ADM8691 Analog Devices, ADM8691 Datasheet - Page 11

• Manufacturer Part Number: ADM8691
• Manufacturer: Analog Devices

Specifications of ADM8691

• Product Description: µP Supv w/ Backup Batt Switch, Adj Reset Period, Adj Watchdog Period, 4.65V Trip point, 100 mA Iout
• Supply Current: 200µA
• Us Price 1000-4999: n/a
• Display In Ist: Yes
• Active For Param Search: Yes

Data Sheet
CIRCUIT INFORMATION
BATTERY SWITCHOVER SECTION
The battery switchover circuit compares V
input and connects V
occurs when V
when V
hysteresis prevents repeated rapid switching if V
slowly or remains nearly equal to the battery voltage.
During normal operation, with V
is internally switched to V
sistor switch. This switch has a typical on resistance of 0.7 Ω
and can supply up to 100 mA at the V
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
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
100 mA or if a lower V
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
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
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
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.
V
BATT
ADM8690/ADM8691/ADM8695
V
CC
CC
is 70 mV greater than V
700
mV
CC
Figure 14. Battery Switchover Schematic
is 50 mV higher than V
100
mV
GATE DRIVE
OUT
CC
to whichever is higher. Switchover
− V
OUT
OUT
through an internal PMOS tran-
INTERNAL
SHUTDOWN SIGNAL
WHEN
V
voltage differential is desired,
BATT
CC
BATT
higher than V
> (V
OUT
as V
BATT
operate with battery
CC
BATT
terminal. V
+ 0.7V)
CC
CC
input to V
to the V
as V
rises. This 20 mV
CC
CC
OUT
BATT
V
BATT ON
(ADM8691,
ADM8695)
falls very
OUT
OUT
falls, and
. The
OUT
OUT
BATT
, V
exceed
BATT
CC
is
during
Rev. C | Page 11 of 24
If the battery switchover section is not used, V
connected to GND and V
POWER-FAIL RESET OUTPUT
RESET is an active low output that provides a RESET signal to
the microprocessor whenever V
V
low. The nominal reset voltage threshold is 4.65 V.
LOW LINE
On power-up, RESET remains low for 50 ms (200 ms for the
ADM8695
old. This allows time for the power supply and microprocessor
to stabilize. On power-down, the
with V
held in a stable shutdown condition.
The RESET active time is adjustable on the
ADM8695
external capacitor to the OSC IN pin. See
through
The guaranteed minimum and maximum reset thresholds for
the
ADM8690/ADM8691/ADM8695
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
V
In addition to RESET , the
active high RESET output. This output is the complement of
RESET and is intended for processors that require an active
high reset signal.
CC
CC
RESET
ADM8690/ADM8691/ADM8695
falls below the reset threshold, the RESET output is forced
should be decoupled close to the device.
V
CC
CC
Figure 20
as low as 1 V. This ensures that the microprocessor is
) after V
by using an external oscillator or by connecting an
V2
CC
ADM8690/ADM8691/ADM8695
t
Figure 15. Power-Fail Reset Timing
.
1
t 1 = RESET TIME
V1 = RESET VOLTAGE THRESHOLD LOW
V2 = RESET VOLTAGE THRESHOLD HIGH
HYSTERESIS = V2 – V1
CC
line that could cause spurious reset pulses,
rises above the appropriate reset thresh-
OUT
ADM8691 ADM8695
V1
should be connected to V
CC
RESET output remains low
are, therefore, compatible with
is at an invalid level. When
/
V2
are 4.5 V and 4.73 V. The
t
Table 5
1
ADM8691
BATT
provide an
and
should be
Figure 17
/
CC
.
V1