FAN5235

REV. 1.3.3 1/3/02

7

Figure 3. FAN5235 5V/3.3V—ALWAYS Internal Block Diagram

Functional Description

The FAN5235 is a high efficiency and high precision DC/DC

controller for notebook and other portable applications. It

provides all of the voltages necessary for system electronics:

5V, 3.3V, 12V, and both 3.3V-ALWAYS and 5V-ALWAYS.

Utilization of both input and output voltage feedback in a

current-mode control allows for fast loop response over a

wide range of input and output variations. Current sense

also permitting the use of a sense resistor for high accuracy.

3.3V and 5V Architecture

The 3.3V and 5V switching regulator outputs of the

FAN5235 are generated from the unregulated input voltage

using synchronous buck converters. Both high side and low-

side MOSFETs are N-channel.

The 3.3V and 5V switchers have pins for current sensing and

for setting of output over-current threshold using MOSFET

a pin that shuts down the converter, and a pin for generating

the boost voltage to drive the high-side MOSFET.

The following discussion of the FAN5235 design will be

done with reference to Figures 1 through 4, showing the

internal block diagram of the IC.

3.3V and 5V PWM Current Sensing

Peak current sensing is done on the low side driver because

of the very low duty-cycle on the high side MOSFET. The

current is sampled 50ns after turn on and the value is held for

current feedback and over-current limit.

3.3V and 5V PWM Loop Compensation

The 3.3V and 5V control loops of the FAN5235 function as

voltage mode with current feedback for stability. They each

have an independent voltage feedback pin, as shown in

Figure 1. They use voltage feed-forward to guarantee loop

rejection of input voltage variation: that is to say that the

PWM (pulse width modulation) ramp amplitude is varied as

a function of the input voltage. Compensation of the control

loops is done entirely internally using current-mode feed-

back compensation. This scheme allows the bandwidth and

phase margin to be almost independent of output capacitance

and ESR.

3.3V and 5V PWM Current Limit

The 3.3V and 5V converters each sense the voltage across

their own low-side MOSFET to determine whether to enter

current limit. If an output current in excess of the current

limit threshold is measured then the converter enters a pulse

skipping mode where Iout is equal to the over-current (OC)

set limit. After 8 clock cycles then the regulator is latched off

(HSD and LSD off). This is the likely scenario in the 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 2µs delay.

Selection of a current-limit set resistor must include the

tolerance of the current-limit trip point, the MOSFET on

resistance and temperature coefficient, and the ripple current,

in addition to the maximum output current.

Example: Maximum DC output current on the 5V is 5A,

MOSFET will have a maximum temperature (ambient +

20m

Ω.

LDO

VIN

VFB5

5V ALWAYS

FAN5235

5V/3.3V-ALWAYS

LDO

3.3V ALWAYS