LM5035ASQ datasheet(24/30 Pages) NSC

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30034137

FIGURE 14. Latched Load Over-Voltage Protection

Figure 15 shows an application of the OVP comparator for

Remote Thermal Protection using a thermistor (or multiple

thermistors) which may be located near the main heat

sources of the power supply. The negative temperature co-

efficient (NTC) thermistor is nearly logarithmic, and in this

example a 100k

Ω thermistor with the β material constant of

4500 kelvins changes to approximately 2 k

Ω at 130°C. Setting

R1 to one-third of this resistance (665

Ω) establishes 130°C

as the desired trip point (for V

REF = 5V). In a temperature band

from 20°C below to 20°C above the OVP threshold, the volt-

age divider is nearly linear with 25 mV per°C sensitivity.

R2 provides temperature hysteresis by raising the OVP com-

parator input by R2 x 23 µA. For example, if a 22k

Ω resistor

is selected for R2, then the OVP pin voltage will increase by

22 k

Ω x 23 µA = 506 mV. The NTC temperature must there-

fore fall by 506mV / 25mV per°C = 20°C before the LM5035A

switches from the standby mode to the normal mode.

30034138

FIGURE 15. Remote Thermal Protection

HICCUP MODE CURRENT LIMIT RESTART (RES)

The basic operation of the hiccup mode current limit restart is

described in the functional description. The delay time to

restart is programmed with the selection of the RES pin ca-

pacitor C

RES as illustrated in Figure 15.

In the case of continuous cycle-by-cycle current limit detec-

tion at the CS pin, the time required for C

RES to reach the 2.5V

hiccup mode threshold is:

For example, if C

RES = 0.01 µF the time t1 is approximately

1.14 ms.

The cool down time, t2 is set by the soft-start capacitor (C

SS)

and the internal 1 µA SS current source, and is equal to:

If C

SS = 0.01 µF t2 is ≊10 ms.

The soft-start time t3 is set by the internal 110 µA current

source, and is equal to:

If C

SS = 0.01 µF t3 is ≊363 µs.

The time t2 provides a periodic cool-down time for the power

converter in the event of a sustained overload or short circuit.

This off time results in lower average input current and lower

power dissipation within the power components. It is recom-

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