lm5027mhx National Semiconductor Corporation, lm5027mhx Datasheet - Page 23

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

Soft-Start
The capacitor on the SS pin determines the time required for
the output duty cycle to increase from zero to its final value
for regulation. The minimum acceptable time is dependent on
the output capacitance and the response of the feedback loop
that controls the COMP pin. If the soft-start time is too quick,
the output could significantly overshoot its intended voltage
before the feedback loop has a chance to regulate the PWM
controller. After power is applied and VCC has passed its up-
per UV threshold (9.5V), the voltage at the SS pin ramps up
as the external capacitor is charged with an internal 20 μA
current source. The voltage at the internal PWM comparator
input node follows the voltage at the SS pin. When the voltage
on the SS pin has reached 1.0V, PWM pulses appear at the
drive output with a very low duty cycle. The voltage at the SS
pin eventually increases to approximately 5.0V. The voltage
at the input to the PWM comparator and the PWM duty cycle
increase to the value required for regulation as determined by
the voltage regulation loop.
FIGURE 15. Resistor Current Sense
23
Hiccup Mode Current Limit Restart
Hiccup mode operation is described in the Restart Time Delay
(Hiccup Mode) section. In the case of continuous current limit
detection at the CS pin, the time required to reach the 1.0V
RES pin threshold is:
For example, if C
approximately 2.14 ms. After the voltage on the RES pin
reaches 1.0V, the 22 µA current source is turned-off and a 5
µA current source is turned-on. The Hiccup Mode time is:
where ΔV is 2.0V. With a C
time is 179 ms. After the Hiccup Mode off time is complete,
the RES pin voltage is pulled low and soft-start capacitor is
released allowing a soft-start sequence to commence.
The soft-start time t
source, and is equal to:
If C
SS
= 0.1 µF, t
restart
RES
restart
= 0.047 µF the time t
is = 6.41 ms
is set by the internal 22 µA current
RES
= 0.047 µF, the Hiccup Mode
CS
30099019
in
Figure 16
www.national.com
is