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FAN5068

DDR-1/DDR-2 plus ACPI Regulator Combo

Fairchild Semiconductor 

FAN5068

COMP

FB

SS/EN

CSS

COMP

FB

SS

1µA

ISS

–

+ E/A

+

VREF

ISNS

RAMP

–

+ PWM

+

PRODUCT SPECIFICATION

Figure 7. SS Clamp and FB Open Protection

4.41K

Reference and

Soft Start

TO

PWM

COMP

S/H

V to I

ISNS

in +

ISNS

in –

ILIM det.

2.5V

I2 =

ILIM*9.6

0.9V

ILIM

mirror

RSENSE

ISNS

LDRV

PGND

ILIM

RILIM

Figure 8. Current Limit / Summing Circuits

Current Processing Section

The following discussion refers to Figure 8.

The current through RSENSE resistor (ISNS) is sampled

shortly after Q2 is turned on. That current is held, and

summed with the output of the error amplifier. This effec-

tively creates a current mode control loop. RSENSE sets the

gain in the current feedback loop. For stable operation, the

voltage induced by the current feedback at the PWM com-

parator input should be set to 30% of the ramp amplitude at

maximum load currrent and line voltage. The following

expression estimates the recommended value of RSENSE as a

function of the maximum load current (ILOAD(MAX)) and the

value of the MOSFET’s RDS(ON).:

RSENSE = I---L---O--3--A-0--D-%---(--M--•---A-0--X-.-1--)-2--•-5---R--•---D-V--S---I(--NO---(-N-M---)--A-•--X--4--)-.-1----k- – 100

(5)

RSENSE must, however, be kept higher than:

RSENSE(MIN) = -I--L---O----A---D----(--M---1--A-5--X-0---µ)---•-A---R-----D---S---(--O----N----) – 100

(6)

Setting the Current Limit

An ISNS is compared to the current established when a 0.9 V

internal reference drives the ILIM pin. RILIM, the RDS(ON) of

Q2, and RSENSE determine the current limit:

RILIM

=

-----9---.--6------

ILIMIT

×

-(--1---0---0----+-----R-----S---E---N---S---E----)

RDS(ON)

(7)

Where ILIMIT is the peak inductor current. Since the toler-

ance on the current limit is largely dependent on the ratio of

the external resistors it is fairly accurate if the voltage drop

on the Switching Node side of RSENSE is an accurate repre-

sentation of the load current. When using the MOSFET as

the sensing element, the variation of RDS(ON) causes propor-

tional variation in the ISNS. This value not only varies from

device to device, but also has a typical junction temperature

coefficient of about 0.4%/°C (consult the MOSFET datasheet

for actual values), so the actual current limit set point will

decrease propotional to increasing MOSFET die tempera-

ture. A factor of 1.6 in the current limit setpoint should

compensate for all MOSFET RDS(ON) variations, assuming

the MOSFET’s heat sinking will keep its operating die

temperature below 125°C.

REV. 1.0.1 9/9/04

11

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