Data Sheet ADM8690/ADM8691/ADM8695
Rev. C | Page 11 of 24
CIRCUIT INFORMATION
BATTERY SWITCHOVER SECTION
The battery switchover circuit compares V
CC
to the V
BATT
input and connects V
OUT
to whichever is higher. Switchover
occurs when V
CC
is 50 mV higher than V
BATT
as V
CC
falls, and
when V
CC
is 70 mV greater than V
BATT
as V
CC
rises. This 20 mV
hysteresis prevents repeated rapid switching if V
CC
falls very
slowly or remains nearly equal to the battery voltage.
GATE DRIVE
V
CC
BATT
V
OUT
BATT ON
(ADM8691,
ADM8695)
100
mV
700
mV
INTERNAL
SHUTDOWN SIGNAL
WHEN
V
BATT
> (V
CC
+ 0.7V)
0093-005
Figure 14. Battery Switchover Schematic
During normal operation, with V
CC
higher than V
BATT
, V
CC
is internally switched to V
OUT
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 V
OUT
terminal. V
OUT
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 V
OUT
. 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 requirements at V
OUT
exceed
100 mA or if a lower V
CC
− V
OUT
voltage differential is desired,
an external PNP pass transistor can be connected in parallel
with the internal transistor. The BATT ON output (ADM8691/
ADM8695) can directly drive the base of the external transistor
(see Figure 24).
A 7 Ω MOSFET switch connects the V
BATT
input to V
OUT
during
battery backup. This MOSFET has very low input-to-output
differential (dropout voltage) at the low current levels required
for battery backup of CMOS RAM or other low power CMOS
circuitry. The supply current in battery backup is typically 0.4 μA.
The ADM8690/ADM8691/ADM8695 operate with battery
voltages from 2.0 V to 4.25 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 V
BATT
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, V
BATT
should be
connected to GND and V
OUT
should be connected to V
CC
.
POWER-FAIL RESET OUTPUT
RESET
is an active low output that provides a
RESET
signal to
the microprocessor whenever V
CC
is at an invalid level. When
V
CC
falls below the reset threshold, the
RESET
output is forced
low. The nominal reset voltage threshold is 4.65 V.
RESET
LOW LINE
V1
V2V2
V1
V
CC
t
1
t
1
t
1
= RESET TIME
V1 = RESET VOLTAGE THRESHOLD LOW
V2 = RESET VOLTAGE THRESHOLD HIGH
HYSTERESIS = V2 – V1
00093-006
Figure 15. Power-Fail Reset Timing
On power-up,
RESET
remains low for 50 ms (200 ms for the
) after V
CC
rises above the appropriate reset thresh-
old. This allows time for the power supply and microprocessor
to stabilize. On power-down, the
ADM8695
RESET
output remains low
with V
CC
as low as 1 V. This ensures that the microprocessor is
held in a stable shutdown condition.
The
RESET
active time is adjustable on the /
by using an external oscillator or by connecting an
external capacitor to the OSC IN pin. See and
through .
ADM8691
ADM8695
Table 5 Figure 17
Figure 20
The guaranteed minimum and maximum reset thresholds for
the ADM8690/ADM8691/ADM8695 are 4.5 V and 4.73 V. The
ADM8690/ADM8691/ADM8695 are, therefore, compatible with
5 V supplies with a +10%, −5% tolerance. The reset threshold
comparator typically has 40 mV of hysteresis. The response time
of the reset voltage comparator is less than 1 μs. If glitches are
present on the V
CC
line that could cause spurious reset pulses,
V
CC
should be decoupled close to the device.
In addition to
RESET
, the / provide an
active high RESET output. This output is the complement of
ADM8691 ADM8695
RESET
and is intended for processors that require an active
high reset signal.
