MAX188 datasheet(14/32 Pages) MAXIM

14 / 32 page

Triple-Output TFT LCD Power Supply

with Fault Protection

______________________________________________________________________________________

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 feed-

back voltage error-amplifier output, slope-compensa-

tion, and current-feedback signals trip the multi-input

PWM comparator, the MOSFET turns off, and the flip-

flop resets. Since the inductor current is continuous, a

transverse potential develops across the inductor that

turns on the diode (D1). The voltage across the inductor

becomes the difference between the output voltage and

the input voltage. This discharge condition forces the

current through the inductor to ramp back down, trans-

ferring the energy to the output capacitor and the load.

The MOSFET remains off for the rest of the clock cycle.

MAIN

≈

ILIM

CURRENT

SENSE

OSC

ILIM

COMPARATOR

RESET DOMINANT

SSDONE

SOFT-START

REFOUT

SSOK

REFIN

CLK

ONMN

REF

SLOPE_COMP

PGND

FAULT M

MAX1889

Figure 3. Main Step-Up Regulator Functional Diagram

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