Datasheet

LM5088, LM5088-Q1

SNVS600H –DECEMBER 2008–REVISED MARCH 2013

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Overload Protection Timer (LM5088-2 Only)

To further protect the external circuitry during a prolonged over current condition, the LM5088-2 provides a

current limit timer to disable the switching regulator and provide a delay before restarting (hiccup mode). The

number of current limit events required to trigger the restart mode is programmed by an external capacitor at the

RES pin. During each PWM cycle, as shown in Figure 22, the LM5088 either sinks current from or sources

current into the RES capacitor. If the emulated current ramp exceeds the 1.2V current limit threshold, the present

PWM cycle is terminated and the LM5088 sources 50 µA into the RES pin capacitor during the next PWM clock

cycle. If a current limit event is not detected in a given PWM cycle, the LM5088 disables the 50 µA source

current and sinks 27 µA from the RES pin capacitor during the next cycle. In an overload condition, the LM5088

protects the converter with cycle-by-cycle current limiting until the voltage at RES pin reaches 1.2V. When RES

reaches 1.2V, a hiccup mode sequence is initiated as follows:

• The SS capacitor is fully discharged.

• The RES capacitor is discharged with 1.2 µA

• Once the RES capacitor reaches 0.2V, a normal soft-start sequence begins. This provides a time delay

before restart.

• If the overload condition persists after restart, the cycle repeats.

• If the overload condition no longer exists after restart, the RES pin is held at ground by the 27 µA discharge

current source and normal operation resumes.

The overload protection timer 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 the RES pin to GND. In

this configuration, the cycle-by-cycle protection will limit the output current indefinitely and no hiccup

sequence will occur.

2. Delayed Hiccup: Connecting a capacitor to the RES pin provides a programmed number of cycle-by-cycle

current limit events before initiating a hiccup mode restart, as previously described. The advantage of this

configuration is 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.

3. Externally Controlled Hiccup: The RES pin can also be used as an input. By externally driving the pin to a

level greater than the 1.2V 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 or an output over-voltage sensor.

Product Folder Links: LM5088 LM5088-Q1