lm5035mhx National Semiconductor Corporation, lm5035mhx Datasheet - Page 12

• Manufacturer Part Number: lm5035mhx
• Description: Pwm Controller With Integrated Half-bridge And Syncfet Drivers
• Manufacturer: National Semiconductor Corporation

www.national.com
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
exceeds 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
depending 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
external 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 reduces 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 criti-
cal for the current sense filter and sense resistor. The ca-
pacitor 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 transformer 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 sensitive low power ground connec-
tions 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 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
12
current source causes the voltage at the RES pin to gradu-
ally increase. The LM5035 protects the converter with cycle-
by-cycle current limiting while the voltage at RES pin in-
creases. If the RES voltage reaches the 2.5V threshold, the
following restart sequence occurs (also see Figure 2):
• The RES capacitor and SS capacitors are fully dis-
• The soft-start current source is reduced from 55µA to 1µA
• The SS capacitor voltage slowly increases. When the SS
• If the overload condition persists after restart, cycle-by-
• If the overload condition no longer exists after restart, the
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 com-
pletely 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 over-
load 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 de-
scribed. 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 dissipa-
tion 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.
charged
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
cycle current limiting will begin to increase the voltage on
the RES capacitor again, repeating the hiccup mode
sequence
RES pin will be held at ground by the 12µA current sink
and normal operation resumes