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19-1788; Rev 0; 10/00
KIT ATION EVALU E AILABL AV
Triple Charge-Pump TFT LCD DC-DC Converter
General Description
MAX1747
Features
o Adjustable Outputs Up to +5.5V Main High-Power Output Up to +35V Positive Charge-Pump Output Down to -35V Negative Charge-Pump Output o 200kHz to 2MHz Adjustable Switching Frequency o +2.7V to +4.5V Input Supply o Internal Power MOSFETs o 0.1A Shutdown Current o Internal Soft-Start o Power-Ready Output o Internal Supply Sequencing o Fast Transient Response o Ultra-Thin Solution (No Inductors) o Thin TSSOP Package (1.1mm max)
The MAX1747 triple charge-pump DC-DC converter provides the regulated voltages required by active matrix thin-film transistor (TFT) liquid-crystal displays (LCDs) in a low-profile TSSOP package. One highpower and two low-power charge pumps convert the +2.7V to +4.5V input supply voltage into three independent output voltages. The primary high-power charge pump generates an output voltage (VOUT) between 4.5V and 5.5V that is regulated within 1%. The low-power BiCMOS control circuitry and the low on-resistance (R ON ) power MOSFETs maximize efficiency. The adjustable switching frequency (200kHz to 2MHz) provides fast transient response and allows the use of small low-profile ceramic capacitors. The dual low-power charge pumps independently regulate one positive output (VPOS) and one negative output (VNEG). These additional outputs use external diode and capacitor multiplier stages (as many stages as required) to regulate output voltages up to +35V and -35V. The constant switching frequency and a proprietary regulation algorithm minimize output ripple and capacitor sizes for all three charge pumps. The MAX1747 is available in the ultra-thin TSSOP package (1.1mm max height).
Ordering Information
PART MAX1747EUP TEMP. RANGE -40C to +85C PIN-PACKAGE 20 TSSOP
Applications
TFT Active-Matrix LCDs Passive-Matrix Displays Personal Digital Assistants (PDAs)
INPUT
Typical Operating Circuit
SUPM IN CXP CXN MAIN OUTPUT TO C SHDN RDY DRVN OUT SUPP SUPN FB
Pin Configuration
TOP VIEW
TGND 1 TGND 2 RDY 3 FB 4 INTG 5 IN 6 GND 7 REF 8 FBP 9 FBN 10 20 OUT 19 CXP 18 SUPM 17 CXN
MAX1747
MAX1747
NEGATIVE OUTPUT
DRVP FBN
16 PGND 15 SUPP 14 DRVP 13 SUPN 12 DRVN 11 SHDN
REF FBP INTG PGND POSITIVE OUTPUT
TGND GND
TSSOP
________________________________________________________________ Maxim Integrated Products
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Triple Charge-Pump TFT LCD DC-DC Converter MAX1747
ABSOLUTE MAXIMUM RATINGS
IN, SUPM, OUT, TGND to GND................................-0.3V to +6V SHDN........................................................................-0.3V to +1V PGND to GND.....................................................................0.3V SUPM to IN .........................................................................0.3V CXN to PGND.........................................-0.3V to (VSUPM + 0.3V) CXP to PGND ............................(VSUPM - 0.3V) to (VOUT + 0.3V) DRVN to GND .........................................-0.3V to (VSUPN + 0.3V) DRVP to GND..........................................-0.3V to (VSUPP + 0.3V) RDY to GND ...........................................................-0.3V to +14V SUPP, SUPN to GND..............................................-0.3V to +14V INTG, REF, FB, FBN, FBP to GND ...............-0.3V to (VIN + 0.3V) Continuous Current into: SUPM, CXN, CXP, OUT ..............................................800mA SUPP, SUPN, DRVN, DRVP........................................200mA SHDN...........................................................................+100A All Other Pins ....................................................................10mA Continuous Power Dissipation (TA = +70C) 20-Pin TSSOP (derate 10.9mW/C above +70C) .......879mW Operating Temperature Range............................-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 = VSUPM = +3.0V, VSUPP = VSUPN = +5V, TGND = PGND = GND, I SHDN = 22A, COUT = 2 4.7F, CREF = 0.22F, CINTG = 1500pF, VOUT = +5V, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Input Supply Range Input Undervoltage Threshold Input Quiescent Supply Current SYMBOL VIN VUVLO IIN + ISUPM VIN falling, 40mV hysteresis (typ) VFB = VFBP = 1.5V, VFBN = -0.2V, VOUT = 5V, no load on DRVN and DRVP; CXN and CXP open VFB = VFBP = 1.5V, VFBN = -0.2V, VOUT = 5V, no load on DRVN and DRVP; CXN and CXP open V SHDN = 0, VSUPM = 5V fOSC VOUT IOUT(MAX) VFB IFB VFB = 1.25V Rising edge Falling edge CX = 0.47F I SHDN = 22A 0.65 4.5 200 1.237 -50 530 1.09 1.125 1.100 4.096 / FOSC VSUPN ISUPN VFBN VFBN = -0.2V, no load on DRVN V SHDN = 0, VSUPN = 13V -50 2.7 0.6 0.1 0 13 0.8 10 +50 1.16 1.248 1.263 +50 CONDITIONS MIN 2.7 2.2 2.4 0.9 TYP MAX 4.5 2.6 1.0 UNITS V V mA
Output Quiescent Supply Current Shutdown Supply Current Operating Frequency MAIN CHARGE PUMP Output Voltage Range Maximum Output Current FB Regulation Voltage FB Input Bias Current Integrator Transconductance FB Power-Ready Trip Level FB Fault Trip Level Main Soft-Start Period
IQ(OUT)
2.5 0.1 1
4.0 20 1.2 5.5
mA A MHz V mA V nA S V V s
NEGATIVE LOW-POWER CHARGE PUMP SUPN Input Supply Range SUPN Quiescent Current SUPN Shutdown Current FBN Regulation Voltage V mA A mV
2
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Triple Charge-Pump TFT LCD DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VSUPM = +3.0V, VSUPP = VSUPN = +5V, TGND = PGND = GND, I SHDN = 22A, COUT = 2 4.7F, CREF = 0.22F, CINTG = 1500pF, VOUT = +5V, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER FBN Input Bias Current DRVN PCH On-Resistance DRVN NCH On-Resistance FBN Power-Ready Trip Level FBN Fault Trip Level Negative Soft-Start Period POSITIVE LOW-POWER CHARGE PUMP SUPP Input Supply Range SUPP Quiescent Current SUPP Shutdown Current FBP Regulation Voltage FBP Input Bias Current DRVP PCH On-Resistance DRVP NCH On-Resistance FBP Power-Ready Trip Level FBP Fault Trip Level Positive Soft-Start Period REFERENCE Reference Voltage Reference Undervoltage Threshold LOGIC SIGNALS SHDN Input Low Voltage SHDN Bias Voltage SHDN Bias Voltage Tempco SHDN Input Current Range RDY Output Low Voltage RDY Output High Leakage I SHDN For 200kHz to 2MHz operation ISINK = 2mA V RDY = 13V 3 0.25 0.01 I SHDN = 22A 580 724 2 65 0.5 1 0.4 830 V mV mV/C A V A VREF -2A < IREF < 50A VREF rising 1.231 0.95 1.25 1.05 1.269 1.18 V V VFBP = 1.20V VFBP = 1.30V Rising edge Falling edge 1.090 VFBP IFBP VFBP = 1.5V VSUPP ISUPP VFBP = 1.5V, no load on DRVP V SHDN = 0, VSUPP = 13V 1.20 -50 3 1.5 20 1.125 1.100 2.048/ FOSC 1.160 2.7 0.6 0.1 1.25 13 0.8 10 1.30 +50 6 5 V mA A V nA k V V s VFBN = 50mV VFBN = -50mV Falling edge Rising edge 20 80 125 140 2.048/ FOSC 165 SYMBOL IFBN VFBN = -50mV CONDITIONS MIN -50 3 1 TYP MAX +50 6 5 UNITS nA k mV mV s
MAX1747
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Triple Charge-Pump TFT LCD DC-DC Converter MAX1747
ELECTRICAL CHARACTERISTICS
(VIN = VSUPM = +3.0V, VSUPP = VSUPN = +5V, TGND = PGND = GND, I SHDN = 22A, COUT = 2 1500pF, VOUT = +5V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input Supply Range Input Undervoltage Threshold Input Quiescent Supply Current SYMBOL VIN VUVLO IIN + ISUPM VIN falling, 40mV hysteresis (typ) VFB = VFBP = 1.5V, VFBN = -0.2V, VOUT = 5V, no load on DRVN and DRVP; CXN and CXP open VFB = VFBP = 1.5V, VFBN = -0.2V, VOUT = 5V, no load on DRVN and DRVP; CXN and CXP open V SHDN = 0, VSUPM = 5V fOSC VOUT IOUT(MAX) VFB IFB VFB = 1.25V Rising edge VSUPN ISUPN VFBN IFBN VFBN = 0 VFBN = 50mV VFBN = -50mV Falling edge VSUPP ISUPP VFBP IFBP VFBP = 1.5V VFBP = 1.20V VFBP = 1.30V Rising edge 20 1.09 1.16 VFBP = 1.5V, no load on DRVP V SHDN = 0, VSUPP = 13V 1.20 -50 20 80 2.7 165 13 0.8 10 1.30 +50 6 5 VFBN = -0.2V, no load on DRVN V SHDN = 0, VSUPN = 13V -50 -50 CX = 0.47F I SHDN = 22A 0.65 4.5 200 1.222 -50 1.09 2.7 1.271 +50 1.16 13 0.8 10 +50 +50 6 5 CONDITIONS
4.7F, CREF = 0.22F, CINTG =
MIN 2.7 2.2
MAX 4.5 2.6 1.0
UNITS V V mA
Output Quiescent Supply Current Input Shutdown Current Operating Frequency MAIN CHARGE PUMP Output Voltage Range Output Current FB Regulation Voltage FB Input Bias Current FB Power-Ready Trip Level
IQ(OUT)
4.0 20 1.2 5.5
mA A MHz V mA V nA V V mA A mV nA k mV V mA A V nA k V
NEGATIVE LOW-POWER CHARGE PUMP SUPN Input Supply Range SUPN Quiescent Current SUPN Shutdown Current FBN Regulation Voltage FBN Input Bias Current DRVN PCH On-Resistance DRVN NCH On-Resistance FBN Power-Ready Trip Level POSITIVE LOW-POWER CHARGE PUMP SUPP Input Supply Range SUPP Quiescent Current SUPP Shutdown Current FBP Regulation Voltage FBP Input Bias Current DRVP PCH On-Resistance DRVP NCH On-Resistance FBP Power-Ready Trip Level
4
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Triple Charge-Pump TFT LCD DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VSUPM = +3.0V, VSUPP = VSUPM = +5V, TGND = PGND = GND, I SHDN = 22A, COUT = 2 1500pF, VOUT = +5V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER REFERENCE Reference Voltage Reference Undervoltage Threshold LOGIC SIGNALS SHDN Input Low Voltage SHDN Bias Voltage SHDN Input Current Range RDY Output Low Voltage RDY Output High Leakage I SHDN 0.4 I SHDN = 22A For 200kHz to 2MHz operation ISINK = 2mA V RDY = 13V 580 3 900 65 0.5 1 V mV A V A VREF -2A < IREF < 50A VREF rising 1.222 0.95 1.269 1.18 V V SYMBOL CONDITIONS
MAX1747
4.7F, CREF = 0.22F, CINTG =
MIN
MAX
UNITS
Note 1: Specifications from 0C to -40C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, VIN = VSUPM = +3.3V, TA = +25C, unless otherwise noted.)
MAIN OUTPUT EFFICIENCY vs. LOAD CURRENT (MAIN CHARGE PUMP ONLY)
MAX1747 toc01
MAIN OUTPUT EFFICIENCY vs. LOAD CURRENT (MAIN CHARGE PUMP ONLY)
MAX1747 toc02
MAIN OUTPUT EFFICIENCY vs. LOAD CURRENT (MAIN CHARGE PUMP ONLY)
VOUT = 5V 90 VIN = 2.8V EFFICIENCY (%) 80 70 60 VIN = 4.0V 50 40 VIN = 3.3V VNEG = -7V WITH INEG = 10mA VPOS = 12V WITH IPOS = 5mA
MAX1747 toc03
5.03 5.02 5.01 VOUT (V) 5.00 VIN = 2.8V 4.99 4.98 4.97 0 100 200 IOUT (mA) 300 VIN = 4.0V
100 VOUT = 5V 90 EFFICIENCY (%) 80 70 VIN = 4.0V 60 50 40 VIN = 2.8V
100
VIN = 3.3V
VIN = 3.3V
400
0
100
200 IOUT (mA)
300
400
0
100
200 IOUT (mA)
300
400
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Triple Charge-Pump TFT LCD DC-DC Converter MAX1747
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = VSUPM = +3.3V, TA = +25C, unless otherwise noted.)
SWITCHING FREQUENCY vs. ISHDN
MAX1747 toc04 MAX1747 toc05
NO-LOAD SUPPLY CURRENT vs. SWITCHING FREQUENCY
16 14 12 FREQUENCY (MHz) 10 8 6 4 2 0 0 0.5 1.0 FREQUENCY (MHz) 1.5 2.0 0 0 1.2 2.0
NO-LOAD SUPPLY CURRENT vs. TEMPERATURE
MAX1747 toc06
12.0 11.5 IIN + ISUPM (mA) 11.0 10.5 10.0
1.6
ICC + IIN (mA)
0.8
0.4
9.5 9.0 10 20 30 40 50 -40 -15 10 35 ISHDN (mA)
VON = 3.3V RFREQ = 120k 60 85
TEMPERATURE (C)
SWITCHING FREQUENCY vs. TEMPERATURE
MAX1747 toc07
NEGATIVE LOW-POWER CHARGE-PUMP EFFICIENCY vs. LOAD CURRENT
MAX1747 toc08
NEGATIVE LOW-POWER CHARGE-PUMP OUTPUT VOLTAGE vs. LOAD CURRENT
-6.4 -6.5 VSUPN = 5V
MAX1747 toc09
1.15 1.10 FREQUENCY (MHz) 1.05 1.00 0.95 0.90 0.85 -40 -15 10 35
100 90 80 EFFICIENCY (%)
VNEG = -7V
-6.3
VSUPN = 5V 60 50 40 VSUPN = 7V VNEG (V) 70 VSUPN = 6V -6.6 -6.7
VSUPN = 6V -6.8 -6.9 -7.0 -7.1 VSUPN = 7V 0 10 20 INEG (mA) 30 40
VON = 3.3V RFREQ = 120k 60 85
30 20 0 10 20 INEG (mA) 30 40
TEMPERATURE (C)
MAXIMUM NEGATIVE CHARGE-PUMP OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
MAX1747 toc10
POSITIVE LOW-POWER CHARGE-PUMP EFFICIENCY vs. LOAD CURRENT
90 80 VPOS (V) VSUPP = 5V
MAX1747 toc11
POSITIVE LOW-POWER CHARGE-PUMP OUTPUT VOLTAGE vs. LOAD CURRENT
12.1 12.0 VSUPP = 7V 11.9 11.8 11.7 11.6 11.5 VSUPP = 5V VSUPP = 6V
MAX1747 toc12
-2
VNEG(NOMINAL) = -20V
100
12.2
-6 INEG = 10mA
EFFICIENCY (%)
VNEG (V)
-10
70 60 50 40 30 VPOS = 12V 20 VSUPP = 6V VSUPP = 7V
-14
INEG = 1mA
-18
-22 3 5 7 9 11 13 VSUPN (V)
11.4 40 0 10 20 IPOS (mA) 30 40
0
10
20 IPOS (mA)
30
6
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Triple Charge-Pump TFT LCD DC-DC Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = VSUPM = +3.3V, TA = +25C, unless otherwise noted.)
MAXIMUM POSITIVE CHARGE-PUMP OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
VPOS(NOMINAL) = 32V IPOS = 1mA
MAX1747 toc13
MAX1747
LOAD TRANSIENT
MAX1747 toc14
34
200mA 100mA 0 IOUT 100mA/div
28
VPOS (V)
22 IPOS = 10mA
16
5.05V 5V 4.95V VOUT 50mV/div
10
4 3 5 7 9 11 13 VSUPP (V) 40s/div VIN = 3.3V, VOUT = 5.0V ROUT = 500 TO 25 CINTG = 1500pF
RIPPLE WAVEFORM
MAX1747 toc15
STARTUP WAVEFORM (NO LOAD)
MAX1747 toc16
VOUT 20mV/div
2V 0 4V
VON 2V/div VCXP 2V/div
VNEG 10mV/div VPOS 10mV/div
2V 4V 2V 0 1ms/div VOUT = 5V, NO LOAD ON CONNECTED TO SHDN THROUGH A 58k RESISTOR VOUT 2V/div
400ns/div VOUT = +5.0V,IOUT = 200mA VNEG = -7V, INEG = 10mA VPOS = +12V, IPOS = 5mA
STARTUP WAVEFORM (200mA LOAD)
MAX1747 toc17
POWER-UP SEQUENCE
MAX1747 toc18
2V 0 4V 2V 4V 2V 0 1ms/div VOUT = 5V, ROUT = 25 (200mA) ON CONNECTED TO SHDN THROUGH A 58k RESISTOR
VON 2V/div VCXP 2V/div
2V 0 5V 0 -10V
VON 2V/div VMAIN 5V/div VNEG 10V/div VPOS 10V/div
VOUT 2V/div
10V 0 2ms/div VMAIN = 5V, VNEG = -7V, VPOS = 12V ON CONNECTED TO SHDN THROUGH A 58k RESISTOR
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Triple Charge-Pump TFT LCD DC-DC Converter MAX1747
Pin Description
PIN 1, 2 3 4 5 6 7 8 9 10 NAME TGND RDY FB INTG IN GND REF FBP FBN FUNCTION Must be connected to ground. Active-Low Open-Drain Output. Indicates all outputs are ready. The RON is 125 (typ). Main Charge-Pump Feedback Input. Regulates to 1.25V nominal. Connect to the center of a feedback resistive divider between the main output (OUT) and analog ground (GND). Main Charge-Pump Integrator Output. If used, connect 1500pF to analog ground (GND). To disable the integrator, connect to GND. Supply Input. +2.7V to +4.5V input range. Powers only the logic and reference. Bypass to analog ground (GND) with a 0.1F capacitor as close to the pin 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. REF is disabled in shutdown. Positive Charge-Pump 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. Connect feedback resistive divider to the reference (REF). Shutdown Input. Drive SHDN through an external resistor. When SHDN is pulled low, the device turns off and draws only 0.1A. OUT is also pulled low through an internal 10 resistor in shutdown mode. When current is sourced into SHDN through RFREQ, the device activates, and the SHDN input current sets the oscillator's switching frequency: RFREQ (k) = 45.5 (MHz / mA) (VON - 0.7V) / fOSC (MHz) Negative Charge-Pump Driver Output. Output high level is VSUPN, and low level is PGND. Negative Charge-Pump Driver Supply Voltage. Bypass to power ground (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 power ground (PGND) with a 0.1F capacitor. Power Ground. Connect to analog ground (GND) underneath the IC. Negative Terminal of the Main Charge-Pump Flying Capacitor Main Charge-Pump Supply Voltage Input Positive Terminal of the Main Charge-Pump Flying Capacitor Main Charge-Pump Output. Bypass to power ground (PGND) with 10F for a 1MHz application (see Output Capacitor Selection). An internal 10 resistor discharges the output when the device is shut down.
11
SHDN
12 13 14 15 16 17 18 19 20
DRVN SUPN DRVP SUPP PGND CXN SUPM CXP OUT
Detailed Description
The MAX1747 is an efficient triple-output power supply for TFT LCD applications. The device contains one high-power charge pump and two low-power charge pumps. The MAX1747 charge pumps switch continuously at a constant frequency, so the output noise contains well-defined frequency components, and the circuit requires much smaller external capacitors for a
8
given output ripple. The adjustable switching frequency is set by the current into the shutdown pin (see Frequency Selection and Shutdown). The main charge pump uses internal MOSFETs with low RON to provide high output current. The adjustable output voltage of the main charge pump can be set up to 5.5V with external resistors. The dual low-power charge pumps independently regulate a positive output
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Triple Charge-Pump TFT LCD DC-DC Converter MAX1747
VIN = 3.0V CIN 10F C1 0.1F CXP SUPM IN CXN OUT SUPP RDY SHDN D3 C5 0.1F DRVN D4 D7 C6 1.0F FB C9 0.1F C3 0.1F DRVP D8 D2 FBN VNEG -7V, 10mA C10 1.0F R5 280k R6 49.9k REF CREF 0.22F TGND GND FBP INTG PGND CINTG 1500pF R4 49.9k R3 432k C7 0.1F D5 C4 1.0F D1 SUPN C11 0.1F C12 0.1F R2 49.9k COUT (2) 4.7F R1 150k CX 0.47F VOUT +5V, 200mA
R7 100k
RFREQ 100k
MAX1747
D6 VPOS +12V, 5mA C8 1.0F
Figure 1. Typical Application Circuit
(VPOS) and a negative output (VNEG). These two outputs use external diode and capacitor stages (as many stages as required) to regulate output voltages above +35V and under -35V. A proprietary regulation algorithm minimizes output ripple as well as capacitor sizes for all three charge pumps. Also included in the MAX1747 are a precision 1.25V reference that sources up to 50A, shutdown, power-up sequencing, fault detection, and an activelow open-drain ready output.
across CX on top of the supply voltage. This transfers the sum of the two voltages to the output capacitor (COUT).
Dual Charge-Pump Regulators
The MAX1747 contains two individual low-power charge pumps. Using a single stage, the first charge pump inverts the supply voltage (VSUPN) and provides a regulated negative output voltage. The second charge pump doubles the supply voltage (VSUPP) and provides a regulated positive output voltage. The MAX1747 contains internal P-channel and N-channel MOSFETs to control the power transfer. The internal MOSFETs switch at a constant frequency set by the current into the shutdown pin (see Frequency Selection and Shutdown).
Main Charge Pump
During the first half-cycle, the MAX1747 charges the flying capacitor (CX) by connecting it between the supply voltage (VSUPM) and ground (Figure 2). This initial charge is controlled by the variable N-channel on-resistance. During the second half-cycle, the MAX1747 level shifts the flying capacitor by stacking the voltage
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9
Triple Charge-Pump TFT LCD DC-DC Converter MAX1747
SUPM OSC VSUPM = VIN 2.7V TO 4.5V C1 OUT CXP CX CXN FB gm CINTG INTG PGND VREF 1.25V REF CREF GND VOUT = [1+ (R1/R2)] VREF VREF = 1.25V R2 R1 VOUT COUT
MAX1747
OSC
SUPN C5 DRVN
VSUPP = 2.7V TO 13V D3 D4 R5 VNEG C6
MAX1747
FBN VREF 1.25V REF GND PGND R6
CREF VNEG = -(R5/R6) VREF VREF = 1.25V
Figure 2. Main Charge-Pump Block Diagram
Figure 3. Negative Charge-Pump Block Diagram
Negative Charge Pump During the first half-cycle, the P-channel MOSFET turns on, and flying capacitor C5 charges to VSUPN minus a diode drop (Figure 3). 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, current flows from C5 to C6 until the diode (D4) turns off. The amount of charge transferred to the output is controlled by the variable N-channel RON. Positive Charge Pump During the first half-cycle, the N-channel MOSFET turns on and charges the flying capacitor, C3 (Figure 4). This initial charge is controlled by the variable N-channel R ON . During the second half-cycle, the N-channel 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 (VC3 + VSUPP), charge flows from C3 to C4 until the diode (D2) turns off.
Frequency Selection and Shutdown
The shutdown pin (SHDN) on the MAX1747 performs a dual function: it shuts down the device and determines the oscillator frequency. The SHDN input looks like a diode to ground and should be driven through a resistor (Figure 5).
Driving SHDN low forces all three MAX1747 converters into shutdown mode. When disabled, the supply current drops to 20A (max) to maximize battery life, and OUT is pulled to ground through an internal 10 resistor. For the low-power charge pumps, the output capacitance and load current determine the rate at which each output voltage will decay. The device activates (see Power-up Sequencing) once SHDN is forward biased (minimum of 3A of current). Do not leave SHDN floating. For a typical application where shutdown is used only to set the switching frequency, connect SHDN to the input (V IN = 3.3V) with a 120k resistor for a 1MHz switching frequency. The bias current into SHDN, programmed with an external resistor, determines the oscillator frequency (see Typical Operating Characteristics). To select the frequency, calculate the external resistor value, RFREQ, using the following formula: RFREQ = 45.5 (MHz / mA) (VON - 0.7V) / fOSC where RFREQ is in k and fOSC is in MHz. Program the frequency in the 200kHz to 2MHz range. This frequency range corresponds to SHDN input currents between 3A to 65A. Proper operation of the oscillator is not guaranteed beyond these limits. Forcing SHDN below 400mV disables the device.
Soft-Start
For the MAX1747, soft-start is achieved by controlling the rise rate of the output voltage, regardless of output capacitance or output load, and limited only by the output impedance of the regulator (see Startup Waveforms
10
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Triple Charge-Pump TFT LCD DC-DC Converter MAX1747
MAX1747
OSC DRVP SUPP VSUPP = 2.7V TO 13V D1 OSC D2 SHDN VPOS GND R4 PGND C4 RFREQ = kFREQ (VON - 0.7V)/fOSC RFREQ IS IN k, kFREQ IS 45.5MHz/mA, AND fOSC IS IN MHz. VON = VIN
MAX1747
IN CIN RFREQ
C3
FBP VREF 1.25V GND
R3
VPOS = [1 + (R3/R4)] VREF VREF = 1.25V
Figure 4. Positive Charge-Pump Block Diagram
Figure 5. Frequency Adjustment
in the Typical Operating Characteristics). The main output voltage is controlled to be in regulation within 4096 clock cycles (1/fOSC). The negative and positive lowpower charge pumps are controlled to be in regulation within 2048 clock cycles.
Power-Up Sequencing
Upon power-up or exiting shutdown, the MAX1747 starts a power-up sequence. First, the reference powers up. Then the primary charge pump powers up with soft-start enabled. Once the main charge pump reaches 90% of its nominal value (VFB > 1.125V), the negative charge pump turns on. When the negative output voltage reaches approximately 90% of its nominal value (VFBN < 125mV), the positive charge pump starts up. Finally, when the positive output voltage reaches 90% of its nominal value (VFBP > 1.125V), the active-low ready signal (RDY) goes low (see Power Ready).
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 Power-Up Sequencing).
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 RON. 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 logic level output.
Voltage Reference
The voltage at REF is nominally 1.25V. The reference can source up to 50mA with excellent load regulation (see Typical Operating Characteristics). Connect a 0.22F bypass capacitor between REF and GND. During shutdown, the reference is disabled.
Fault Detection
Once RDY is low, and if any output falls below its fault detection threshold, RDY goes high impedance. For the reference, the fault threshold is 1.05V. For the main charge pump, 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 < 1.1V). 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 faultdetection threshold, the main charge pump remains
Design Procedure
Efficiency Considerations
The efficiency characteristics of the MAX1747 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). Therefore, the maximum efficiency may be approximated by: Efficiency VOUT / (2 VSUPM) for the main charge pump Efficiency - VNEG / (VSUPN N) for the negative low-power charge pump
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Triple Charge-Pump TFT LCD DC-DC Converter MAX1747
Efficiency VPOS / [VSUPP (N+1)] for the positive low-power charge pump where N is the number of charge-pump stages. For the low-power charge pumps, the output capacitor should be anywhere from 5-times to 20-times larger than the flying capacitor, depending on the ripple tolerance. Increasing the output capacitance or decreasing the ESR reduces the output ripple voltage and the peak-to-peak transient voltage.
Output Voltage Selection
Adjust the main output voltage by connecting a voltage-divider from the output (VOUT) to FB and GND (see Typical Operating Circuit). Adjust the negative lowpower output voltage by connecting a voltage-divider from the output (VNEG) to FBN to REF. Adjust the positive low-power output voltage by connecting a voltagedivider from the output (VPOS) to FBP to GND. Select R2, R4, and R6 in the 10k to 200k range. Calculate the remaining resistors with the following equations: R1 = R2 [(VOUT / VREF) - 1] R3 = R4 [(VPOS / VREF) - 1] R5 = R6 |VNEG / VREF| where VREF = 1.25V. VOUT may range from 4.5V to 5.5V, VPOS may range from VSUPP to +35V, and VNEG may range from 0 to -35V.
Input Capacitors
Using an input capacitor with a value equal to or greater than the output capacitor is recommended. Place the capacitor as close to the IC as possible. If the source impedance or inductance of the input supply is large, additional input bypassing may be required. For the low-power charge-pump inputs (SUPN and SUPP), using bypass capacitors with values equal to or greater than the flying capacitors is recommended. Place these capacitors as close to the supply voltage inputs as possible.
Rectifier Diodes
Use Schottky diodes with a current rating greater than 4 times the average output current, and with a voltage rating of 1.5 times VSUPP for the positive charge pump and VSUPN for the negative charge pump.
Flying Capacitors
Increasing the flying capacitor's value increases the output-current capability. Above a certain point, larger capacitor values lower the secondary pole formed by the transfer capacitor and switch RON, which destabilizes the output. For the main charge pump, use a ceramic capacitor based on the following equation: CX 0.47F x MHz fOSC
Integrator Capacitor
The MAX1747 contains an internal current integrator that improves the DC load regulation but increases the peak-to-peak transient voltage (see Load-Transient Waveform in the Typical Operating Characteristics). Connect a ceramic capacitor between INTG and GND based on the following equation: CINTG 150Hz x COUT fOSC
For the low-power charge pumps, a 0.1F ceramic capacitor works well in most applications. Smaller values may be used for lower current applications. Component suppliers are listed in Table 1.
Table 1. Component Suppliers
SUPPLIER CAPACITORS AVX Kemet Sanyo Taiyo Yuden DIODES Central International Rectifier Motorola Nihon 516-435-1110 310-322-3331 602-303-5454 847-843-7500 516-435-1824 310-322-3332 602-994-6430 847-843-2798 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 FAX
Output Capacitors
For the main charge pump, use a ceramic capacitor based on the following equation: 20 2F x MHz COUT x CX x fOSC AND fOSC MHz For low-frequency applications (close to 200kHz), selection of the output capacitor is limited solely by the switching frequency. However, for high-frequency applications (close to 2MHz), selection of the output capacitor is limited by the secondary pole formed by the flying capacitor and switch on-resistance.
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Triple Charge-Pump TFT LCD DC-DC Converter
PC Board Layout and Grounding
Careful printed circuit layout is important to minimize ground bounce and noise. First, place the main chargepump flying capacitor less than 0.2in (5mm) from the CXP and CXN 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 charge-pump 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 analog and power ground areas connected at only one point under the IC. To maximize output power and efficiency, and 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 MAX1747 evaluation kit for an example of proper board layout.
MAX1747
Chip Information
TRANSISTOR COUNT: 2534
Package Information
TSSOP,NO PADS.EPS
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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