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FN8092.5

January 12, 2009

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 (see

Figure 1).

.

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 Equation 1 (see Figure 2).

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

user-set) threshold voltage.

.

If using a voltage divider on the PFI input to critique an

external voltage and intending to use the MR input to initiate

resets then avoid having the PFI voltage less than PFI Vth

+2.2V as unintended PFO transition may occur when MR is

transitioning high.

On the ISL88707 and ISL88708, users can adjust the

200ms to ~1.4s. Care should be taken in PCB layout and

capacitor placement in order to reduce stray capacitance as

FIGURE 1. POWER-SUPPLY MONITORING TIMING DIAGRAM (WDI TRI-STATED)

MR

RST

1V

RST

PFI V

TH

1.25

R

1

R

2

+

R

2

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

⎝⎠

⎜⎟

⎛⎞

=

(EQ. 1)

PFI

ISL8870x

ISL88705, ISL88706, ISL88707, ISL88708, ISL88716, ISL88813