Compensation Compensation on the feedback node is required to have enough margin for stability. Add a pole-zero pair from FB to GND in the form of a compensation resistor (RCOMP in Figures 5 and 6) in series with a compensation capacitor (CCOMP in Figures 5 and 6). For continuous conduction operation, select RCOMP to be 1/2 the value of R2, the low-side feedback resistor. For discontinuous-conduction operation, select RCOMP to be 1/5th the value of R2. Start with a compensation capacitor value of (220pF RCOMP)/10k. Increase this value to improve the DC stability as necessary. Larger compensation values slow down the converter's response time. Check the startup waveform for excessive overshoot each time the compensation capacitor value is increased.
MAX1779
COUT >
For configurations that need less output current, the MAX1779 allows lower output capacitance when operating in discontinuous-conduction mode throughout the load range. Under these conditions, at least 10F is recommended, as shown in Figure 6. In both discontinuous and continuous operation, additional feedback compensation is required (see the Feedback Compensation section) to increase the margin for stability by reducing the bandwidth further. In cases where the output capacitance is sufficiently large, additional feedback compensation will not be necessary. However, in certain applications that require benign load transients and constantly operate in discontinuous-conduction mode, output capacitance less than 10F may be used. Output voltage 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(C) + VRIPPLE(ESR) For low-value ceramic capacitors, the output voltage ripple is dominated by VRIPPLE(C). Integrator Capacitor The MAX1779 contains an internal current integrator that improves the DC load regulation but increases the peak-to-peak transient voltage (see the Load Transient Waveforms in the Typical Operating Characteristics). For highly accurate DC load regulation, enable the integrator by connecting a capacitor to INTG. The minimum capacitor value should be COUT/10k or 1nF, whichever is greater. Alternatively, to minimize the peak-to-peak transient voltage at the expense of DC load regulation, disable the integrator by connecting INTG to REF and adding a 100k resistor to GND.
Charge Pump
Efficiency Considerations The efficiency characteristics of the MAX1779 regulated charge pumps are similar to a linear regulator. They are dominated by quiescent current at low output currents and by the input voltage at higher output currents (see Typical Operating Characteristics). So the maximum efficiency may be approximated by: Efficiency IVNEGI / [VIN N]; for the negative charge pump Efficiency VPOS / [VIN (N + 1)]; for the positive charge pump where N is the number of charge-pump stages. Output Voltage Selection Adjust the positive output voltage by connecting a voltage-divider from the output (VPOS) to FBP to GND (see Typical Operating Circuit). Adjust the negative output voltage by connecting a voltage-divider from the output (VNEG) to FBN to REF. Select R4 and R6 in the 50k to 100k range. Higher resistor values improve efficiency at low output current but increase output voltage error due to the feedback input bias current. Calculate the remaining resistors with the following equations: R3 = R4 [(VPOS / VREF) - 1] R5 = R6 (IVNEG / VREFI) where VREF = 1.25V. VPOS may range from VSUPP to +40V, and VNEG may range from 0 to -40V. Flying Capacitor Increasing the flying capacitor's value increases the output current capability. Above a certain point, increasing the capacitance has a negligible effect because the output current capability becomes domi13
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Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
nated by the internal switch resistance and the diode impedance. Start with 0.1F ceramic capacitors. Smaller values may be used for low-current applications. Charge-Pump Output Capacitor Increasing the output capacitance or decreasing the ESR reduces the output ripple voltage and the peak-topeak transient voltage. Use the following equation to approximate the required capacitor value: CPUMP [IPUMP / (125kHz VRIPPLE)] Charge-Pump Input Capacitor Use a bypass capacitor with a value equal to or greater than the flying capacitor. Place the capacitor as close to the IC as possible. Connect directly to PGND. Rectifier Diode Use Schottky diodes with a current rating equal to or greater than 4 times the average output current, and a voltage rating at least 1.5 times VSUPP for the positive charge pump and VSUPN for the negative charge pump. mum load current that the LX charge pump can provide and is limited by the following formula: ILXPUMP = ((N + 1) IPOS) + (M + INEG) 5mA where N is the number of stages in the positive lowpower charge pump, and M is the number of stages in the negative charge pump. Applications requiring more output current should not use the LX charge pump, so they will require extra stages on both low-power charge pumps. The output capacitor of this unregulated charge pump needs to be stacked on top of the main output in order to keep the main regulator stable. Increasing the integrator capacitor may also be required to compensate for the additional charge-pump capacitance on the main regulator loop. The output capacitor of this unregulated charge pump needs to be stacked on top of the main output in order to keep the main regulator stable. Increasing the integrator capacitor may also be required to compensate for the additional charge-pump capacitance on the main regulator loop.
PC Board Layout and Grounding
Carefully printed circuit layout is extremely important to minimize ground bounce and noise. First, place the main boost converter output diode and output capacitor less than 0.2in (5mm) from the LX and PGND pins with wide traces and no vias. Then place 0.1F ceramic bypass capacitors near the charge-pump input pins (SUPP and SUPN) to the PGND pin. Keep the chargepump circuitry as close to the IC as possible, using wide traces and avoiding vias when possible. Locate all feedback resistive dividers as close to their respective feedback pins as possible. The PC board should feature separate GND and PGND areas connected at only one point under the IC. To maximize output power and efficiency and to minimize output power ripple voltage, use extra wide power ground traces and solder the IC's power ground pin directly to it. Avoid having sensitive traces near the switching nodes and high-current lines. Refer to the MAX1779 evaluation kit for an example of proper board layout.
Table 1. Component Suppliers
SUPPLIER INDUCTORS Coilcraft Coiltronics Sumida USA Toko CAPACITORS AVX Kemet Sanyo Taiyo Yuden DIODES Central Semiconductor International Rectifier Motorola Nihon Zetex 516-435-1110 310-322-3331 602-303-5454 847-843-7500 516-543-7100 516-435-1824 310-322-3332 602-994-6430 847-843-2798 516-864-7630 803-946-0690 408-986-0424 619-661-6835 408-573-4150 803-626-3123 408-986-1442 619-661-1055 408-573-4159 PHONE 847-639-6400 561-241-7876 847-956-0666 847-297-0070 FAX 847-639-1469 561-241-9339 847-956-0702 847-699-1194
Applications Information
LX Charge Pump
Some applications require multiple charge-pump stages due to low supply voltages. In order to reduce the circuit's size and component count, an unregulated charge pump may be added onto the LX switching node. The configuration shown in Figure 4 works well for most applications. The maximum output current of the low-power charge pumps depends on the maxi14
Chip Information
TRANSISTOR COUNT: 2846
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0.47F 1.0F 10H VMAIN = +5V IN 100k SHDN FB RDY R2 50k DRVN 0.1F SUPN SUPP FBN 1.0F R6 49.9k REF CREF 0.22F DRVP 0.1F R5 320k RCOMP 10k CCOMP 220pF 0.1F R1 150k COUT (2) 4.7F LX
VIN = +3.0V
Figure 4. Minimizing the Number of Charge-Pump Stages
(2) 4.7F
MAX1779
VNEG = -8V, 1mA
VPOS = +15V, 1mA INTG CINTG 3300pF GND TGND FBP PGND R3 549k R4 49.9k 1.0F
Low-Power Triple-Output TFT LCD DC-DC Converter
MAX1779
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15
MAX1779
Figure 5. Typical Operating Circuit (L = 33H)
Low-Power Triple-Output TFT LCD DC-DC Converter
16
VIN = +3.3V CIN 10F RRDY 100k IN C11 0.1F SHDN FB R2 50k CCOMP 470pF RDY INTG CINTG 2200pF SUPN RCOMP 24k LX R1 150k COUT 22F 33H VMAIN = +5.0V C5 0.1F SUPP DRVN
MAX1779
DRVP C6 0.47F C9 0.22F FBN C10 2.2F R6 49.9k REF CREF 0.22F PGND R5 320k C3 0.1F
C3 0.47F
VNEG -8V, 5mA
C7 0.22F FBP TGND GND R4 49.9k C8 430k
VPOS +12V, 5mA
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C8 2.2F
VIN = +3.3V CIN (2) 4.7F RRDY 100k IN C11 0.1F SHDN RDY R2 50k CCOMP 220pF INTG CINTG 2200pF SUPN C5 0.1F SUPP DRVN FB RCOMP 10k LX R1 150k COUT (2) 4.7F
Figure 6. Typical Operating Circuit (L = 10H)
10H VMAIN = +5.0V
MAX1779
DRVP C3 0.1F C9 0.22F FBN C10 2.2F R6 49.9k REF CREF 0.22F PGND R5 320k C7 0.22F FBP TGND GND R4 49.9k R3 430k C8 2.2F
C6 0.47F
C4 0.47F
Low-Power Triple-Output TFT LCD DC-DC Converter
MAX1779
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VPOS +12V, 5mA
VNEG -8V, 5mA
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Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779
Package Information
TSSOP.EPS
Note: The MAX1779 16-pin TSSOP package does not have an exposed pad.
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
18 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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