FAN5232

PRODUCT SPECIFICATION

6

REV. 1.1.1 10/7/02

Loop Compensation

The switcher regulator control loop of the FAN5232 is

current-mode with voltage feed-forward. It uses voltage

feed-forward to guarantee loop rejection of input voltage

variation: the ramp amplitude is varied as a function of the

input voltage. Compensation of the control loop is done

entirely internally using current-mode compensation. This

scheme allows the bandwidth and phase margin to be almost

independent of output capacitance and capacitors’ ESR. Use

of a current sense resistor other than the recommended 1K

Ω

may affect the converter’s stability.

Current Sensing

Current sensing is done by measuring the voltage across the

low side MOSFET 50nsec after it is turned on. This value is

then held for current feedback and over-current limit. The

gain is set by an external resistor from the drain to the ISNS

pin, which is normally set to be 1K

Ω.

Current Limit

The converter senses the voltage across its low-side

MOSFET to determine when to enter current limit. If output

current in excess of the current limit threshold is measured,

over-current (OC) limit. If this situation persists for 8 clock

cycles then the regulator is latched off (HSD and LSD off).

This is the likely scenario in case of a “soft” short. If the

short is “hard”, it will instantly trigger the under-voltage

protection, which again will latch the regulator off (HSD and

LSD off) after a 2µsec delay.

Selection of a current-limit set resistor must include the tol-

erance of the current-limit trip point, the MOSFET resis-

tance and temperature coefficient, and the ripple current, in

addition to the maximum output current.

Example: Maximum DC output current on the 12V is 8A,

MOSFET will have a maximum temperature (ambient +

24m

Ω.

Peak current is DC output current plus peak ripple current:

input voltage, and L is the inductance. This current generates

a voltage on the low-side MOSFET of 11A • 24m

Ω =

254mV. The current limit threshold is typically 150mV

(worst-case 135mV) with R2 = 1K

Ω, and so this value must

be decreased to (135/254) • 1K

Ω = 531Ω.

Precision Current Limit

Precision current limiting can be achieved by placing a dis-

crete sense resistor between the source of the low-side

MOSFET and ground. Sensing is then accomplished with

the 1K

Ω resistor between the sense resistor and the IFBSW

pin, as shown in Figure 2. In this case, current limit accuracy

is set by the tolerance of the IC, ±10%.

Figure 2. Precision Current Sensing

Softstart

Softstart of the switcher is accomplished by means of an

external capacitor between pins SDNADJ and ground.

Overvoltage Protection (Soft Crowbar)

When the output voltage of the switcher exceeds approxi-

mately 115% of nominal, it enters into over-voltage (OV)

protection, with the goal of protecting the load from damage.

During operation, severe load dump or a short of an upper

MOSFET can cause the output voltage to increase signifi-

cantly over normal operation range. When the output

exceeds the over-voltage threshold of 115%, the over-voltage

comparator forces the lower gate driver high and turns the

lower MOSFET on. This will pull down the output voltage

and eventually may blow the battery fuse. As soon as output

voltage drops below the threshold, the OVP comparator is

disengaged.

This OVP scheme provides a soft crowbar function (bang-

bang control followed by blow of the fuse), which helps to

tackle severe load transients and does not invert the output

voltage when activated – common problem for OVP schemes

with a latch. The prevention of the output inversion saves the

use of a Schottky diode across the load.

Undervoltage Protection

When the output voltage of the switcher falls below 75% of

nominal value, after a 2usec delay it goes into under-voltage

protection. In under-voltage protection, the high and low side

MOSFETs are turned off. Once under-voltage protection is

triggered, it remains on until power is recycled.

5V/3.3V-ALWAYS Operation

The 5V-ALWAYS supply is generated from the on-chip

linear regulator off the input supply voltage. The

3.3V-ALWAYS is generated from a linear regulator attached

internally to the 5V-ALWAYS.

I

pk

I

DC

TV

O

V

in

V

o

–

()

•

2LV

in

•

•

--------------------------------------------

+

≈

8A

4

µs 12V

19V

12V

–

()

•

•

2

4.7

µH12V

•

•

----------------------------------------------------------------11A

=

+

=

LSD

ISNS

PGND