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ADM8691ARN Datasheet(PDF) 5 Page - Analog Devices

Part No. ADM8691ARN
Description  Microprocessor Supervisory Circuits
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ADM8691ARN Datasheet(HTML) 5 Page - Analog Devices

 
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ADM8690–ADM8695
REV. 0
–5–
PIN CONFIGURATIONS
1
2
3
4
8
7
6
5
TOP VIEW
(Not to Scale)
ADM8690
ADM8692
ADM8694
VOUT
PFO
WDI
RESET
VBATT
VCC
GND
PFI
14
13
12
11
16
15
10
9
8
1
2
3
4
7
6
5
TOP VIEW
(Not to Scale)
ADM8691
ADM8693
ADM8695
VBATT
CEIN
WDO
RESET
RESET
VOUT
VCC
GND
PFO
WDI
CEOUT
BATT ON
LOW LINE
OSC IN
OSC SEL
PFI
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.
VCC
VBATT
VOUT
BATT ON
(ADM8690,
ADM8695)
100
mV
700
mV
INTERNAL
SHUTDOWN SIGNAL
WHEN
VBATT > (VCC + 0.7V)
GATE DRIVE
Figure 1. Battery Switchover Schematic
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 0.7
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 (ADM8691/
ADM8693/ADM8695) can directly drive the base of the exter-
nal transistor.
A 7
Ω MOSFET switch connects the V
BATT input to VOUT dur-
ing battery backup. This MOSFET has very low input-to-out-
put 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 typi-
cally 0.4
µA.
The ADM8690/ADM8691/ADM8694/ADM8695 operates with
battery voltages from 2.0 V to 4.25 V, and the ADM8692/
ADM8693 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 backup. A small charging current of typically 10 nA
(0.1
µA max) 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 com-
pensating for its self discharge current. Also note that this cur-
rent 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.
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
ADM8690
50 ms
4.65 V
1.6 s
Yes
No
No
ADM8691
50 ms or ADJ
4.65 V
100 ms, 1.6 s, ADJ
Yes
Yes
Yes
ADM8692
50 ms
4.4 V
1.6 s
Yes
No
No
ADM8693
50 ms or ADJ
4.4 V
100 ms, 1.6 s, ADJ
Yes
Yes
Yes
ADM8694
200 ms
4.65 V
1.6 s
Yes
No
No
ADM8695
200 ms or ADJ
4.65 V
100 ms, 1.6 s, ADJ
Yes
Yes
Yes


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