SC2620_08 Datasheet, PDF (16/26 Pages) Semtech Corporation – Dual 2A, 30V Step-down Regulator with Programmable Frequency up to 1.4MHz

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 < 2V), the overload
SS
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_FB is still below 0.7V, then the regulator will
undergo shutdown and restart. The soft-start process
should allow the output voltage to reach 70% of its final
value before C_SS is charged above 2V. Figures 9(a) and
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
SS1
PGOOD1
M1
CSS1
SC2620
SS2
M2
CSS2
OFF ON
CSS1
SS1
SC2620
PGOOD1
SS2
CSS2
CONTROL1
CONTROL2
OFF ON
OFF
ON
TD
(a)
(b)
Figure 10. Sequencing the Outputs by (a) Delaying Release of one Channel Relative to the
Other and (b) Using PGOOD1 to Control Channel 2.
 2008 Semtech Corp.
16
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