Philips Semiconductors

PIP401

OR-ing Intelligent Switch

Product data

Rev. 03 — 5 November 2002

11 of 16

9397 750 10578

© Koninklijke Philips Electronics N.V. 2002. All rights reserved.

11.3 Overtemperature detection

Figure 8 shows how the thermal flag output of the PIP401 is used to shut down a

power supply in the event of overtemperature. In this circuit, there are pull-up

resistors from TFL to both the 5 V and 12 V supplies.

11.3.1

On-state

The overtemperature flag can be used to detect excessive load current. The

overtemperature flag will operate when the power dissipation due to load current is

sufficient to raise the junction temperature to approximately 160

°C. The typical

on-state voltage of the PIP401 at 25

°C is 70 mV. To calculate the on-state voltage at

160

°C, the 25 °C value is multiplied by the normalized on-state voltage factor given

in Figure 6. This factor is 1.6 giving an on-state voltage at 160

°C of 112 mV. Using a

thermal resistance from junction to ambient of 15 K/W and an ambient temperature of

55

°C, the overtemperature flag will operate when the current exceeds:

(1)

One possible use of the overtemperature flag is to detect failure of the cooling fan, in

applications where forced air cooling is used.

If the thermal resistance from junction to ambient is higher, then the temperature flag

will operate at a lower value of output current.

11.3.2

Off-state

cause the PIP401 to turn off, and the voltage drop to increase from a few millivolts to

approximately 0.7 V. The increased dissipation in the PIP401 will cause its junction

temperature to rise. Taking a junction to ambient thermal resistance of 15 K/W and an

ambient temperature of 55

°C, the overtemperature flag will operate when the

dissipation in the PIP401 is greater than:

(2)

Fig 8.

Overtemperature detection.

VCC

VRS

GND

LOAD

03ah74

5 V

SUPPLY

shutdown

12 V

SUPPLY

VO

VI

2.2

k

Ω

1

k

Ω

TFL

I

O

T

TFL

T

amb

–

∆V

ON

R

th j

a

–

()

×

-----------------------------------------

160

55

–

0.112

15

×

-------------------------

62.5 A

==

=

P

tot

160

55

–

15

---------------------

7W

==