6

the clamped error amplifier output voltage. As the internal

soft-start voltage increases, the pulse-width on the PHASE

pin increases to reach its steady-state duty cycle at time T2.

Also at time T2, the error amplifier references of the linear

controllers, ramp to their final value bringing all outputs

within regulation limits.

Overcurrent Protection

All outputs are protected against excessive overcurrents.

The PWM controller uses the upper MOSFET’s

against a shorted output. All linear controllers monitor their

respective FB pins for undervoltage events to protect against

excessive currents.

A sustained overload (undervoltage on linears or overcurrent

on the PWM) on any output results in an independent

shutdown of the respective output, followed by subsequent

individual re-start attempts performed at an interval equivalent

to 3 soft-start intervals. Figure 2 describes the protection

feature. At time T0, an overcurrent event sensed across the

internal soft-start initiates a number of soft-start cycles. After a

three-cycle wait, the fourth soft-start initiates a ramp-up

attempt of the failed output, at time T2, bringing the output in

regulation at time T4.

To exemplify a UV event on one of the linears, at time T1,

overcurrent event, resulting in a UV condition. Similarly, after

three soft-start periods, the fourth cycle initiates a ramp-up of

this linear output at time T3. One soft-start period after T3,

the linear output is within regulation limits. UV glitches less

than 1

µs (typically) in duration are ignored.

Overcurrent protection is performed on the synchronous

switching regulator on a cycle-by-cycle basis. OC monitoring

is active as long as the regulator is operational. Since the

overcurrent protection on the linear regulators is performed

through undervoltage monitoring at the feedback pins (FB2,

FB3, and FB4), this feature is activated approximately 25%

into the soft-start interval (see Figure 2).

the PWM converter. As shown in Figure 3, the internal

40

signal enables the overcurrent comparator (OCC). When

overcurrent function will trip at a peak inductor current

FIGURE 1. SOFT-START INTERVAL

0V

0V

TIME

(0.5V/DIV)

T1

T2

T0

(1V/DIV)

+5V

FIGURE 2. OVERCURRENT/UNDERVOLTAGE PROTECTION

RESPONSE

0V

TIME

(0.5V/DIV.)

T1

T2

T3

T0

UV MONITORING

INACTIVE

SOFT-START

FUNCTION

ACTIVE

T4

I

I

OCSET

R

OCSET

×

r

DS ON

()

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

ISL6521