19-1795; Rev 0; 11/00 Low-Power Triple-Output TFT LCD DC-DC Converter 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 (1Ω) 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 ♦ Three Integrated DC-DC Converters ♦ 250kHz Current-Mode PWM Boost Regulator Up to +13V Main High-Power Output ±1% Accuracy High Efficiency (91%) ♦ Dual Charge-Pump Outputs Up to +40V Positive Charge-Pump Output Down to -40V Negative Charge-Pump Output ♦ Internal Supply Sequencing ♦ Internal Power MOSFETs ♦ +2.7V to +5.5V Input Supply ♦ 0.1µA Shutdown Current ♦ 0.5mA Quiescent Current ♦ Internal Soft-Start ♦ Power-Ready Output ♦ Ultra-Small External Components ♦ Thin TSSOP Package (1.1mm max) Ordering Information PART MAX1779EUE TEMP. RANGE PIN-PACKAGE -40°C to +85°C 16 TSSOP Pin Configuration ________________________Applications TFT Active-Matrix LCD Displays Passive-Matrix LCD Displays TOP VIEW 16 TGND RDY 1 FB 2 PDAs Digital-Still Cameras INTG 3 IN 4 Camcorders 15 LX 14 PGND MAX1779 13 SUPP GND 5 12 DRVP REF 6 11 SUPN FBP 7 10 DRVN FBN 8 9 SHDN TSSOP Typical Operating Circuit appears at end of data sheet. ________________________________________________________________ Maxim Integrated Products 1 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. MAX1779 General Description MAX1779 Low-Power Triple-Output TFT LCD DC-DC Converter 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 = +70°C) 16-Pin TSSOP (derate 9.4mW/°C above +70°C) ..........755mW Operating Temperature Range MAX1779EUE ..................................................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = +10V, TGND = PGND = GND, CREF = 0.22µF, CINTG = 2200pF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Input Supply Range SYMBOL CONDITIONS VIN Input Undervoltage Threshold VUVLO IN Quiescent Supply Current IIN MIN TYP 2.7 VIN rising, 40mV hysteresis (typ) 2.2 MAX UNITS 5.5 V 2.4 2.6 V VFB = VFBP = +1.5V, VFBN = -0.2V 0.5 1 mA SUPP Quiescent Current ISUPP VFBP = +1.5V 0.25 0.55 mA SUPN Quiescent Current ISUPN VFBN = -0.1V 0.25 0.55 mA V SHDN = 0, VIN = +5V 0.1 10 µA IN Shutdown Current SUPP Shutdown Current V SHDN = 0, VSUPP = +13V 0.1 10 µA SUPN Shutdown Current V SHDN = 0, VSUPN = +13V 0.1 10 µA 13 V 1.248 1.261 V 50 nA 212 250 288 kHz 79 85 92 % MAIN BOOST CONVERTER Output Voltage Range VMAIN VIN FB Regulation Voltage VFB 1.235 FB Input Bias Current IFB Operating Frequency fOSC VFB = +1.25V, INTG = GND Oscillator Maximum Duty Cycle Load Regulation -50 0.1 % Line Regulation IMAIN = 0 to 50mA, VMAIN = +5V 0.1 %/V Integrator Gm 320 µs RLX(ON) ILX = 100mA 1.0 2.0 Ω LX Leakage Current ILX VLX = +13V 0.01 20 µA LX Current Limit ILIM 450 650 mA LX Switch On-Resistance 350 Maximum RMS LX Current 250 FB Fault Trip Level POSITIVE CHARGE PUMP VSUPP Input Supply Range 2 Falling edge VSUPP 1.07 1.1 2.7 _______________________________________________________________________________________ mA 1.14 V 13 V Low-Power Triple-Output TFT LCD DC-DC Converter (VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = +10V, TGND = PGND = GND, CREF = 0.22µF, CINTG = 2200pF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN MAX 0.5 × fOSC Operating Frequency FBP Regulation Voltage FBP Input Bias Current DRVP PCH On-Resistance TYP VFBP IFBP VFBP = +1.5V 1.20 -50 VFBP = +1.200V DRVP NCH On-Resistance VFBP = +1.300V FBP Power-Ready Trip Level Rising edge FBP Fault Trip Level Falling edge Hz 1.25 1.30 50 V nA 3 10 Ω 1.5 5 20 1.09 Maximum RMS DRVP Current UNITS Ω kΩ 1.13 1.16 V 1.11 V 0.1 A NEGATIVE CHARGE PUMP VSUPN Input Supply Range VSUPN 2.7 Operating Frequency FBN Regulation Voltage FBN Input Bias Current DRVN PCH On-Resistance 13 0.5 × fOSC VFBN IFBN VFBN = -0.05V -50 -50 DRVN NCH On-Resistance VFBN = +0.050V VFBN = -0.050V 20 FBN Power-Ready Trip Level Falling edge 80 FBN Fault Trip Level Rising edge Maximum RMS DRVN Current V Hz 0 50 50 mV nA 3 1.5 10 5 Ω Ω kΩ 120 165 mV 140 mV 0.1 A REFERENCE Reference Voltage VREF Reference Undervoltage Threshold LOGIC SIGNALS -2µA < IREF < 50µA VREF rising SHDN Input Low Voltage 1.25 1.269 V 0.9 1.05 1.2 V 0.25V hysteresis (typ) SHDN Input High Voltage SHDN Input Current 1.231 0.9 2.1 I SHDN V V µA 0.01 1 RDY Output Low Voltage ISINK = 2mA 0.25 0.5 V RDY Output High Voltage V RDY = +13V 0.01 1 µA _______________________________________________________________________________________ 3 MAX1779 ELECTRICAL CHARACTERISTICS (continued) MAX1779 Low-Power Triple-Output TFT LCD DC-DC Converter ELECTRICAL CHARACTERISTICS (VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = +10V, TGND = PGND = GND, CREF = 0.22µF, CINTG = 2200pF, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER Input Supply Range SYMBOL CONDITIONS MIN MAX UNITS 2.7 5.5 V 2.2 2.6 V 1 mA VFBP = +1.5V 0.55 mA VFBN = -0.1V 0.55 mA VIN Input Undervoltage Threshold VUVLO IN Quiescent Supply Current IIN SUPP Quiescent Current ISUPP SUPN Quiescent Current ISUPN VIN rising, 40mV hysteresis (typ) VFB = VFBP = +1.5V, VFBN = -0.2V IN Shutdown Current V SHDN = 0, VIN = +5V 10 µA SUPP Shutdown Current V SHDN = 0, VSUPP = +13V 10 µA SUPN Shutdown Current V SHDN = 0, VSUPN = +13V 10 µA VIN 13 V 1.225 1.271 V -50 50 nA 195 305 kHz 79 92 % MAIN BOOST CONVERTER Output Voltage Range VMAIN FB Regulation Voltage VFB FB Input Bias Current IFB Operating Frequency fOSC VFB = +1.25V, INTG = GND Oscillator Maximum Duty Cycle LX Switch On-Resistance LX Leakage Current LX Current Limit RLX(ON) ILX ILX = 100mA 2.0 Ω VLX = +13V 20 µA 350 700 mA 1.07 1.14 V 2.7 13 V 1.20 1.30 V -50 50 nA 10 Ω ILIM FB Fault Trip Level Falling edge POSITIVE CHARGE PUMP SUPP Input Supply Range VSUPP FBP Regulation Voltage VFBP FBP Input Bias Current IFBP VFBP = +1.5V DRVP PCH On-Resistance VFBP = +1.200V DRVP NCH On-Resistance VFBP = +1.300V FBP Power-Ready Trip Level Rising edge 5 20 Ω kΩ 1.09 1.16 V NEGATIVE CHARGE PUMP SUPN Input Supply Range VSUPN 2.7 13 V FBN Regulation Voltage VFBN -50 50 mV FBN Input Bias Current DRVN PCH On-Resistance IFBN 50 nA VFBN = -0.05V -50 VFBN = +0.050V DRVN NCH On-Resistance FBN Power-Ready Trip Level 10 Ω 5 Ω VFBN = -0.050V 20 Falling edge 80 165 mV 1.223 1.269 V 0.9 1.2 V kΩ REFERENCE Reference Voltage Reference Undervoltage 4 VREF -2µA < IREF < 50µA VREF rising _______________________________________________________________________________________ Low-Power Triple-Output TFT LCD DC-DC Converter (VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = +10V, TGND = PGND = GND, CREF = 0.22µF, CINTG = 2200pF, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS 0.9 V 1 µA LOGIC SIGNALS SHDN Input Low Voltage 0.25V hysteresis (typ) SHDN Input High Voltage 2.1 SHDN Input Current V I SHDN RDY Output Low Voltage ISINK = 2mA 0.5 V RDY Output High Leakage V RDY = +13V 1 µA Note 1: Specifications to -40°C are guaranteed by design, not production tested. Typical Operating Characteristics (Circuit of Figure 5, VIN = +3.3V, TA = +25°C, unless otherwise noted.) MAIN STEP-UP CONVERTER EFFICIENCY vs. LOAD CURRENT (L = 10µH, 5V OUTPUT) 5.01 VIN = +4.2V 5.00 4.99 VIN = +3.0V VIN = +3.0V 80 VMAIN (V) EFFICIENCY (%) VIN = +3.0V 70 FIGURE 6 4.98 50 100 150 200 FIGURE 5 4.98 0 50 100 150 200 0 50 100 150 200 250 300 IMAIN (mA) IMAIN (mA) IMAIN (mA) MAIN STEP-UP CONVERTER EFFICIENCY vs. LOAD CURRENT (L = 33µH, 5V OUTPUT) MAIN OUTPUT VOLTAGE vs. LOAD CURRENT (L = 33µH, 10V OUTPUT) MAIN STEP-UP CONVERTER EFFICIENCY vs. LOAD CURRENT (L = 33µH, 10V OUTPUT) 10.02 80 VMAIN (V) VIN = +3.0V VIN = +3.3V VIN = +5.0V 10.00 70 9.98 FIGURE 5 0 50 100 150 IMAIN (mA) 200 250 300 MAX1779-06 VIN = +5.5V 90 VIN = +3.3V 80 70 60 60 50 100 MAX1779-05 VIN = +4.2V 90 10.04 MAX1779-04 100 EFFICIENCY (%) FIGURE 6 50 0 VIN = +4.2V 5.00 4.99 60 EFFICIENCY (%) VMAIN (V) VIN = +4.2V 90 5.01 5.02 MAX1779-02 100 MAX1779-01 5.02 MAIN OUTPUT VOLTAGE vs. LOAD CURRENT (L = 33µH, 5V OUTPUT) MAX1779-03 MAIN OUTPUT VOLTAGE vs. LOAD CURRENT (L = 10µH, 5V OUTPUT) FIGURE 5 9.96 0 50 100 IMAIN (mA) 150 FIGURE 5 50 0 50 100 150 IMAIN (mA) _______________________________________________________________________________________ 5 MAX1779 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (continued) (Circuit of Figure 5, VIN = +3.3V, TA = +25°C, unless otherwise noted.) EFFICIENCY vs. LOAD CURRENT (BOOST CONVERTER AND CHARGE PUMPS) -7.88 -7.92 -7.96 VSUPN = +7V 60 50 100 150 200 40 -8.08 30 VNEG = -8V 0 250 5 10 POSITIVE CHARGE-PUMP OUTPUT VOLTAGE vs. LOAD CURRENT 15 20 INEG (mA) 10 INEG (mA) POSITIVE CHARGE-PUMP EFFICIENCY vs. LOAD CURRENT SWITCHING FREQUENCY vs. INPUT VOLTAGE IMAIN (mA) 0 90 12.12 VSUPP = +5V 12.00 VPOS (V) VPOS (V) 80 VSUPP = +7V 11.88 70 VSUPP = +6V 60 VSUPP = +7V 50 11.76 VSUPP = +6V VSUPP = +5V VPOS = +12V 30 5 10 15 20 25 15 280 260 240 220 0 30 5 10 15 20 25 200 2.5 30 3.0 3.5 4.0 4.5 IPOS (mA) IPOS (mA) INPUT VOLTAGE (V) REFERENCE VOLTAGE vs. REFERENCE LOAD CURRENT RIPPLE WAVEFORMS LOAD TRANSIENT (L = 10µH, 500µs PULSE) MAX1779-14 1.254 5V A 5.0V -8V B 4.9V 5.0 MAX1779-15 5.1V MAX1779-13 1.256 20 40 11.64 0 5 300 MAX1779-11 100 MAX1779-10 12.24 VSUPN = +7V MAX1779-12 0 VSUPN = +6V 60 -8.04 SWITCHING FREQUENCY (kHz) 50 70 50 -8.00 VNEG = -8V, INEG = 1mA VPOS = +12V, IPOS = 1mA VSUPN = +5V 80 EFFICIENCY (%) VMAIN = +10V SINGLE-STAGE CHARGE PUMPS 70 90 -7.84 VSUPN = +6V 80 100 MAX1779-09 VSUPN = +5V -7.80 VNEG (V) EFFICIENCY (%) 90 -7.76 MAX1779-08 VMAIN = +5V TWO-STAGE CHARGE PUMPS NEGATIVE CHARGE-PUMP EFFICIENCY vs. LOAD CURRENT NEGATIVE CHARGE-PUMP OUTPUT VOLTAGE vs. LOAD CURRENT MAX1779-07 100 A 1.252 VREF (V) MAX1779 Low-Power Triple-Output TFT LCD DC-DC Converter 1.250 50mA 1.248 C 12V 1.246 B 0 1.244 0 10 20 30 IREF (µA) 6 40 50 4.0µs/div A. VMAIN = 5V, IMAIN = 100mA, 10mV/div B. VNEG = -8V, INEG = 1mA, 5mV/div C. VPOS = 12V, IPOS = 1mA, 5mV/div, FIGURE 5 100µs/div A. VMAIN = 5V, 50mV/div B. VMAIN = 5mA to 50mA, 25mA/div FIGURE 6 _______________________________________________________________________________________ Low-Power Triple-Output TFT LCD DC-DC Converter LOAD TRANSIENT WITHOUT INTEGRATOR (L = 10µH, 5µs PULSE) LOAD TRANSIENT WITHOUT INTEGRATOR (L = 10µH, 500µs PULSE) LOAD TRANSIENT (L = 33µH, 500µs PULSE) MAX1779-17 MAX1779-16 5.0V 5.0V A 5V A A 400mA 4.9V 200mA B 0 50mA B 4.9V 100mA 100mA B C 0 0 0 10µs/div A. VMAIN = 5V, 100mV/div B. IL, 200mA/div C. IMAIN = 10mA to 100mA, 100mA/div INTG = REF, FIGURE 6 100µs/div A. VMAIN = 5V, 50mV/div B. VMAIN = 5mA to 50mA, 25mA/div INTG = REF, FIGURE 6 100µs/div A. VMAIN = 5V, 50mV/div B. IMAIN = 10mA to 100mA, 50mA/div FIGURE 5 LOAD TRANSIENT (L = 33µH, 5µs PULSE) LOAD TRANSIENT WITHOUT INTEGRATOR (L = 33µH, 500µs PULSE) 5.1V MAX1779-18 5.1V MAX1779-19 5.1V STARTUP WAVEFORM (L = 10µH) MAX1779-20 MAX1779-21 2V 5.0V A 5.0V A A 0 5V 4.9V 4.9V 100mA 200mA B 3V B B 0 0 100µs/div A. VMAIN = 5V, 50mV/div B. IMAIN = 10mA to 100mA, 50mA/div INTG = REF, FIGURE 5 500mA C 0 10µs/div A. VMAIN = 5V, 50mV/div B. IMAIN = 20mA to 200mA, 100mA/div FIGURE 5 200µs/div A. VSHDN = 0 to 2V, 2V/div B. VMAIN = 5V, 1V/div C. IL, 500 mA/div FIGURE 6, RMAIN = 100Ω _______________________________________________________________________________________ 7 MAX1779 Typical Operating Characteristics (continued) (Circuit of Figure 5, VIN = +3.3V, TA = +25°C, unless otherwise noted.) MAX1779 Low-Power Triple-Output TFT LCD DC-DC Converter Typical Operating Characteristics (continued) (Circuit of Figure 5, VIN = +3.3V, TA = +25°C, unless otherwise noted.) STARTUP WAVEFORM (L = 33µH) POWER-UP SEQUENCING MAX1779-23 MAX1779-22 2V A 2V A 0 0 5V 5V B B 3V 0 500mA -10V C 0 C 10V D 0 200µs/div A. VSHDN = 0 to 2V, 2V/div B. VMAIN = 5V, 1V/div C. IL, 500mA/div RMAIN = 50Ω 4ms/div A. VSHDN = 0 to 2V, 2V/div B. VMAIN = 5V, RMAIN = 50Ω, 2.5V/div C. VNEG = -8V, RNEG = 8kΩ, 10V/div D. VPOS = +12V, RPOS = 12kΩ, 10V/div Pin Description 8 PIN NAME FUNCTION 1 RDY Active-Low Open-Drain Output. Indicates all outputs are ready. The on-resistance is 125Ω (typ). 2 FB Main Boost Regulator Feedback Input. Regulates to 1.25V nominal. Connect feedback resistive divider to analog ground (GND). 3 INTG 4 IN 5 GND Analog Ground. Connect to power ground (PGND) underneath the IC. 6 REF Internal Reference Bypass Terminal. Connect a 0.22µF capacitor from this terminal to analog ground (GND). External load capability to 50µA. 7 FBP Positive Charge-Pump Regulator Feedback Input. Regulates to 1.25V nominal. Connect feedback resistive divider to analog ground (GND). 8 FBN Negative Charge-Pump Regulator Feedback Input. Regulates to 0V nominal. 9 SHDN 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 0.1µF capacitor between IN and GND, as close to the pins as possible. Active-Low Logic-Level Shutdown Input. Connect SHDN to IN for normal operation. _______________________________________________________________________________________ Low-Power Triple-Output TFT LCD DC-DC Converter PIN NAME FUNCTION 10 DRVN Negative Charge-Pump Driver Output. Output high level is VSUPN, and low level is PGND. 11 SUPN Negative Charge-Pump Driver Supply Voltage. Bypass to PGND with a 0.1µF capacitor. 12 DRVP Positive Charge-Pump Driver Output. Output high level is VSUPP, and low level is PGND. 13 SUPP Positive Charge-Pump Driver Supply Voltage. Bypass to PGND with a 0.1µF capacitor. 14 PGND Power Ground. Connect to GND underneath the IC. 15 LX 16 TGND 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. 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 50µA, logic shutdown, soft-start, power-up sequencing, fault detection, and an active-low open-drain ready output. 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 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 MAX1779 Pin Description (continued) MAX1779 Low-Power Triple-Output TFT LCD DC-DC Converter L1 VOUT = [1 + (R1 / R2)] x VREF VREF = 1.25V VIN = 2.7V TO 5.5V IN OSC LX S R VMAIN (UP TO 13V) D1 Q R1 + + + - PGND C1 ILIM Σ - SLOPE COMP + RCOMP FB + + Gm INTG CINTG REF + MAX1779 GND R2 C2 CCOMP 1.25V Figure 1. PWM Boost Converter Block Diagram (VC3 + VSUPP), charge flows from C3 to C4 until the diode (D3) turns off. Soft-Start 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). Shutdown A logic-low level on SHDN disables all three MAX1779 converters and the reference. When shut down, the supply current drops to 0.1µA to maximize battery life and the reference is pulled to ground. The output 10 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 > ______________________________________________________________________________________ Low-Power Triple-Output TFT LCD DC-DC Converter MAX1779 VSUPN = 2.7V TO 13V SUPN OSC D4 C5 DRVN D5 R5 FBN + VNEG C6 + R6 VREF 1.25V REF MAX1779 CREF 0.22µF PGND GND ( ) VNEG = - R5 VREF R6 VREF = 1.25V Figure 2. Negative Charge-Pump Block Diagram VSUPP = 2.7V TO 13V SUPP OSC D2 C3 DRVP D3 R3 FBP + VPOS C4 + - R4 VREF 1.25V MAX1779 GND PGND ( )V VPOS = 1 + R3 R4 VREF = 1.25V REF Figure 3. Positive Charge-Pump Block Diagram ______________________________________________________________________________________ 11 MAX1779 Low-Power Triple-Output TFT LCD DC-DC Converter 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 125Ω on-resistance. If a fault is detected, the internal open-drain MOSFET appears as a high impedance. Connect a 100kΩ 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 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). Voltage Reference The voltage at REF is nominally 1.25V. The reference can source up to 50µA with good load regulation (see Typical Operating Characteristics). Connect a 0.22µF bypass capacitor between REF and GND. 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 10µH and 33µH work best with the controller’s switching frequency. The inductor value depends on the maximum output load the application must support, input voltage, and 12 IMAIN(MAX) ≥ 1 VIN(MIN) ILIM(MIN) 2 VMAIN then VMAIN - VIN(MIN) 2 V 1 1 IN(MIN) VIN(MIN) L≥ I -I 2 ƒ VMAIN VMAIN LIM(MIN) MAIN(MAX) B. Discontinuous-conduction: if IMAIN(MAX) < 1 VIN(MIN) ILIM(MIN) 2 VMAIN then ( 1 IMAIN(MAX) VMAIN - VIN(MIN) L ≥ 2 ILIM(MIN)2 ƒ ) where I LIM(MIN) = 350mA and ƒ = 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: 2 I × VMAIN PLR ≅ MAIN × RL VIN ______________________________________________________________________________________ Low-Power Triple-Output TFT LCD DC-DC Converter 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: COUT > 60 × L × IMAIN(MAX) VMAIN × VIN(MIN) 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 10µF 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 10µF 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 100kΩ resistor to GND. 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)/10kΩ. 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. 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 50kΩ to 100kΩ 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 domi- ______________________________________________________________________________________ 13 MAX1779 where RL is the inductor’s series resistance. For best performance, select inductors with resistance less than the internal N-channel MOSFET on-resistance (1Ω typ). MAX1779 Low-Power Triple-Output TFT LCD DC-DC Converter nated by the internal switch resistance and the diode impedance. Start with 0.1µF 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. 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 0.1µF 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. 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. Table 1. Component Suppliers SUPPLIER INDUCTORS PHONE FAX Coilcraft 847-639-6400 847-639-1469 Coiltronics 561-241-7876 561-241-9339 Sumida USA 847-956-0666 847-956-0702 Toko 847-297-0070 847-699-1194 CAPACITORS AVX 803-946-0690 803-626-3123 Kemet 408-986-0424 408-986-1442 Sanyo 619-661-6835 619-661-1055 Taiyo Yuden 408-573-4150 408-573-4159 Central Semiconductor 516-435-1110 516-435-1824 International Rectifier 310-322-3331 310-322-3332 Motorola 602-303-5454 602-994-6430 LX Charge Pump Nihon 847-843-7500 847-843-2798 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 maxi- Zetex 516-543-7100 516-864-7630 Refer to the MAX1779 evaluation kit for an example of proper board layout. Applications Information 14 DIODES Chip Information TRANSISTOR COUNT: 2846 ______________________________________________________________________________________ ______________________________________________________________________________________ 1.0µF (2) 4.7µF VNEG = -8V, 1mA VIN = +3.0V CINTG 3300pF 100k R5 320k CREF 0.22µF R6 49.9k 0.1µF 0.1µF GND TGND INTG REF FBN DRVN RDY SHDN IN MAX1779 10µH PGND FBP DRVP SUPP SUPN FB LX RCOMP 10k R4 49.9k 0.1µF R2 50k R1 150k R3 549k CCOMP 220pF COUT (2) 4.7µF VMAIN = +5V 1.0µF 1.0µF VPOS = +15V, 1mA MAX1779 0.47µF Low-Power Triple-Output TFT LCD DC-DC Converter Figure 4. Minimizing the Number of Charge-Pump Stages 15 16 VNEG -8V, 5mA C6 0.47µF C10 2.2µF CIN 10µF VIN = +3.3V CREF 0.22µF R5 320k R6 49.9k C9 0.22µF CINTG 2200pF C11 0.1µF C5 0.1µF RRDY 100k PGND REF FBN DRVN INTG RDY SHDN IN MAX1779 33µH GND TGND FBP DRVP SUPP SUPN FB LX C7 0.22µF C3 0.1µF R2 50k R1 150k CCOMP 470pF RCOMP 24k R4 49.9k C8 430k C3 0.47µF C8 2.2µF VPOS +12V, 5mA COUT 22µF VMAIN = +5.0V MAX1779 Low-Power Triple-Output TFT LCD DC-DC Converter Figure 5. Typical Operating Circuit (L = 33µH) ______________________________________________________________________________________ ______________________________________________________________________________________ VNEG -8V, 5mA C6 0.47µF CIN (2) 4.7µF CREF 0.22µF C10 2.2µF R5 320k R6 49.9k C9 0.22µF CINTG 2200pF C11 0.1µF C5 0.1µF RRDY 100k PGND REF FBN DRVN INTG RDY SHDN IN MAX1779 10µH GND TGND FBP DRVP SUPP SUPN FB LX C7 0.22µF C3 0.1µF R2 50k R1 150k CCOMP 220pF RCOMP 10k R4 49.9k R3 430k C4 0.47µF C8 2.2µF VPOS +12V, 5mA COUT (2) 4.7µF VMAIN = +5.0V MAX1779 VIN = +3.3V Low-Power Triple-Output TFT LCD DC-DC Converter Figure 6. Typical Operating Circuit (L = 10µH) 17 Low-Power Triple-Output TFT LCD DC-DC Converter TSSOP.EPS MAX1779 Package Information 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 © 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.