lm5027mhx National Semiconductor Corporation, lm5027mhx Datasheet - Page 17

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

PWM Comparators
The pulse width modulator (PWM) comparator compares the
voltage ramp signal at the RAMP pin to the loop error signal.
The loop error signal is received from the external feedback
and isolation circuit in the form of a control current into the
matched pair of NPN transistors which sink current through a
5 kΩ resistor connected to the 5V reference. The resulting
control voltage is compared at the PWM input to a 1V level
shifted ramp signal. An opto-coupler detector can be con-
nected directly between the REF pin and the COMP pin. Since
the COMP pin is a current mirror input, the potential difference
across the opto-coupler detector is nearly constant. The
bandwidth limiting phase delay which is normally introduced
by the significant capacitance of the opto-coupler is thereby
greatly reduced. Higher loop bandwidths can be realized
since the bandwidth-limiting pole associated with the opto-
coupler is now at a much higher frequency. The PWM com-
parator polarity is configured such that with no current into the
COMP pin, the controller produces maximum duty cycle at the
main gate drive output (OUTA).
SSSR <2.0V
Fault/Event
UVLO<0.4V
2.0V<UVLO
SS< 1V
<1.25V
>0.4V
OTP
>6.2V
>6.2V
>6.2V
>6.2V
>6.2V
Vcc
UVLO
>2.0V
>2.0V
>2.0V
-
-
>1.25V
>1.25V
>1.25V
>1.25V
TABLE 1. Fault/Event Summary
OTP
-
SSSR<2V
SSSR<2V
discharge
discharge
discharge
>4.0V
after
after
Fast
Fast
Fast
SS
17
-
Feed-Forward Ramp
An external resistor (R
VIN, AGND, and the RAMP pins is required to create the
PWM ramp signal as shown in
nal at the RAMP pin will vary in proportion to the input line
voltage. This varying slope provides line feed-forward infor-
mation necessary to improve line transient response with
voltage mode control. The RAMP signal is compared to the
error signal by the pulse width modulator comparator to con-
trol the duty cycle of the outputs. With a constant error signal,
the on-time (t
to stabilize the volt-second product of the transformer prima-
ry. The power path gain of the conventional voltage-mode
pulse with modulator (oscillator generated ramp) varies di-
rectly with input voltage. The use of a line generated ramp
(input voltage feed-forward) nearly eliminates the gain varia-
tion. As a result, the feedback loop is only required to make
very small corrections for large changes in input voltage. At
the end of each clock period, an internal MOSFET with an
R
voltage to ground.
DS(ON)
discharge
discharge
discharge
discharge
SSSR
Slow
Slow
of 10Ω (typical) is enabled to reset the C
Fast
Fast
-
ON
) varies inversely with the input voltage (VIN)
Switching
OUTA
Low
Low
Low
Low
FF
) and capacitor (C
Figure
Switching
OUTB
Low
Low
Low
Low
8. The slope of the sig-
FF
) connected to
www.national.com
FF
OUTSR
Low
Low
Low
Low
Low
capacitor