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MAX1513 Datasheet(PDF) 24 Page - Maxim Integrated Products |
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MAX1513 Datasheet(HTML) 24 Page - Maxim Integrated Products |
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24 / 28 page  TFT-LCD Power-Supply Controllers 24 ______________________________________________________________________________________ Selecting the Number of Charge-Pump Stages For highest efficiency, always choose the lowest num- ber of charge-pump stages that meet the output volt- age requirement. Figures 13 and 14 show the positive and negative charge-pump output voltages for a given VMAIN for one-, two-, and three-stage charge pumps. The number of positive charge-pump stages is given by: where nPOS is the number of positive charge-pump stages, VGON is the gate-on linear-regulator REG P out- put, VMAIN is the main step-up regulator output, VD is the forward-voltage drop of the charge-pump diode, and VDROPOUT is the dropout margin for the linear reg- ulator. Use VDROPOUT = 0.3V. The number of negative charge-pump stages is given by: where nNEG is the number of negative charge-pump stages, VGOFF is the gate-off linear-regulator REG N output, VMAIN is the main step-up regulator output, VD is the forward-voltage drop of the charge-pump diode, and VDROPOUT is the dropout margin for the linear reg- ulator. Use VDROPOUT = 0.3V. The above equations are derived based on the assumption that the first stage of the positive charge pump is connected to VMAIN and the first stage of the negative charge pump is connected to ground. Sometimes fractional stages are more desirable for bet- ter efficiency. This can be done by connecting the first stage to VIN or another available supply. If the first charge-pump stage is powered from VIN, then the above equations become: Flying Capacitors Increasing the flying capacitor (CX) value lowers the effective source impedance and increases the output- current capability. Increasing the capacitance indefinite- ly has a negligible effect on output-current capability because the switch resistance and the diode impedance place a lower limit on the source impedance. A 0.1µF ceramic capacitor works well in most low-current appli- cations. The flying capacitor’s voltage rating must exceed the following: where n is the stage number in which the flying capaci- tor appears, and VMAIN is the output voltage of the main step-up regulator. Charge-Pump Output Capacitor Increasing the output capacitance or decreasing the ESR reduces the output ripple voltage and the peak-to- peak voltage during load transients. With ceramic Vn V CX MAIN >× n VV V VV n VV V VV POS GON DROPOUT IN MAIN D NEG GOFF DROPOUT IN MAIN D = + × = + × - - -+ - 2 2 n VV VV NEG GOFF DROPOUT MAIN D = + × - - 2 n VV V VV POS GON DROPOUT MAIN MAIN D = + × - - 2 POSITIVE CHARGE-PUMP OUTPUT VOLTAGE vs. VMAIN VMAIN (V) 12 10 8 6 4 10 20 30 40 50 60 0 214 2-STAGE CHARGE-PUMP 3-STAGE CHARGE-PUMP VD = 0.3V TO 1V 1-STAGE CHARGE-PUMP Figure 13. Positive Charge-Pump Output Voltage vs. VMAIN NEGATIVE CHARGE-PUMP OUTPUT VOLTAGE vs. VMAIN VMAIN (V) 12 10 8 6 4 -40 -35 -30 -25 -20 -15 -10 -5 -0 -45 214 1-STAGE CHARGE-PUMP 2-STAGE CHARGE-PUMP 3-STAGE CHARGE-PUMP VD = 0.3V TO 1V Figure 14. Negative Charge-Pump Output Voltage vs. VMAIN |
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