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ADM8692 Datasheet(PDF) 10 Page - Analog Devices

Part No. ADM8692
Description  Microprocessor Supervisory Circuits
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ADM8692 Datasheet(HTML) 10 Page - Analog Devices

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ADM8690/ADM8691/ADM8692/ADM8693/ADM8695
Rev. B | Page 10 of 20
CIRCUIT INFORMATION
BATTERY SWITCHOVER SECTION
The battery switchover circuit compares VCC to the VBATT input,
and connects VOUT to whichever is higher. Switchover occurs
when VCC is 50 mV higher than VBATT as VCC falls, and when VCC
is 70 mV greater than VBATT as VCC rises. This 20 mV of
hysteresis prevents repeated rapid switching if VCC falls very
slowly or remains nearly equal to the battery voltage.
GATE DRIVE
VCC
VBATT
VOUT
BATT ON
(ADM8690,
ADM8695)
100
mV
700
mV
INTERNAL
SHUTDOWN SIGNAL
WHEN
VBATT > (VCC + 0.7V)
Figure 14. Battery Switchover Schematic
During normal operation, with VCC higher than VBATT, VCC
is internally switched to VOUT through an internal PMOS tran-
sistor switch. This switch has a typical on resistance of 0.7 Ω
and can supply up to 100 mA at the VOUT terminal. VOUT is
normally used to drive a RAM memory bank, requiring
instantaneous currents of greater than 100 mA. If this is the
case, a bypass capacitor should be connected to VOUT. The
capacitor provides the peak current transients to the RAM.
A capacitance value of 0.1 μF or greater can be used.
If the continuous output current requirement at VOUT exceeds
100 mA, or if a lower VCC − VOUT voltage differential is desired,
an external PNP pass transistor can be connected in parallel
with the internal transistor. The BATT ON output (ADM8691/
ADM8693/ADM8695) can directly drive the base of the
external transistor.
A 7 Ω MOSFET switch connects the VBATT input to VOUT during
battery backup. This MOSFET has very low input-to-output
differential (dropout voltage) at the low current levels required
for battery back up of CMOS RAM or other low power CMOS
circuitry. The supply current in battery back up is typically 0.4 μA.
The ADM8690/ADM8691/ADM8695 operate with battery
voltages from 2.0 V to 4.25 V, and the ADM8692/
ADM8693 operate with battery voltages from 2.0 V to 4.0 V.
High value capacitors, either standard electrolytic or the farad-
size, double-layer capacitors, can also be used for short-term
memory backup. A small charging current of typically 10 nA
(0.1 μA maximum) flows out of the VBATT terminal. This current
is useful for maintaining rechargeable batteries in a fully
charged condition. This extends the life of the backup battery by
compensating for its self-discharge current. Also note that this
current poses no problem when lithium batteries are used for
backup because the maximum charging current (0.1 μA) is safe
for even the smallest lithium cells.
If the battery switchover section is not used, VBATT should be
connected to GND and VOUT should be connected to VCC.
POWER-FAIL RESET OUTPUT
RESET is an active low output that provides a RESET signal to
the microprocessor whenever VCC is at an invalid level. When
VCC falls below the reset threshold, the RESET output is forced
low. The nominal reset voltage threshold is 4.65 V (ADM8690/
ADM8691/ADM8695) or 4.4 V (ADM8692/ADM8693).
RESET
LOW LINE
V1
V2
V2
V1
VCC
t1
t1
t1 = RESET TIME
V1 = RESET VOLTAGE THRESHOLD LOW
V2 = RESET VOLTAGE THRESHOLD HIGH
HYSTERESIS = V2–V1
Figure 15. Power-Fail Reset Timing
On power-up, RESET remains low for 50 ms (200 ms for
ADM8695) after VCC rises above the appropriate reset threshold.
This allows time for the power supply and microprocessor to
stabilize. On power-down, the RESET output remains low with
VCC as low as 1 V. This ensures that the microprocessor is held
in a stable shutdown condition.
This RESET active time is adjustable on the ADM8691/ADM8693/
ADM8695 by using an external oscillator or by connecting an
external capacitor to the OSC IN pin. Refer to Table 5 and
Figure 17, Figure 18, Figure 19, and Figure 20.
The guaranteed minimum and maximum thresholds of the
ADM8690/ADM8691/ADM8695 are 4.5 V and 4.73 V, and the
guaranteed thresholds of the ADM8692/ADM8693 are 4.25 V and
4.48 V. The ADM8690/ADM8691/ADM8695 are, therefore,
compatible with 5 V supplies with a +10%, −5% tolerance and
the ADM8692/ADM8693 are compatible with 5 V ± 10%
supplies. The reset threshold comparator has approximately
50 mV of hysteresis. The response time of the reset voltage
comparator is less than1 μs. If glitches are present on the VCC line
that could cause spurious reset pulses, VCC should be decoupled
close to the device.


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