lm5035bsqx National Semiconductor Corporation, lm5035bsqx Datasheet - Page 12

• Manufacturer Part Number: lm5035bsqx
• Description: Pwm Controller With Integrated Half-bridge And Syncfet Drivers
• Manufacturer: National Semiconductor Corporation

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Cycle-by-Cycle Current Limit
The CS pin is driven by a signal representative of the trans-
former primary current. If the voltage sensed at CS pin ex-
ceeds 0.25V, the current sense comparator terminates the
HO or LO output driver pulse. If the high current condition
persists, the controller operates in a cycle-by-cycle current
limit mode with duty cycle determined by the current sense
comparator instead of the PWM comparator. Cycle-by-cycle
current limiting may trigger the hiccup mode restart cycle de-
pending on the configuration of the RES pin (see below).
A small R-C filter connect to the CS pin and located near the
controller is recommended to suppress noise. An internal
32Ω MOSFET connected to the CS input discharges the ex-
ternal current sense filter capacitor at the conclusion of every
cycle. The discharge MOSFET remains on for an additional
50 ns after the HO or LO 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 re-
duces the filtering requirements and improves the current
sense response time.
The current sense comparator is very fast and responds to
short duration noise pulses. Layout considerations are critical
for the current sense filter and sense resistor. The capacitor
associated with the CS filter must be placed very close to the
device and connected directly to the CS and AGND pins. If a
current sense transformer is used, both leads of the trans-
former secondary should be routed to the filter network, which
should be located close to the IC. If a sense resistor located
in the source of the main MOSFET switch is used for current
sensing, a low inductance type of resistor is required. When
designing with a current sense resistor, all of the noise sen-
sitive low power ground connections should be connected
together near the AGND pin, and a single connection should
be made to the power ground (sense resistor ground point).
Overload Protection Timer
The LM5035B 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 LM5035B either sources or
sinks current from the RES pin capacitor. If no current limit is
detected during a cycle, a 12 µA discharge current sink is en-
abled to pull the RES pin to ground. If a current limit is
detected, the 12 µA sink current is disabled and a 22µA cur-
12
rent source causes the voltage at the RES pin to gradually
increase. The LM5035B 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 2):
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•
•
•
•
The overload timer function is very versatile and can be con-
figured for the following modes of protection:
1. Cycle-by-cycle only: The hiccup mode can be completely
disabled by connecting a zero to 50 kΩ resistor from the RES
pin to AGND. In this configuration, the cycle-by-cycle protec-
tion 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 be-
fore 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 ex-
tended 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.
The RES capacitor and SS capacitors are fully discharged
The soft-start current source is reduced from 110 µA to 1
µA
The SS capacitor voltage slowly increases. When the SS
voltage reaches
the first narrow output pulse. After the first pulse occurs,
the SS source current reverts to the normal 110 µA 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 12 µA current sink
and normal operation resumes
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1V, the PWM comparator will produce