lm5039mhx National Semiconductor Corporation, lm5039mhx Datasheet - Page 13

• Manufacturer Part Number: lm5039mhx
• Description: Half-bridge Pwm Controller With Average Current Limit
• Manufacturer: National Semiconductor Corporation

Frequency Foldback
Ideally, a power converter will have the characteristics of a
constant current source while operating in current limit. In re-
ality, the current limit level tends to increase as the output
voltage decreases. In a hard-short condition, avoiding an in-
crease of the average output current requires extremely low
duty cycles. However, the minimum achievable on-time is
limited due to propagation and turn-off delays. In a fixed fre-
quency converter, the peak output inductor current creeps up
during the minimum on-time and does not have enough off-
time to come back down. Therefore, the average output cur-
rent increases. The propagation delay of the LM5039 has
been optimized to about 50ns and the turn-off time mainly
depends on the total gate charge of the external power FET.
To avoid the output current tail when the power converter is
in average current limit, the LM5039 oscillator frequency is
proportionally decreased. In a hard-short condition, the oscil-
lator frequency is reduced to 1/3
by the RT resistor. The frequency foldback is implemented
only in the average current limit condition. and it does not af-
fect the ac response of the control loop.
Overload Protection Timer
The LM5039 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
current limit events required to trigger the restart is pro-
grammable by the external capacitor at the RES pin. During
each PWM cycle, the LM5039 either sources or sinks current
from the RES pin capacitor. If no current limit is detected dur-
ing a cycle, an 12 µA discharge current sink is enabled to pull
the RES pin to ground. If a current limit is detected, the 12 µA
sink current is disabled and a 22µA current source causes the
voltage at the RES pin to gradually increase. The LM5039
protects the converter with peak cycle-by-cycle and average
current limiting while the voltage at RES pin increases. If the
RES voltage reaches the 2.5V threshold, the following restart
sequence occurs:
•
•
•
The RES, ACL and SS capacitors are fully discharged
The soft-start current source is reduced from 110 µA to 1.2
µA
The SS capacitor voltage slowly increases. When the SS
voltage reaches
≊
1V, the PWM comparator will produce
rd
the oscillator frequency set
FIGURE 3. Current Limit Restart Timing
13
•
•
The overload timer function is very versatile and can be con-
figured for the following modes of protection:
1. Continuous Current limit 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 peak
cycle-by-cycle/average current-limit protection will limit the
output current indefinitely and no hiccup sequences will oc-
cur.
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 peak cycle-by-cycle and
average current 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.
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, peak cycle-
by-cycle and average 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. Restart timer is initiated
as the signal at CS pin crosses 500mV and works the
same if the controller is in average current limit mode or
pure peak cycle-by-cycle current limiting.
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