lm5027mhx National Semiconductor Corporation, lm5027mhx Datasheet - Page 14

• Manufacturer Part Number: lm5027mhx
• Description: Voltage Mode Active Clamp Controller
• Manufacturer: National Semiconductor Corporation

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Soft-Start
The soft-start circuit allows the regulator to gradually increase
its output voltage until the steady state operating point is
reached; thereby reducing start-up stresses and surge cur-
rents. When bias power is supplied to the LM5027, the SS pin
capacitor is discharged by an internal MOSFET. When the
voltages on the UVLO, OTP, VCC, and REF pins reach the
operating thresholds, the soft-start capacitor is released and
is charged with a 22 µA current source. When the SS pin volt-
age reaches 1V, output pulses commence with a slowly in-
creasing duty cycle (refer to
5). The voltage on the SS pin eventually increases to 5V, while
the voltage at the PWM comparator is limited to the level re-
quired for regulation as determined by the voltage feedback
loop via the COMP pin. When the soft-start voltage reaches
4.0V, the capacitor on the SSSR pin is released and charged
with a 25 µA current source (refer to
Figure
ly 2.5V (refer to
PWM circuit gradually increases the synchronous rectifier
drive duty cycle (OUTSR) in proportion the rising SSSR pin
voltage. Delaying the start of the SSSR gate drive pulses until
5) . When the SSSR pin voltage reaches approximate-
Figure
6), the internal synchronous rectifier
Figure
3,
Figure
Figure 4
3,
Figure 4
and
FIGURE 3. Soft-Start Timing
Figure
and
14
after the main soft-start is completed allows the output voltage
to reach regulation before the synchronous rectifiers begins
operation. This delay prevents the synchronous rectifier from
sinking current from the output in applications where the out-
put voltage may be pre-biased.
Soft-Stop
If the UVLO pin voltage falls below the 2.0V standby thresh-
old, but above the 0.4V shutdown threshold, the synchronous
rectification soft-start capacitor is discharged with a 20 µA
current source which gradually disables the synchronous rec-
tifiers (refer to
discharged to 2.0V the soft-start and synchronous rectifica-
tion soft-start capacitors are quickly discharged to ground to
terminate PWM pulses at OUTA ,OUTB, and OUTSR. The
PWM pulses may cease before the SSSR voltage reduces
the synchronous rectifier duty cycle if the VCC or REF voltage
drops below the respective under-voltage thresholds during
the soft-stop process. This soft-stop method of turning off the
converter prevents oscillations in the synchronous rectifiers
during a shutdown sequence.
Figure
4). After the SSSR capacitor has been
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