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ADM690ADM695 Datasheet(PDF) 5 Page - Analog Devices

Part No. ADM690ADM695
Description  Microprocessor Supervisory Circuits
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ADM690ADM695 Datasheet(HTML) 5 Page - Analog Devices

 
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ADM690–ADM695
REV. A
–5–
PIN CONFIGURATIONS
PRODUCT SELECTION GUIDE
Part
Nominal Reset
Nominal VCC
Nominal Watchdog
Battery Backup
Base Drive
Chip Enable
Number
Time
Reset Threshold
Timeout Period
Switching
Ext PNP
Signals
ADM690
50 ms
4.65 V
1.6 s
Yes
No
No
ADM691
50 ms or ADJ
4.65 V
100 ms, 1.6 s, ADJ
Yes
Yes
Yes
ADM692
50 ms
4.4 V
1.6 s
Yes
No
No
ADM693
50 ms or ADJ
4.4 V
100 ms, 1.6 s, ADJ
Yes
Yes
Yes
ADM694
200 ms
4.65 V
1.6 s
Yes
No
No
ADM695
200 ms or ADJ
4.65 V
100 ms, 1.6 s, ADJ
Yes
Yes
Yes
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.
Figure 1. Battery Switchover Schematic
During normal operation with VCC higher than VBATT, VCC is in-
ternally 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 (ADM691/
ADM693/ADM695) 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 back up of CMOS RAM or other low
power CMOS circuitry. The supply current in battery back up
is typically 0.6
µA.
The ADM690/ADM691/ADM694/ADM695 operates with
battery voltages from 2.0 V to 4.25 V and the ADM692/ADM693
operates 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 back
up. A small charging current of typically 10 nA (0.1
µA max)
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 back up battery by compensating
for its self discharge current. Also note that this current poses
no problem when lithium batteries are used for back up 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.
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
TOP VIEW
(Not to Scale)
ADM691
ADM693
ADM695
GND
V
BATT
V
OUT
PFI
PFO
WDO
V
CC
RESET
BATT ON
LOW LINE
OSC IN
OSC SEL
RESET
CE
IN
CE
OUT
WDI
GND
V
BATT
V
OUT
PFI
PFO
WDI
RESET
V
CC
1
2
3
4
8
7
6
5
TOP VIEW
(Not to Scale)
ADM690
ADM692
ADM694


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