LM5035 datasheet(23/28 Pages) NSC

23 / 28 page

Applications Information (Continued)

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 2k

Ω at 130˚C. Set-

ting R1 to one-third of this resistance (665

Ω) establishes

130˚C as the desired trip point (for V

REF = 5V). In a tempera-

ture band from 20˚C below to 20˚C above the OVP thresh-

old, the voltage divider is nearly linear with 25mV per˚C

sensitivity.

R2 provides temperature hysteresis by raising the OVP com-

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

Ω resistor

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

22k

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

fall by 506mV / 25mV per˚C = 20˚C before the LM5035

switches from the standby mode to the normal mode.

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

capacitor C

RES as illustrated in Figure 15.

20177537

FIGURE 14. Latched Load Over-Voltage Protection

20177538

FIGURE 15. Remote Thermal Protection

LM5035 Datasheet(PDF) 23 Page - National Semiconductor (TI)