7

FN8092.3

December 6, 2006

Low Voltage Monitoring

auxiliary voltage on PFI.

voltage is greater than the specific IC fixed-voltage trip point

again asserted, i.e. RST is high and RST is low.

Power Failure Monitor

These devices also have a Power-Failure Monitor that helps

to monitor an additional critical voltage on the Power-Fail

Input (PFI) pin. For example, the PFI pin could be used to

provide an early power-fail warning, detect a low-battery

condition, presence detection or simply monitor a power

supply other than +5V. The 1.25V threshold detector can be

adjusted using an external resistor divider network to provide

custom voltage monitoring of voltages greater than 1.25V,

according to the following formula (See Figure 1):

PFO goes low whenever PFI is less than the 1.25V (or user-

set) threshold voltage.

On the ISL88707 and ISL88708, users can adjust the Power

ground (see Figure 3). For example, connecting a 50pF

Care should be taken in PCB layout and capacitor placement in

order to reduce stray capacitance as much as possible, which

PFI

ISL8870X

FIGURE 2. POWER-SUPPLY MONITORING TIMING DIAGRAM (WDI TRI-STATED)

MR

RST

1V

RST

ISL88707, ISL88708

0

2

4

6

8

10

12

14

0

10

20

3040

50

6070

80

90

100

OPEN = 200ms

ISL88705, ISL88706, ISL88707, ISL88708, 8SL88716, ISL88813