PRODUCT SPECIFICATION

FAN5233

REV. 1.0.6 1/22/02

9

Functional Description

The FAN5233 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 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

gives maximum efficiency, while 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

FAN5233 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.

If the 5V switcher is not used, connect SDN5 (pin 17) to

SGND (pin 14). If the 3.3V switcher is not used, connect

SDN3.3 (pin 11) to SGND (pin 14).

The following discussion of the FAN5233 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 FAN5233 function as

voltage mode with current feedback for stability. They each

have an independent voltage feedback pin, as shown in Fig-

ure 1. They use voltage feed-forward to guarantee loop rejec-

tion of input voltage variation: that is to say that the PWM

(pulse width modulation) ramp amplitude is varied as a func-

tion of the input voltage. Compensation of the control loops

is done entirely internally using current-mode feedback com-

pensation. This scheme allows the bandwidth and phase mar-

gin 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,

inductor is 5µH at a current of 5A. Because of the low

perature (ambient + self-heating) of only 75°C, at which its

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 7A • 20m

Ω = 140mV.

The current limit threshold is typically 150mV (worst-case

135mV) with R2 = 1K

Ω, and so this value is suitable. R2

could be increased a further 10% if additional noise margin

is deemed necessary.

Precision Current Limit

Precision current limiting can be achieved by placing a

discrete sense resistor between the source of the low-side

MOSFET and ground.

In this case, current limit accuracy is set by the tolerance of

the IC, +10%.

Figure 4. Using a Precision Current Sense Resistor

Shutdown (SDWN)

The SDWN pin turns off all 4 converters (+5V, +3.3V, and

+12V, 5V-ALWAYS) and puts the FAN5233 into a low-

power mode (Shutdown mode).

This mode of operation implies the use of a push button

switch between SDWN and Vin. Pushing the button allows

I

pk

I

DC

VIN

V

O

–

()T

2L

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

V

O

VIN

-----------

•

+

5A

16V

5V

–

()4µsec

2.5

µH

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

5V

16V

----------

•

+

6.4A

==

=

HSD

SW

LSD

ISEN

GND