LM5021 datasheet(11/23 Pages) TI

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Toverload =

CSS x (VSS_OCV - VHC)

IOVCS

CSS x 0.6V

10 PA

Toff =

CSS x (VHC - VRST)

IDTCS

CSS x (4.6V - 0.3V)

0.25 PA

LM5021

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SNVS359D – MAY 2005 – REVISED MARCH 2013

Figure 10, Figure 11, and Figure 12 illustrate the equivalent circuit of the hiccup mode for LM5021 and the

relevant waveforms. During start-up and in normal operation, the external soft-start capacitor Css is pulled up by

a current source that delivers 22 µA to the SS pin capacitor. In normal operation, the soft-start capacitor

continues to charge and eventually reaches the saturation voltage of the current source (VSS_OCV, nominally

5.2V). During start-up the COMP pin voltage follows the SS capacitor voltage and gradually increases the peak

current delivered by the power supply. When the output of the switching power supply reaches the desired

voltage, the voltage feedback amplifier takes control of the COMP signal (via the opto-coupler). In normal

operation the COMP level is held at an intermediate voltage between 1.25V and 2.75V controlled by the voltage

regulation loop. When the COMP pin voltage is below 1.25V, the duty-cycle is zero. When the COMP level is

above 2.75V, the duty cycle will be limited by the 0.5V threshold of cycle-by-cycle current limit comparator.

If the output of the power supply is overloaded, the voltage regulation loop demands more current by increasing

the COMP pin control voltage. When the COMP pin exceeds the over voltage detection threshold (VOVLD,

nominally 4.6V), the SS capacitor Css will be discharged by a 10 µA overload detection timer current source,

IOVCS. If COMP remains above VOVLD long enough for the SS capacitor to discharge to the Hiccup mode

threshold (VHIC, nominally 4.6V), the controller enters the hiccup mode. The OUT pin is then latched low and the

SS capacitor discharge current source is reduced from 10 µA to 0.25 µA, the dead-time current source, IDTCS.

The SS pin voltage is slowly reduced until it reaches the Restart threshold (VRST, nominally 0.3V). Then a new

start-up sequence commences with 22 µA current source charging the capacitor CSS. The slow discharge of the

SS capacitor from the Hiccup threshold to the Restart threshold provides an extended off time that reduces the

overheating of components including diodes and MOSFETs due to the continuous overload. The off time during

the hiccup mode can be calculated from the following equation:

(8)

Example:

Toff = 808 ms, assuming the CSS capacitor value is 0.047 µF

Short duration intermittent overloads will not trigger the hiccup mode. The overload duration required to trigger

the hiccup response is set by the capacitor CSS, the 10 µA discharge current source and voltage difference

between the saturation level of the SS pin and the Hiccup mode threshold. Figure 12 shows the waveform of SS

pin with a short duration overload condition. The overload time required to enter the hiccup mode can be

calculated from the following equation:

(9)

Example:

Toverload = 2.82 ms, assuming the CSS capacitor value is 0.047 µF

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