SC2542EVB SEMTECH [Semtech Corporation], SC2542EVB Datasheet - Page 12

• Manufacturer Part Number: SC2542EVB
• Description: High Performance Wide Input Range Dual Synchronous Buck Controller
• Manufacturer: SEMTECH [Semtech Corporation]

pin high), an internal 84uA current source, Iss, (soft start
current) will charge the soft-start capacitor gradually. The
PWM output starts pulsing when the soft start voltage
reaches 1V.
This soft start scheme will ensure the duty cycle increases
slowly, therefore limiting the in rush current into the out-
put capacitor and also ensuring the inductor does not
saturate. The soft start capacitor will eventually be
charged up to 2.5V.
The soft-start sequence is initiated when EN pin or STBY
pin is high and Vcc > 5.5V or during recovery from a fault
condition ( OCP, OVP, or UVLO).
The period of start up can be programmed by the soft
start capacitor:
Shutdown
When the EN pin or STBY pin is pulled low, an internal
15uA current source discharges the soft-start capacitor
and DRVH/DRVL signals stop pulsing. The output volt-
age ramps down at a rate determined by the load condi-
tion.
The SC2542 can also be shutdown by pulling down di-
rectly on the SS pin. The designer needs to consider the
slope of the SS pin voltage and choose a suitable pull
down resistor to prevent the output from undershooting.
Shutdown can also be triggered under an OCP condition.
When an OCP condition is detected, DRVH and DRVL will
stop pulsing and enter a “tri-state shutdown” with the
output voltage ramping down at a rate determined by
the load condition. The internal 15uA current source will
begin discharging the soft-start capacitor and when the
soft-start voltage reaches 0.65V, DRVL will go high.
Over Current Protection (OCP)
The inductor current is sensed by using the low side
MOSFET Rds(on). After low side MOSFET is turned on,
the OCP comparator starts monitoring the voltage drop
across the MOSFET. The OCP trip level is programmed
by a resistor from the ILIM pin to the phase node. There
© 2005 Semtech Corp.
POWER MANAGEMENT
Applications Information (Cont.)
T
SS
=
C
SS
84
×
µ
2.5
A
V
12
is an internal current source that flows out of the ILIM
pin which will generate a voltage drop on the setting resis-
tor. When the sum of the setting resistor voltage and
the MOSFET drain to source voltage is less then zero,
the OCP condition will be flagged. This functionality is
depicted in Figure 8.
The following formula is used to set the OCP level:
When OCP is tripped, both high side and low side MOS-
FETs will be turned off and this condition is latched. At
the same time, the soft start cap will be discharged by
the internal current source of 15uA. When the Vss drops
bellow 0.65V, the DRVL pin will go high again.
To avoid switching noise during the phase node commu-
tation, a 100nS blanking time is built in after the low
side MOSFET is turned on, as shown in Fig. 9.
TG
TG
Figure 9. OCP comparator timing chart.
Figure 8. Block diagram of over current protection.
I
I
L
L
OCP
10
Out
µ
100nS
100nS
Blanking
Blanking
A R
×
+
-
10uA
ILIM
VCC
=
ILIM
DRVL
DRVH
OCP
OCP
I
L PEAK
_
×
R
DS ON
(
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SC2542
)
OUTPUT