2-303

Applications Guidelines for a procedure to determine the

soft-start interval.

Fault Protection

All four outputs are monitored and protected against extreme

overload. A sustained overload on any output or an over-

drives the FAULT/RT pin to VCC.

Figure 4 shows a simplified schematic of the fault logic. An

over-voltage detected on VSEN1 immediately sets the fault

latch. A sequence of three over-current fault signals also

sets the fault latch. The over-current latch is set dependent

upon the states of the over-current (OC), linear under-

voltage (LUV) and the soft-start signals. A window

fully charged to 4V (UP signal). An under-voltage on either

linear output (VSEN2, VSEN3, or VSEN4) is ignored until

after the soft-start interval (T4 in Figure 3). This allows

then on) resets the counter and the fault latch.

Over-Voltage Protection

During operation, a short on the upper MOSFET of the PWM

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

over-voltage comparator trips to set the fault latch and turns

fault latch raises the FAULT/RT pin to VCC.

A separate over-voltage circuit provides protection during

the initial application of power. For voltages on the VCC pin

below the power-on reset (and above ~4V), the output level

is monitored for voltages above 1.3V. Should VSEN1 exceed

this level, the lower MOSFET, Q2 is driven on.

Over-Current Protection

All outputs are protected against excessive over-currents.

The PWM controller uses the upper MOSFET’s

against shorted output. All linear controllers monitor their

respective VSEN pins for under-voltage events to protect

against excessive currents.

Figure 5 illustrates the over-current protection with an

overload on OUT1. The overload is applied at T0 and the

the OVER-CURRENT comparator trips when the voltage

ROCSET. This inhibits all outputs, discharges the soft-start

cycle with the error amplifiers clamped by soft-start. With

OUT1 still overloaded, the inductor current increases to trip

the over-current comparator. Again, this inhibits all outputs,

but the soft-start voltage continues increasing to 4V before

discharging. The counter increments to 2. The soft-start

cycle repeats at T3 and trips the over-current comparator.

The SS pin voltage increases to 4V at T4 and the counter

increments to 3. This sets the fault latch to disable the

converter. The fault is reported on the FAULT/RT pin.

The linear controllers operate in the same way as the PWM

in response to over-current faults. The differentiating factor

for the linear controllers is that they monitor the VSEN pins

for under-voltage events. Should excessive currents cause

the voltage at the VSEN pins to fall below the linear under-

voltage threshold, the LUV signal sets the over-current

above the under-voltage threshold during normal operation.

Cycling the bias input power off then on resets the counter

and the fault latch.

FIGURE 3. SOFT-START INTERVAL

0V

0V

0V

TIME

PGOOD

SOFT-START

(1V/DIV)

OUTPUT

(0.5V/DIV)

VOLTAGES

T1

T2

T3

T0

T4

FAULT

LATCH

S

R

Q

POR

COUNTER

OC1

OV

LUV

+

-

+

-

0.15V

4V

SS

VCC

FAULT

R

FIGURE 4. FAULT LOGIC - SIMPLIFIED SCHEMATIC

UP

OVER-

CURRENT

LATCH

INHIBIT

S

R

Q

HIP6021