16

 2006 Semtech Corp.

www.semtech.com

SC2620

POWER MANAGEMENT

Applications Information

increases to 3.3mA. The current flowing out of the other

SS pin (which is still pulled low) increases to 105µA. The

fast charging circuit quickly pulls the released soft-start

capacitor to 1V (slightly below the switching threshold).

The fast charging circuit is then disabled. A 1.8µA current

source continues to charge the soft-start capacitor (Figure

3). The soft-start voltage ramp at the SS pin clamps the

error amplifier output (Figure 2). During regulator start-

up, COMP voltage follows the SS voltage. The converter

starts to switch when its COMP voltage exceeds 1.1V. The

peak inductor current gradually increases until the

converter output comes into regulation. Proper soft-start

prevents output overshoot during start-up. Current drawn

from the input supply is also well controlled. Notice that

the inductor current, not the converter output voltage, is

ramped during soft-start.

Both soft-start capacitors are charged to a final voltage of

about 2.4V.

Overload / Short-Circuit Protection

Each current limit comparator in the SC2620 limits the

peak inductor current to 3.2A (typical). The regulator

output voltage will fall if the load is increased above the

current limit. If overload is detected (the output voltage

falls below 70% of the set voltage), then the regulator will

be shut off. An internal 0.8µA current sink starts to

discharge the soft-start capacitor. As the soft-start

capacitor is discharged below 1V, the discharge current

source turns off and the soft-start capacitor is recharged

with a 1.8µA current source. The regulator undergoes soft-

start. During soft-start (1V < V

shutdown latch in Figure 3 cannot be set. When V

SS

exceeds 2V, the set input of the overload latch is no longer

blanked. If V

undergo shutdown and restart. The soft-start process

should allow the output voltage to reach 70% of its final

value before C

9(b) show the timing diagrams of successful and failed

start-up waveforms respectively. The soft-start interval

should also be made sufficiently long so that the output

voltage rises monotonically and it does not overshoot its

final voltage by more than 5%.

During normal soft-start, both the COMP voltage and the

switch current limit gradually increase until the converter

becomes regulated. If the regulator output is shorted to

CONTROL1

CONTROL2

(b)

ON

OFF

SC2620

PGOOD1

SS1

SS2

CSS1

CSS2

(a)

SC2620

PGOOD1

SS1

SS2

CSS1

CSS2

ON

OFF

CONTROL1

ON

OFF

CONTROL2

TD

M2

M1

Figure 10. Sequencing the Outputs by (a) Delaying Release of one Channel Relative to the

Other and (b) Using PGOOD1 to Control Channel 2.