MAX1997 datasheet(16/31 Pages) MAXIM

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Quintuple/Triple-Output TFT LCD Power Supplies

with Fault Protection and VCOM Buffer

______________________________________________________________________________________

In addition, all outputs are monitored for fault conditions

that last longer than the fault timer period. The device

goes into a latched shutdown state, if the junction tem-

perature of the device exceeds +160

°C.

Main Step-Up Controller

The main step-up regulator switches at up to 1.5MHz,

and employs a current-mode control architecture to

maximize loop bandwidth and provide fast transient

response to pulsed loads found in source drivers for

TFT LCD panels. In addition, the high switching fre-

quency allows the use of low-profile inductors and

ceramic capacitors to minimize the thickness of LCD

panel designs. The integrated high-efficiency MOSFET

reduces the number of external components. The IC’s

built-in soft-start function controls the inrush current.

Depending on the input-to-output voltage ratio, the reg-

ulator controls the output voltage and the power deliv-

ered to the output by modulating the duty cycle (D) of

the power MOSFET in each switching cycle.

The duty cycle of the MOSFET is approximated by:

On the rising edge of the internal clock, the controller sets

a flip-flop, which turns on the N-channel MOSFET (Figure

3). The input voltage is applied across the inductor. The

inductor current ramps up linearly, storing energy in a

magnetic field. Once the sum of the feedback voltage,

slope-compensation, and current-feedback signals trip

the multi-input PWM comparator, the MOSFET turns off,

and the flip-flop resets. Since the inductor current is con-

tinuous, a transverse potential develops across the induc-

tor that turns on the diode (D1). The voltage across the

inductor becomes the difference between the output volt-

age and the input voltage. This discharge condition

forces the current through the inductor to ramp back

down, transferring the energy stored in the magnetic field

to the output capacitor and the load. The MOSFET

remains off for the rest of the clock cycle.

V- V

MAIN

≈

Figure 3. Main Step-Up Converter Functional Diagram

TO FAULT

LOGIC

1.0V

REFOUT

REFIN

CLK

∑

PGND

SOFT-START

RESET DOMINANT

ILIM COMPARATOR

FROM

OSCILLATOR

CURRENT

SENSE

SLOPE_COMP

VLIMIT

REF

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