Alliance Semiconductor
2575 Augustine Drive . Santa Clara, CA 95054 . Tel: 408.855.4900 . Fax: 408.855.4999 . www.alsc.com
Notice: The information in this document is subject to change without notice
rev 1.0
P Power Supply Supervisor With Battery Backup Switch
ASM690A / 692A
ASM802L / 802M
ASM805L
October 2003
General Description
The AS690A / AS692A / AS802L / AS802M / AS805L offers
complete single chip solutions for power supply monitoring and
control battery functions in microprocessor systems. Each
device implements four functions: Reset control, watchdog
monitoring, battery-backup switching and power-failure
monitoring. In addition to microprocessor reset under power-up
and power-down conditions, these devices provide battery-
backup switching to maintain control in power loss and brown-
out situations. Additional monitoring capabilities can provide an
early warning of unregulated power supply loss before the
voltage regulator drops out. The important features of these
four functions are:
1.6 second watchdog timer to keep microprocessor
responsive
4.40V or 4.65V V
CC
threshold for microprocessor reset at
power-up and power-down
SPDT (Single-pole, Double-throw) PMOS switch connects
backup power to RAM if V
CC
fails
1.25V threshold detector for power loss or general purpose
voltage monitoring
These features are pin-compatible with the industry standard
power-supply supervisors. Short-circuit and thermal protection
have also been added. The AS690A / AS802L / AS805L
generate a reset pulse when the supply voltage drops below
4.65V and the AS692A / AS802M generate a reset below
4.40V. The ASM802L / ASM802M have power-fail accuracy to
2%. The ASM805L is the same as the ASM690A except that
RESET is provided instead of RESET.
Features
Two precision supply-voltage monitor options
4.65V (AS690A / AS802L / AS805L)
4.40V (AS692A / AS802M )
Battery-backup power switch on-chip
Watchdog timer: 1.6 second timeout
Power failure / low battery detection
Short circuit protection and thermal limiting
Small 8-pin SO package
No external components
Specified over full temperature range
Applications
Embedded control systems
Portable/Battery operated systems
Intelligent instruments
Wireless instruments
Wireless communication systems
PDAs and hand-held equipments
P / C power supply monitoring
Safety system
Typical Operating Circuit
Block Diagram
BUS
+
_
R
1
R
2
0.1 F
3.6 V
Lithium
Battery
ASM690A
Regulated +5V
Unregulated DC
V
BATT
GND
V
OUT
WDI
PFO
PFI
RESET
V
CC
V
CC
RESET
GND
GND
NMI
I/O LINE
V
CC
CMOS
RAM
Battery-Switchover Circuit
Reset
Generator
Watchdog
Timer
V
BATT
V
CC
WDI
PFI
V
OUT
RESET
(RESET)
PFO
1.25V
3.5V
0.8V
1.25V
GND
1
2
3
4
5
6
7
8
|+
|+
|+
|+
-
+
-
+
-
+
-
+
ASM690A, ASM692A, ASM802L, ASM802M, (ASM805L)
Battery-Switchover Circuit
Reset
Generator
Watchdog
Timer
V
BATT
V
CC
WDI
PFI
V
OUT
RESET
(RESET)
PFO
1.25V
3.5V
0.8V
1.25V
1
2
4
5
6
7
8
|+
|+
|+
|+
-
+
-
+
-
+
-
+
2 of 13
Notice: The information in this document is subject to change without notice
ASM690A / 692A
ASM802L / 802M
ASM805L
P Power Supply Supervisor With Battery Backup Switch
rev 1.0
October 2003
Pin Configuration
1
2
3
4
5
6
7
8
V
OUT
V
CC
GND
PFI
V
BATT
RESET (RESET)
WDI
PFO
Plastic/CerDip/SO
ASM690A
ASM692A
ASM802L
ASM802M
(ASM805L)
Pin Description
Pin Number
Name
Function
ASM690A /
ASM692A
ASM802L /
ASM802M
ASM805L
1
1
V
OUT
Voltage supply for RAM. When V
CC
is above the reset threshold, V
OUT
connects to
V
CC
through a P-Channel MOS device. If V
CC
falls below the reset threshold, this
output will be connected to the backup supply at V
BATT
(or V
CC
, whichever is
higher) through the MOS switch to provide continuous power to the CMOS RAM.
2
2
V
CC
+5V power supply input.
3
3
GND
Ground
4
4
PFI
Power failure monitor input. PFI is connected to the internal power fail comparator
which is referenced to 1.25V. The power fail output (PFO) is active LOW but
remains HIGH if PFI is above 1.25V. If this feature is unused, the PFI pin should be
connected to GND or V
OUT
.
5
5
PFO
Power-fail output. PFO is active LOW whenever the PFI pin is less than 1.25V.
6
6
WDI
Watchdog input. The WDI input monitors microprocessor activity. An internal timer
is reset with each transition of the WDI input. If the WDI is held HIGH or LOW for
longer than the watchdog timeout period, typically 1.6 seconds, RESET (or RESET)
is asserted for the reset pulse width time, t
RS
, of 140ms, minimum.
7
-
RESET
Active-LOW reset output. When triggered by V
CC
falling below the reset threshold
or by watchdog timer timeout, RESET (or RESET) pulses low for the reset pulse
width t
RS
, typically 200ms. It will remain low if V
CC
is below the reset threshold
(4.65V in ASM690A / ASM802L and 4.4V in the ASM692A / ASM802L) and
remains low for 200ms after V
CC
rises above the reset threshold.
-
7
RESET
Active-HIGH reset output. The inverse of RESET.
8
8
V
BATT
Auxiliary power or backup-battery input. V
BATT
should be connected to GND if the
function is not used. The input has about 40mV of hysteresis to prevent rapid tog-
gling between V
CC
and V
BATT
.
3 of 13
Notice: The information in this document is subject to change without notice
ASM690A / 692A
ASM802L / 802M
ASM805L
P Power Supply Supervisor With Battery Backup Switch
rev 1.0
October 2003
Detailed Description
It is important to initialize a microprocessor to a known state
in response to specific events that could create code
execution errors and "lock-up". The reset output of these
supervisory circuits send a reset pulse to the microprocessor
in response to power-up, power-down/power-loss or a
watchdog time-out.
RESET/RESET Timing
Power-up reset occurs when a rising V
CC
reaches the reset
threshold, V
RT
, forcing a reset condition in which the reset
output is asserted in the appropriate logic state for the
duration of t
RS
. The reset pulse width, t
RS
, is typically around
200ms and is LOW for the ASM690A, ASM692A, ASM802
and HIGH for the ASM805L. Figure 1 shows the reset pin
timing.
Power-loss or "brown-out" reset occurs when V
CC
dips below
the reset threshold resulting in a reset assertion for the
duration of t
RS
. The reset signal remains asserted as long as
V
CC
is between V
RT
and 1.1V, the lowest V
CC
for which these
devices can provide a guaranteed logic-low output. To ensure
logic inputs connected to the ASM690A / ASM692A/ASM802
RESET pin are in a known state when V
CC
is under 1.1V, a
100k
pull-down resistor at RESET is needed: the logic-high
ASM805L will need a pull-up resistor to V
CC
.
Watchdog Timer
A Watchdog time-out reset occurs when a logic "1" or logic
"0" is continuously applied to the WDI pin for more than 1.6
seconds. After the duration of the reset interval, the watchdog
timer starts a new 1.6 second timing interval; the
microprocessor must service the watchdog input by changing
states or by floating the WDI pin before this interval is
finished. If the WDI pin is held either HIGH or LOW, a reset
pulse will be triggered every 1.8 seconds (the 1.6 second
timing interval plus the reset pulse width t
RS
).
Application Information
Microprocessor Interface
The ASM690 has logic-LOW RESET output while the
ASM805 has an inverted logic-HIGH RESET output.
Microprocessors with bidirectional reset pins can pose a
problem when the supervisory circuit and the microprocessor
output pins attempt to go to opposite logic states. The
problem can be resolved by placing a 4.7k
resistor between
the RESET output and the microprocessor reset pin. This is
shown in Figure 2. Since the series resistor limits drive
capabilities, the reset signal to other devices should be
buffered.
Figure 1: RESET/RESET Timing
Figure 2: Interfacing with bi-directional
microprocessor reset inputs
V
CC
RESET
RESET
GND
GND
V
CC
ASM690A
Power Supply
Bi-directional I/O pin
RESET
Buffered
BUF
4.7K
4 of 13
Notice: The information in this document is subject to change without notice
ASM690A / 692A
ASM802L / 802M
ASM805L
P Power Supply Supervisor With Battery Backup Switch
rev 1.0
October 2003
Watchdog Input
As discussed in the Reset section, the Watchdog input is
used to monitor microprocessor activity. It can be used to
insure that the microprocessor is in a continually responsive
state by requiring that the WDI pin be toggled every second.
If the WDI pin is not toggled within the 1.6 second window
(minimum t
WD
+ t
RS
), a reset pulse will be asserted to return
the microprocessor to the initial start-up state. Pulses as
short as 50ns can be applied to the WDI pin. If this feature is
not used, the WDI pin should be open circuited or the logic
placed into a high-impedance state to allow the pin to float.
Backup-Battery Switchover
A power loss can be made less severe if the system RAM
contents are preserved. This is achieved in the ASM690/692/
802/805 by switching from the failed V
CC
to an alternate
power source connected at V
BATT
when V
CC
is less than the
reset threshold voltage (V
CC
< V
RT
), and V
CC
is less than
V
BATT
. The V
OUT
pin is normally connected to V
CC
through a
2
PMOS switch but a brown-out or loss of V
CC
will cause a
switchover to V
BATT
by means of a 20
PMOS switch.
Although both conditions (V
CC
< V
RT
and V
CC
<V
BATT
) must
occur for the switchover to V
BATT
to occur, V
OUT
will be
switched back to V
CC
when V
CC
exceeds V
RT
irrespective of
the voltage at V
BATT
. It should be noted that an internal
device diode (D1 in Figure 3) will be forward biased if V
BATT
exceeds V
CC
by more than a diode drop when V
CC
is
switched to V
OUT
. Because of this it is recommended that
V
BATT
be no greater than V
RT
+0.6V.
Table 1. Pin Connections in Battery Backup Mode
During the backup power mode, the internal circuitry of the
supervisory circuit draws power from the battery supply.
While V
CC
is still alive, the comparator circuits remain alive
and the current drawn by the device is typically 35A. When
V
CC
drops more than 1.1V below V
BATT
, the internal
switchover comparator, the PFI comparator and WDI
comparator will shut off, reducing the quiescent current drawn
by the IC to less than 1A.
Condition
SW1/SW2
SW3/SW4
V
CC
> Reset Threshold
open
closed
V
CC
< Reset Threshold
V
CC
> V
BATT
open
closed
V
CC
< Reset Threshold
V
CC
< V
BATT
closed
open
ASM690A/802A/805L Reset Threshold = 4.65V
ASM692A /ASM802M Reset Threshold = 4.4V
Pin
Connection
V
OUT
Connected to V
BATT
through internal PMOS
switch
V
BATT
Connected to V
OUT
PFI
Disabled
PFO
Logic-LOW
RESET
Logic-LOW (except on ASM805 where it is
HIGH)
WDI
Watchdog timer disabled
SW1 SW2
D1
D2
SW3 SW4
D3
ASM690A
ASM692A
ASM802L
ASM802M
ASM805L
V
OUT
V
CC
V
BATT
Figure 3: Internal device configuration of battery
switch-over function
5 of 13
Notice: The information in this document is subject to change without notice
ASM690A / 692A
ASM802L / 802M
ASM805L
P Power Supply Supervisor With Battery Backup Switch
rev 1.0
October 2003
Backup Power Sources - Batteries
Battery voltage selection is important to insure that the
battery does not discharge through the parasitic device diode
D1 (see Figure 3) when V
CC
is less than V
BATT
and V
CC
>
V
RT
.
Table 2: Maximum Battery Voltages
Although most batteries that meet the requirements of Table
2 are acceptable, lithium batteries are very effective backup
source due to their high-energy density and very low self-
discharge rates.
Battery replacement while Powered
Batteries can be replaced even when the device is in a
powered state as long as V
CC
remains above the reset
threshold voltage V
RT
. In the ASM devices, a floating V
BATT
pin will not cause a powersupply switchover as can occur in
some other supervisory circuits. If V
BATT
is not used, the pin
should be grounded.
Backup Power Sources - SuperCapTM
Capacitor storage, with very high values of capacitance, can
be used as a back-up power source instead of batteries.
SuperCap
TM
are capacitors with capacities in the fractional
farad range. A 0.1 farad SuperCapTM would provide a useful
backup power source. Like the battery supply, it is important
that the capacitor voltage remain below the maximum
voltages shown in Table 2. Although the circuit of Figure 4
shows the most simple way to connect the SuperCapTM, this
circuit cannot insure that an over voltage condition will not
occur since the capacitor will ultimately charge up to V
CC
. To
insure that an over voltage condition does not occur, the
circuit of Figure 5 is preferred. In this circuit configuration, the
diode-resistor pair clamps the capacitor voltage at one diode
drop below V
CC
. V
CC
itself should be regulated within 5% of
5V for the ASM692A/802M or within 10% of 5V for the
ASM690A/802L/805L to insure that the storage capacitor
does not achieve an over voltage state.
Note: SuperCap
TM
is a trademark of Baknor Industries
Part Number
MAXIMUM Battery Voltage
ASM690A
4.80
ASM802L
4.80
ASM805L
4.80
ASM692A
4.55
ASM802M
4.55
100K
ASM692A
ASM802M
V
CC
V
OUT
V
BATT
RESET
(RESET)
GND
To P
+
0.1F
Figure 5: Capacitor as a backup power source
Voltage clamped to 0.5V below V
CC
V
CC
V
OUT
V
BATT
RESET
(RESET)
GND
To P
+5V
0.1F
+
Figure 4: Capacitor as a backup power source
D2
D1
To
SRAM
D1
ASM692A
ASM802M
To
SRAM
+5V
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