lm5034mtcx National Semiconductor Corporation, lm5034mtcx Datasheet - Page 16

• Manufacturer Part Number: lm5034mtcx
• Description: High Voltage Dual Interleaved Current Mode Controller With Active Clamp
• Manufacturer: National Semiconductor Corporation

www.national.com
Cycle-by-Cycle Current Limit
Each CS pin is designed to accept a signal representative of
its transformer primary current. If the voltage at CS exceeds
0.5V the current sense comparator terminates the present
main output driver (OUT pin) pulse. If the high current fault
persists, the controller operates with constant peak switch
current in a cycle-by-cycle current limit mode, and a Hiccup
Mode Current Limit Restart cycle begins (see below).
Each CS pin is internally connect to ground through a 30Ω
resistor during the main output off time to discharge external
filter capacitance. The discharge device remains on for an
additional 50 ns after the main output driver switches high to
blank leading edge transients in the current sensing circuit.
Discharging the CS pin filter each cycle and blanking leading
edge spikes reduces the filter requirement which improves
the current sense response time.
The current sense comparators are fast and respond to short
duration noise pulses. The external circuitry at each CS pin
should include an R-C filter to suppress noise. Layout con-
siderations are critical for the current sense filter and the
sense resistor. Refer to the Applications Information section
for PC board layout guidelines.
Hiccup Mode Current Limit Restart
If cycle-by-cycle current limiting continues in either or both
controllers for a sufficient period of time, the Current Limit
Restart circuit disables both regulators and initiates a soft-
start sequence after a programmable delay. The duration of
cycle-by-cycle current limiting before turn-off occurs is pro-
grammed by the value of the external capacitor at the RES
pin. The dwell time before output switching resumes is pro-
grammed by the value of the Soft-start capacitor(s). The
circuit is detailed in Figure 20 and the timing is shown in
Figure 21. A description of this circuit’s operation is as fol-
lows:
a) No current limit detected:
The 10 µA discharge current source at RES is enabled
pulling the RES pin to ground.
b) Current limit repeatedly detected at both CS inputs:
The 20 µA current source at RES is enabled continuously to
charge the RES pin capacitor as shown in Figure 21. The
current limit comparators also terminate the PWM output
16
pulses to provide a cycle-by-cycle current limiting. When the
voltage on the RES capacitor reaches the 2.55V restart
comparator threshold, the comparator sets the Restart Latch
which produces the following restart sequence:
• The SS1 and SS2 pin charging currents are reduced from
• An internal MOSFET is turned on to discharge the RES
• The internal MOSFETs at SS1 and SS2 are turned on to
• COMP1 and COMP2 follow SS1 and SS2 respectively
• When the voltages at the SS pins fall below 200mV, the
• When either SS pin reaches )1.5V its PWM controller
If the overload condition still exists, the voltage at RES
begins to increase again and repeat the restart cycle as
shown in Figure 21. If the overload condition has been
cleared, the RES pin is held at ground by the 10 µA current
source.
c) Current limit repeatedly detected at one of the two CS
inputs:
In this condition the RES pin capacitor is charged by the 20
µA current source once each clock cycle of the current
limited regulator (CLK1 or CLK2), and discharged by the 10
µA current source once each clock cycle of the unaffected
regulator. The voltage at the RES pin increases one fourth
as fast as in case b) described above. The current limited
regulator operates in a cycle-by-cycle current limit mode until
the voltage at RES reaches the 2.55V threshold. When the
Restart Comparator output switches high the Restart Latch
is set, both SS pin capacitors are discharged to disable the
regulator channels, and a restart sequence begins as de-
scribed in case b) above.
To determine the value of the RES pin capacitor, see the
Applications Information section.
50µA to 1 µA,
pin capacitor.
discharge the Soft-start capacitors.
and reduce the PWM duty cycles to zero
internal MOSFETs at the SS pins are turned off allowing
the SS pins to be charged by the 1µA current sources.
produces the first pulse of a soft-start sequence which
resets the Restart Latch. The SS charging currents are
increased to 50 µA and the soft-start sequence continues
at the normal rate.