MAX1997 datasheet(21/31 Pages) MAXIM

21 / 31 page

Quintuple/Triple-Output TFT LCD Power Supplies

with Fault Protection and VCOM Buffer

______________________________________________________________________________________

The maximum inductor current, input voltage, output

voltage, and switching frequency determine the induc-

tor value. To ensure an adequate inductor current-

sense signal in the IC, always calculate the inductor

value with the maximum guaranteed inductor current

even though the actual operating current may be much

lower. For the MAX1997/MAX1998, the maximum guar-

anteed inductor current is the minimum value of the

internal LX current limit (1.6A, see the Electrical

Characteristics). The equations provided here include a

constant defined as LIR, which is the ratio of the peak-

to-peak inductor current ripple to the average DC

inductor current. For a good compromise between the

size of the inductor, power loss, and output voltage rip-

ple, select an LIR of 0.3 to 0.5. The inductance value is

then given by:

where fOSC is the oscillator frequency (see Elec-

trical Characteristics), and IL(MAX) is 1.6A. Considering

the typical application circuit, the typical input voltage

is 3.3V, the main output voltage is 9V, and the switching

frequency is 1.5MHz. Based on the above equations,

the inductance value is 4.3µH for an LIR of 0.2. The

inductance value is 1.7µH for an LIR of 0.5. The induc-

tance in the standard application circuit is chosen to be

3.3µH.

The inductor’s peak current rating should be higher

than the expected peak inductor current throughout the

normal operating range. The expected peak inductor

current is given by:

where

η is the efficiency of the regulator. For most

applications, the efficiency is between 75% and 85%.

Under fault conditions, the inductor current may reach

the internal LX current limit (see Electrical Character-

istics). However, soft saturation inductors and the con-

troller’s fast current-limit circuitry protect the device

from failure during such a fault condition.

The inductor’s DC resistance can significantly affect

efficiency due to the resistive power loss (PLR), which

can be approximated by the following equation:

where ILAVG is the average inductor current and RL is

the inductor’s series resistance. For best performance,

select inductors with resistance less than the internal

N-channel MOSFET’s on-resistance (0.25

Ω typ). To

minimize radiated noise in sensitive applications, use a

shielded inductor.

LAVG

MAIN

=≅









V- V

PEAK

MAIN(MAX) MAIN

IN(MIN)

MAIN

IN(MIN)

OSC













































V- V

LIR

IN(TYP)

MAIN

IN(TYP)

L(MAX) OSC





























Figure 8. Startup and Fault Protection Logic

SHDN

ONDC

GATE

ENABLES REG 1 LINEAR-REGULATOR

ENABLES REG 2 LINEAR-REGULATOR

ENABLES STEP-UP REGULATOR

ENABLES REG P LINEAR-REGULATOR

ENABLES REG N LINEAR-REGULATOR

UVLO

REFERENCE READY

THERMAL FAULT

OVERCURRENT FAULT

REG 2 FAULT

REG 1 FAULT

REG P FAULT

STEP-UP REGULATOR FAULT

REG N FAULT

fOSC/128

FREQ

CLK

GATE

READY

STEP-UP REGULATOR

SOFT-START DONE

VCT > VON2

VCT > VONP

VCT > VONN

RIPPLE

COUNTER

PFLT

FAULT TIMER

FAULT

LATCH

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