Data Sheet ADM8641/ADM8642
Rev. B | 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 ref-
erence allows the user to monitor specific voltage levels accurately
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.
Figure 17. ADM8641 Typical Application Circuit
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.
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.
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
guaranteed to be valid as soon as V
CC
rises above 0.9 V. For the
ADM8642, the output is guaranteed to be logic low from when
V
CC
= 0.9 V to when the device exits ULVO.
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.
Figure 20.
DIS
Input Timing
ADM8641
MICRO-
PROCESSOR
VCC
OUT
INPUT
GND
DIS
3.3V
VIO
12781-117
ADM8642
MICRO-
PROCESSOR
VCC
VIN
OUT
GND
12V
3.3V
INPUT
V
IO
DIS
12781-118
ADM8642
MICRO-
PROCESSOR
VCC
VIN
OUT
GND
0.8V
3.3V
INPUT
V
IO
DIS
12781-119
VCC
OUT
V
TH
V
TH
+ V
HYST
t
PD_VCC
DIS EXTERNALLY
DRIVEN LOW
DIS
t
PD_VCC
t
D_DIS
t
D_DIS
12781-017
