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ADM696 Datasheet(PDF) 5 Page - Analog Devices

Part No. ADM696
Description  Microprocessor Supervisory Circuits
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ADM696 Datasheet(HTML) 5 Page - Analog Devices

 
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ADM696/ADM697
REV. 0
–5–
Low Line RESET OUTPUT
RESET
is an active low output which provides a RESET signal
to the microprocessor whenever the Low Line Input (LLIN) is
below 1.3 V. The LLIN input is normally used to monitor the
power supply voltage. An internal timer holds RESET low for
50 ms after the voltage on LLIN rises above 1.3 V. This is in-
tended as a power-on RESET signal for the processor. It 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.
The LLIN comparator has approximately 12 mV of hysteresis
for enhanced noise immunity.
In addition to RESET, an active high RESET output is also
available. This is the complement of RESET and is useful for
processors requiring an active high RESET.
t
1
t
1
= RESET TIME
V1 = RESET VOLTAGE THRESHOLD LOW
V2 = RESET VOLTAGE THRESHOLD HIGH
HYSTERESIS = V2–V1
V2
V2
V1
V1
t
1
LL
IN
LOW LINE
RESET
Figure 2. Power-Fail Reset Timing
Watchdog Timer RESET
The watchdog timer circuit monitors the activity of the micro-
processor in order to check that it is not stalled in an indefinite
loop. An output line on the processor is used to toggle the
Watchdog Input (WDI) line. If this line is not toggled within
the selected timeout period, a RESET pulse is generated. The
ADM696/ADM697 may be configured for either a fixed
“short” 100 ms or a “long” 1.6 second timeout period or for an
adjustable timeout period. If the “short” period is selected some
systems may be unable to service the watchdog timer immedi-
ately after a reset, so a “long” timeout is automatically initiated
directly after a reset is issued. The watchdog timer is restarted
at the end of Reset, whether the Reset was caused by lack of ac-
tivity on WDI or by LLIN falling below the reset threshold.
The normal (short) timeout period becomes effective following
the first transition of WDI after RESET has gone inactive. The
watchdog timeout period restarts with each transition on the
WDI pin. To ensure that the watchdog timer does not time out,
either a high-to-low or low-to high transition on the WDI pin
must occur at or less than the minimum timeout period. If WDI
remains permanently either high or low, reset pulses will be is-
sued after each timeout period (1.6 s). The watchdog monitor
can be deactivated by floating the Watchdog Input (WDI) or by
connecting it to midsupply.
CIRCUIT INFORMATION
Battery-Switchover Section (ADM696)
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.
During normal operation with VCC higher than VBATT, VCC is
internally switched to VOUT via an internal PMOS transistor
switch. This switch has a typical on resistance of 1.5
Ω and can
supply up to 100 mA at the VOUT terminal. VOUT is normally
used to drive a RAM memory bank which may require instanta-
neous currents of greater than 100 mA. If this is the case, then
a bypass capacitor should be connected to VOUT. The capacitor
will provide the peak current transients to the RAM. A capaci-
tance value of 0.1
µF or greater may 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 may be connected in parallel
with the internal transistor. The BATT ON output can directly
drive the base of the external transistor.
A 20
Ω MOSFET switch connects the V
BATT 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 backup of CMOS RAM or other low power
CMOS circuitry. The supply current in battery backup is typi-
cally 0.6
µA.
The ADM696 operates with battery voltages from 2.0 V to VCC
–0.3 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 max) flows out of the V
BATT terminal. This cur-
rent 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 since 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.
VBATT
VCC
BATT ON
(ADM691, ADM693,
ADM695, ADM696)
VOUT
700
mV
GATE DRIVE
100
mV
INTERNAL
SHUT DOWN SIGNAL
WHEN
VBATT > (VCC + 0.7V)
Figure 1. Battery Switchover Schematic


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