FAN5235

REV. 1.3.3 1/3/02

8

Peak current is DC output current plus peak ripple current:

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 5 converters (+5V, +3.3V, and

+12V, 5V/3.3V-ALWAYS) and puts the FAN5235 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

(for the duration of the contact) to power the 3.3V-ALWAYS

and 5V-ALWAYS long enough for the uC to power up and in

turn latch the SDWN pin high.

Once the SDWN is high then the ALWAYS voltages are

enabled to go high if the respective SDN3.3 and SDN5 go

high.

MAIN 3.3V and 5V Softstart, Sequencing and

Stand-by

Softstart of the 3.3V and 5V converters is accomplished by

means of an external capacitor between pins SDN3.3 (SDN5)

and ground.

The 3.3V (5V) main converter is turned ON if SDWN and

SDN3.3 (SDN5) are both high and is turned off if either SDWN

or SDN3.3 (SDN5) is low.

Stand-by mode is defined as the condition by which V-Mains

are OFF and V-ALWAYS are ON (SDWN=1 and

SDN3.3=SDN5=0).

ALWAYS mode of Operation

If it is desired that 5V-ALWAYS and 3.3V-ALWAYS are always

ON then the SDWN pin must be connected to Vin permanently.

This way the two ALWAYS regulators come up as soon as there

is power while the state of the Main regulators can be controlled

via the SDN5 and SDN3.3 pins.

Sequencing Table

3.3V Voltage Adjustment

The output voltage of the 3.3V converter can be increased by

as much as 10% by inserting a resistor divider in the feedback

line. The feedback pin impedance is about 66K

Ω. Thus, for

example, to increase the output of the 3.3V converter by 10%,

use a 2.21K

Ω/33.2KΩ divider.

Note that the output of the 5V regulator cannot be adjusted.

The feedback line of the 5V regulator is used internally as a

5V supply and, therefore, cannot tolerate any impedance in

series with it.

3.3V and 5V Main Overvoltage Protection

(Soft Crowbar)

When the output voltage of the 3.3V (or the 5V) converter

exceeds approximately 115% of nominal, the converter enters

the over-voltage (OV) protection mode, with the goal of pro-

tecting the load from damage. During operation, severe load

dump or a short of an upper MOSFET could cause the output

voltage to increase significantly over normal operation range

without circuit protection. When the output exceeds the over-

voltage threshold, 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 thresh-

old, OVP comparator is disengaged.

The OVP scheme also provides a soft crowbar function

(bang-bang control followed by blow of the fuse) which

helps to tackle severe load transients but does not invert out-

put voltage when activated—a common problem for OVP

schemes with a latch. The prevention of output inversion

eliminates the need for a Schottky diode across the load.

2L

= 5A +

4µsec • 5V

2 • 5µH

= 7A

≈

HSD

SW

LSD

ISEN

GND

SDN5

SDN3.3

SDWN

3V&5V

ALWAYS

5V

MAIN

3.3V

MAIN

X

X

0

0

0

0

00

1

1

0

0

101

1

1

0

01

1

1

0

1

1

1

1

1

1

1