Data Sheet

ADM8641/ADM8642

Rev. D | Page 9 of 11

THEORY OF OPERATION

The ADM8641 and ADM8642 ultralow power voltage detectors

are especially suited for battery-powered applications due to the

190 nA quiescent current (maximum). The internal precision

reference allows the user to monitor specific voltage levels accur-

ately from 0.5 V to 4.63 V. These devices feature internal input

hysteresis and an open-drain output. The output remains logic

high after the monitored input is above the preset threshold. The

output changes to logic low after the input voltage falls below

the threshold. The devices keep the output in a logic low state

whenever the supply voltage on the VCC pin is below the

UVLO threshold. The output disable input can also keep the

output low regardless of the status on the input.

ADM8641

MICRO-

PROCESSOR

VCC

OUT

INPUT

GND

DIS

3.3V

VIO

Figure 17. ADM8641 Typical Application Circuit

ADM8642

MICRO-

PROCESSOR

VCC

VIN

OUT

GND

12V

3.3V

INPUT

DIS

Figure 18. ADM8642 Typical Application Circuit

VOLTAGE MONITORING INPUT

The VCC pin on the ADM8641 acts as both a device power

input node and a voltage monitoring input node. The ADM8642

uses separate pins for supply and voltage monitoring to achieve

a low voltage monitoring threshold to 0.5 V. It is recommended

to place a 0.1 μF decoupling capacitor between the VCC pin

and the GND pin.

VCC Slew Rate Consideration

A fast VCC ramp (μs range) on power-up can cause the device

to behave in an irregular manner. In applications where a high

slew rate on VCC is possible, for example, powering up using a

battery pack, it is recommended that an RC filter is used to

reduce the slew rate. RC combinations of 3.3 kΩ + 2.2 μF and

1 kΩ + 10 μF have been tested and identified as safe options.

VIN AS AN ADJUSTABLE INPUT

Due to the low leakage nature of the VIN pin, the ADM8642 can

be used as a device with an adjustable threshold. Use an

external resistor divider circuit to program the desired voltage

monitoring threshold based on the VIN threshold, as shown in

Figure 19.

ADM8642

MICRO-

PROCESSOR

VCC

VIN

OUT

GND

0.8V

3.3V

INPUT

DIS

Figure 19. ADM8642 as an Adjustable Threshold Device

TRANSIENT IMMUNITY

To avoid unnecessary output state changes caused by fast

power supply transients, an input glitch filter is added to the

VCC pin of the ADM8641 and the VIN pin of the ADM8642 to

filter out the transient glitches on these pins.

Figure 11 shows the comparator overdrive (that is, the maximum

magnitude of positive and negative going pulses with respect to

the typical threshold) vs. the pulse duration without changing

the state of the output.

OUTPUT

Both the ADM8641 and ADM8642 voltage detectors have an

open-drain output. For the ADM8641, the state of the output is

ADM8642, the output is guaranteed to be logic low from when

When the monitored voltage falls below its associated threshold,

the OUT pin asserts low after 23 μs to 26 μs (typical). When the

monitored voltage rises above the threshold plus hysteresis, the

OUT pin asserts high after 36 μs to 39.5 μs (typical).

DISABLE INPUT

The ADM8641/ADM8642 feature a disable input (DIS). Drive the

DIS pin low to assert the output low. The DIS input has a 0.6 MΩ

internal pull-up resistor so that the input is always high when

unconnected. To drive the DIS input, use an external signal or

a push-button switch to ground; debounce circuitry is integrated

on-chip for this purpose. Noise immunity is provided on the

DIS input, and fast, negative going transients of up to 0.4 μs

(typical) are ignored. If required, a 0.1 μF capacitor between

the DIS pin and ground provides additional noise immunity.

VCC

OUT

DIS EXTERNALLY

DRIVEN LOW

DIS

Figure 20. DIS Input Timing