MAX1997 datasheet(24/31 Pages) MAXIM

24 / 31 page

Quintuple/Triple-Output TFT LCD Power Supplies

with Fault Protection and VCOM Buffer

______________________________________________________________________________________

Input P-Channel MOSFET

Select the input P-channel MOSFET based on current

rating, voltage rating, gate threshold, and on-resis-

tance. The MOSFET must be able to handle the peak

input current (see the Inductor Selection section). The

drain-to-source voltage rating of the input MOSFET

should be higher than the maximum input voltage.

Because the MOSFET conducts the full input current,

its on-resistance should be low enough for good effi-

ciency. Use a logic-level or low-threshold MOSFET to

ensure that the switch is fully enhanced at the lowest

input voltage.

Setting the Input Overcurrent Threshold

The high-side comparator of the MAX1997/MAX1998

provides input overcurrent protection when used in con-

junction with an external P-channel MOSFET P1. The

accuracy of the overcurrent threshold is affected by

many factors, including comparator offset, resistor toler-

ance, input voltage range, and variations in MOSFET

RDS(ON). The input overcurrent comparator is only

intended to protect against catastrophic failures. This

function is similar to an input fuse.

To minimize the impact of the comparator’s input offset

on the current-sense accuracy, the sense voltage

should be close to the upper limit of the comparator’s

common-mode range (same as the operating range),

which extends up to 80% of the input voltage. The

resistive voltage divider R4/R5, combined with the on-

state resistance of P1, sets the overcurrent threshold.

The center of R4/R5 is connected to the inverting input

(OCN) as shown in Figure 10.

If the comparator and resistors are ideal, the threshold

is at the current where both inputs are equal:

IL(MAX) is the average inductor current at maximum load

condition and minimum input voltage, and is given by:

where

η is the efficiency of the main step-up regulator.

If the step-up regulator’s minimum input voltage is 2.7V,

the output voltage is 9V, and the maximum load current is

0.3A. Assuming 80% efficiency, the maximum average

inductor current is:

RDS(MAX) is the maximum on-state drain-to-source

resistance of P1. The maximum RDS(ON) at +25

°C can

be found in the MOSFET manufacturer’s data sheet, but

that number does not include the MOSFET’s tempera-

ture coefficient. Since the resistance temperature coef-

ficient is 0.5%/

°C, RDS(MAX) can be calculated with the

following equation:

RDS(MAX) = RDS_25C ✕ [1 + 0.005 ✕ (TJ - 25)]

where TJ is the actual MOSFET junction temperature in

normal operation due to ambient temperature and self-

heating caused by power dissipation. As an example,

consider the Fairchild FDN304P, which has a maximum

RDS(ON) at room temperature of 70m

Ω. If the junction

temperature is +100

°C, the maximum on-state resis-

tance over temperature is:

RDS(MAX) = 70m

Ω [1 + 0.005 ✕ (100 - 25)] = 100mΩ

For given R2 and R3 values, the ideal ratio of R4/R5

can be determined:

- I

PEAK(MAX)

DS(MAX)

0.8

2.7V

0.3A = 25A

L(MAX) =

L(MAX)

OUT

IN(MIN)

LOAD(MAX)

- I

L(MAX)

DS(MAX)

=×

()×

Figure 10. Setting the Overcurrent Threshold

VIN

RDS(ON)

OC COMP

OCP

OCN

100pF

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