Triple Charge-Pump TFT LCD

DC-DC Converter

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11

in the Typical Operating Characteristics). The main out-

put voltage is controlled to be in regulation within 4096

power 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 pow-

ers up. Then the primary charge pump powers up with

soft-start enabled. Once the main charge pump reach-

tive charge pump turns on. When the negative output

voltage reaches approximately 90% of its nominal value

Finally, when the positive output voltage reaches 90%

ready signal (RDY) goes low (see Power Ready).

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

pump, the fault threshold is approximately 88% of its

pump, the fault threshold is 88% of its nominal value

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 nega-

tive charge-pump output voltage falls below the fault-

detection threshold, the main charge pump remains

active while the positive charge pump stops switching

and its output voltage decays, depending on output

capacitance and load. The positive charge-pump out-

put 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 power-

up sequence is properly completed, the MOSFET turns

on and pulls RDY low with a typical 125

is detected, the internal open-drain MOSFET appears

as a high impedance. Connect a 100k

Ω 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

0.22µF bypass capacitor between REF and GND.

During shutdown, the reference is disabled.

Design Procedure

Efficiency Considerations

The efficiency characteristics of the MAX1747 regulat-

ed charge pumps are similar to a linear regulator. They

are dominated by quiescent current at low output cur-

rents and by the input voltage at higher output currents

(see Typical Operating Characteristics). Therefore, the

maximum efficiency may be approximated by:

Efficiency

charge pump

Efficiency

low-power charge pump

OSC

SUPP

DRVP

FBP

PGND

1.25V

MAX1747

GND

C3

D1

D2

R3

R4

C4

Figure 4. Positive Charge-Pump Block Diagram

MAX1747

IN

SHDN

GND

OSC

Figure 5. Frequency Adjustment