LM5039

www.ti.com

SNVS621D – FEBRUARY 2010 – REVISED MARCH 2013

A small R-C filter connect to the CS pin and located near the controller is recommended to suppress noise. An

internal 36

Ω 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.

Frequency Foldback

Ideally, a power converter will have the characteristics of a constant current source while operating in current

limit. In reality, the current limit level tends to increase as the output voltage decreases. In a hard-short condition,

avoiding an increase 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 frequency 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 current 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

oscillator frequency set by the RT resistor. The frequency foldback is implemented only in the average current

limit condition. and it does not affect 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 programmable 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 during 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 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.

The overload timer function is very versatile and can be configured for the following modes of protection:

1. Continuous Current limit only: The hiccup mode can be completely disabled by connecting a zero to 50

k

Ω 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 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 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.

Copyright © 2010–2013, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LM5039