Datasheet

CLK
Q
S
R
Drivers Off
SS
PWM
COMP
CS
RES
LM5035
SS
Drivers Off
Voltage
Feedback
Current
Sense
Circuit
C
RES
Restart
Comparator
Restart
Latch
2.5V
5V
Restart
Current
Source
Logic
SS
Logic
To Output
Drivers
Current
Limit
0.25V
C
SS
Soft-start
100 mV
55 PA
55 PA
1 PA
22 PA
12 PA
LM5035
SNVS428G –JANUARY 2006–REVISED MARCH 2013 www.ti.com
Overload Protection Timer
The LM5035 provides a current limit restart timer to disable the outputs and force a delayed restart (hiccup
mode) if a current limit condition is repeatedly sensed. The number of cycle-by-cycle current limit events required
to trigger the restart is programmable by the external capacitor at the RES pin. During each PWM cycle, the
LM5035 either sources or sinks current from the RES pin capacitor. If no current limit is detected during a cycle,
an 12µA discharge current sink is enabled to pull the RES pin to ground. If a current limit is detected, the 12µA
sink current is disabled and a 22µA current source causes the voltage at the RES pin to gradually increase. The
LM5035 protects the converter with cycle-by-cycle current limiting while the voltage at RES pin increases. If the
RES voltage reaches the 2.5V threshold, the following restart sequence occurs (also see Figure 12):
• The RES capacitor and SS capacitors are fully discharged
• The soft-start current source is reduced from 55µA to 1µA
• The SS capacitor voltage slowly increases. When the SS voltage reaches ≊1V, the PWM comparator will
produce the first narrow output pulse. After the first pulse occurs, the SS source current reverts to the normal
55µA level. The SS voltage increases at its normal rate, gradually increasing the duty cycle of the output
drivers
• 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 12µA current
sink and normal operation resumes
The overload timer function is very versatile and can be configured for the following modes of protection:
1. Cycle-by-cycle only: The hiccup mode can be completely disabled by connecting a zero to 50kΩ resistor
from the RES pin to AGND. In this configuration, the cycle-by-cycle protection will limit the output current
indefinitely and no hiccup sequences will occur.
2. Hiccup only: The timer can be configured for immediate activation of a hiccup sequence upon detection of
an overload by leaving the RES pin open circuit.
3. Delayed Hiccup: Connecting a capacitor to the RES pin provides a programmed interval of cycle-by-cycle
limiting before initiating a hiccup mode restart, as previously described. The dual advantages of this
configuration are that a short term overload will not cause a hiccup mode restart but during extended
overload conditions, the average dissipation of the power converter will be very low.
4. Externally Controlled Hiccup: The RES pin can also be used as an input. By externally driving the pin to a
level greater than the 2.5V hiccup threshold, the controller will be forced into the delayed restart sequence.
For example, the external trigger for a delayed restart sequence could come from an over-temperature
protection circuit or an output over-voltage sensor.
Figure 12. Current Limit Restart Circuit
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