lm5027mhx National Semiconductor Corporation, lm5027mhx Datasheet - Page 13

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

the voltage at the RES pin to gradually increase. If the RES
voltage reaches the 1.0V threshold, the following restart se-
quence occurs (also see
• The SS and SSSR capacitors are fully discharged.
• The RES 20 µA current source is turned-off and the 5 µA
current source is turned-on.
• The voltage on the RES pin is allowed to charge up to 4.0V.
• When voltage on the RES pin reaches 4.0V the 5 µA current
source is turned-off and a 5 µA current sink is turned-on,
ramping the voltage on the RES capacitor down to 2.0V.
• The RES capacitor voltage is ramped between 4.0V and
2.0V eight times.
• When the counter reaches eight, the RES pin voltage is
pulled low and the Soft-Start capacitor is released to begin a
Figure
2).
FIGURE 2. Restart and Soft-Start Delay Timing
13
soft-start sequence. The SS capacitor voltage slowly increas-
es. When the SS voltage reaches 1.0V, the PWM comparator
will produce the first narrow output pulse at OUTA.
• When the SS voltage reaches 4.0V the capacitor on the
SSSR pin is released and is charged with a 25 µA current
source, soft-starting the free-wheeling synchronous rectifier.
• If the overload condition persists after restart, cycle-by-cycle
current limiting will begin to increase the voltage on the RES
capacitor again, repeating the hiccup mode sequence.
• If the overload condition no longer exists after restart, the
RES pin will be held at ground by the 5 µA current sink and
normal operation resumes.
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