FAN5234 View Datasheet(PDF) - Fairchild Semiconductor

Part Name

Description

Manufacturer

FAN5234

Mobile-Friendly PWM/PFM Controller

Fairchild Semiconductor 

FAN5234

PRODUCT SPECIFICATION

The hysteretic comparator causes HDRV turn-on when the

output voltage (at VSEN) falls below the lower threshold

(10mV below VREF) and terminates the PFM signal when

VSEN rises over the higher threshold (5mV above VREF).

The switching frequency is primarily a function of:

1. Spread between the two hysteretic thresholds

2. ILOAD

3. Output Inductor and Capacitor ESR

A transition back to PWM (Continuous Conduction Mode or

CCM) mode occurs when the inductor current rises suffi-

ciently to stay positive for 8 consecutive cycles. This occurs

when:

ILOAD(CCM)

=

-∆----V----H----Y----S---T---E---R---E----S---I--S-

2 ESR

(3)

where ∆VHYSTERESIS = 15mV and ESR is the equivalent

series resistance of COUT.

Because of the different control mechanisms, the value of the

load current where transition into PWM operation takes

place is typically higher compared to the load level at which

transition into hysteretic mode occurs. Hysteretic mode

can be disabled by setting the FPWM pin HIGH.

Current Processing Section

The following discussion refers to Figure 4.

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. The resistor con-

nected to ISNS pin (RSENSE) sets the gain in the current

feedback loop. For stable operation, the voltage induced by

the current feedback at the PWM comparator input should be

set to 30% of the ramp amplitude at maximum load currrent

and line voltage. The following expression estimates the rec-

ommended 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--ON---N-(--M-)---×A----X-4---).-1----K--- – 100

(4a)

RSENSE must, however, be kept higher than:

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

(4b)

Setting the Current Limit

A ratio of ISNS is also compared to the current established

when a 0.9 V internal reference drives the ILIM pin:

RILIM

=

----1---1---.-2----- × (---1---0---0----+-----R-----S---E---N---S---E----)

ILIMIT

RDS(ON)

(5)

Since the tolerance 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 representation of the load current. When using the

MOSFET as the sensing element, the variation of RDS(ON)

causes proportional 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 temperature. 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.

VSEN

SS

CSS

300K

0.17pf

1.5M 17pf

4.14K

TO PWM COMP

S/H

V to I

in +

ISNS

ISNS

in –

Reference and

Soft Start

ILIM det.

I2 =

ILIM*11.2

2.5V

0.9V

ILIM

ISNS RSENSE

LDRV

PGND

ILIM

R ILIM

Figure 4. Current Limit / Summing Circuits

REV. 1.0.10 5/3/04

7

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