5

FN8227.2

June 9, 2009

variety of applications needing multi-voltage monitoring.

See Figure 1 for the typical application diagram..

Low Voltage Monitoring

During normal operation, the ISL88031 monitors the voltage

voltage on any of these five inputs falls below their respective

voltage trip points, a reset is asserted (RST = low) to prevent

the microprocessor from operating during a power failure or

brownout condition. This reset signal remains low until the

voltages exceeds the voltage threshold settings for the reset

The ISL88031 allows users to customize the minimum voltage

sense level for three of the five monitored voltages. For

for V3MON, V4MON and V5MON inputs. To do this, connect

an external resistor divider network to the VxMON pin in order

to set the trip point to some other voltage above 600mV

according to Equation 1:

Power-On Reset (POR)

Applying power to the ISL88031 activates a POR circuit, which

makes the reset pin(s) active (i.e. RST goes high while RST

goes low). These signals provide several benefits:

• It prevents the system microprocessor from starting to

operate with insufficient voltage.

• It prevents the processor from operating prior to stabilization

of the oscillator.

• It ensures that the monitored device is held out of operation

until internal registers are properly loaded.

• It allows time for an FPGA to download its configuration prior

to initialization of the circuit.

ensures that the supply voltage has stabilized to sufficient

operating levels.

Manual Reset

The manual-reset input (MR) allows the user to trigger a reset

by using a push-button switch or by signaling the input low. The

MR input is an active low debounced input. Reset is asserted if

the MR pin is pulled low to less than 100mV for the minimum

MR pulse width or longer while the push-button is closed. After

(200ms) and then is released.

Figures 2 and 3 illustrate the ISL88031’s operation. Figure 4

shows the ISL88031EVAL1, the evaluation platform for this

family of voltage monitors. Figures 5 and 6 illustrate the RST

output response times.

The ISL88031EVAL1 and Applications

The ISL88031EVAL1 supports all variants of the ISL88031

devices, enabling evaluation of basic functional operation

and common application implementations. Figure 4

illustrates the ISL88031EVAL1 in schematic and

photographic forms. The ISL88031EVAL1 has two isolated

circuits; the left circuit is populated with the

to customize to provide a specific voltage monitoring solution

with the accompanying loose packed variants.

With adequate bias on the two preset and the three

adjustable monitor inputs, the RST output will release to pull

high indicating that all supplies are compliant for a minimum

V2MON nominal thresholds are as previously noted with the

voltage thresholds being monitored by V3MON, V4MON and

V5MON being nominally 1.990V, 1.44V and 0.95V

respectively.

Special Application Considerations

Using good decoupling practices on bias and other

monitoring inputs will prevent transients (i.e. due to switching

noises and short duration droops in the supply voltage) from

causing unwanted resets.

In unusually noisy environments or situations where

unwanted signals may be injected into adjustable VMONx

pins, lowering the node impedance and/or positioning a

small valued filter capacitor as close to the pin as possible

can increase noise immunity.

Although the internal ISL88031 threshold references are

guaranteed over the full temp range, accuracy errors due to

external component tolerances and distribution losses will

occur. High tolerance resistors and layout for extreme

accuracy and critical performance must be considered.

FIGURE 1. TYPICAL APPLICATION DIAGRAM

V3MON

MR

PB

GND

RST

RESET

V2MON

ISL88031

SIGNAL

V4MON

V5MON

V

TRIP

0.6V

=

R

1

R

+

×

R

2

(EQ. 1)

ISL88031