October 2003
rev 1.0
ASM690A / 692A
ASM802L / 802M
ASM805L
µP Power Supply Supervisor With Battery Backup Switch
Applications
•
•
•
•
•
•
•
•
Embedded control systems
Portable/Battery operated systems
Intelligent instruments
Wireless instruments
Wireless communication systems
PDAs and hand-held equipments
µP / µC power supply monitoring
Safety system
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:
Typical Operating Circuit
Unregulated DC
Regulated +5V
V
CC
RESET
PFI
R
2
V
BATT
PFO
WDI
V
OUT
RESET
NMI
V
CC
R
1
0.1 µF
•
•
•
•
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
I/O LINE
GND
+
_
ASM690A
V
CC
CMOS
RAM
GND
These features are pin-compatible with the industry standard
power-supply supervisors. Short-circuit and thermal protection
have also been added. The AS690A / AS802L /
4.65V and the AS692A / AS802M
AS805L
generate a reset pulse when the supply voltage drops below
generate a reset below
V
BATT
V
CC
Block Diagram
8
2
Reset
Generator
+
|+
-
1
Battery-Switchover Circuit
V
OUT
7
RESET
(RESET)
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
WDI
6
1.25V
3.5V
|+
+
-
-
|+
+
Watchdog
Timer
0.8V
PFI
4
1.25V
|+
-
+
5
PFO
ASM690A, ASM692A, ASM802L, ASM802M, (ASM805L)
3
GND
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
BUS
3.6 V
Lithium
Battery
GND
October 2003
rev 1.0
ASM690A / 692A
ASM802L / 802M
ASM805L
Pin Configuration
Plastic/CerDip/SO
V
OUT
V
CC
GND
PFI
1
2
3
4
ASM690A
ASM692A
ASM802L
ASM802M
(ASM805L)
8
7
6
5
V
BATT
RESET (RESET)
WDI
PFO
Pin Description
Pin Number
ASM690A /
ASM692A
ASM802L /
ASM802M
Name
ASM805L
Function
Voltage supply for RAM. When V
CC
is above the reset threshold, V
OUT
connects to
1
1
V
OUT
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
3
2
3
V
CC
GND
+5V power supply input.
Ground
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
.
Power-fail output. PFO is active LOW whenever the PFI pin is less than 1.25V.
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.
Active-LOW reset output. When triggered by V
CC
falling below the reset threshold
7
-
RESET
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
V
BATT
Active-HIGH reset output. The inverse of RESET.
Auxiliary power or backup-battery input. V
BATT
should be connected to GND if the
8
8
function is not used. The input has about 40mV of hysteresis to prevent rapid tog-
gling between V
CC
and V
BATT
.
4
4
PFI
5
5
PFO
6
6
WDI
µP Power Supply Supervisor With Battery Backup Switch
Notice: The information in this document is subject to change without notice
2 of 13
October 2003
rev 1.0
ASM690A / 692A
ASM802L / 802M
ASM805L
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 4.7kΩ 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.
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
100kΩ 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
).
Figure 1: RESET/RESET Timing
BUF
Buffered
RESET
V
CC
4.7K
Ω
Power Supply
RESET
V
CC
RESET
ASM690A
GND
Bi-directional I/O pin
GND
Figure 2: Interfacing with bi-directional
microprocessor reset inputs
µP Power Supply Supervisor With Battery Backup Switch
Notice: The information in this document is subject to change without notice
3 of 13
October 2003
rev 1.0
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
2Ω PMOS switch but a brown-out or loss of V
CC
will cause a
switchover to V
BATT
by means of a 20Ω 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.
SW1 SW2
V
BATT
ASM690A / 692A
ASM802L / 802M
ASM805L
V
CC
D1
D2
SW3 SW4
ASM690A
ASM692A
ASM802M
ASM805L
D3
ASM802L
V
OUT
Figure 3: Internal device configuration of battery
switch-over function
Table 1. Pin Connections in Battery Backup Mode
Pin
V
OUT
V
BATT
PFI
PFO
RESET
WDI
Connection
Connected to V
BATT
through internal PMOS
switch
Connected to V
OUT
Disabled
Logic-LOW
Logic-LOW (except on ASM805 where it is
HIGH)
Watchdog timer disabled
Condition
V
CC
> Reset Threshold
V
CC
< Reset Threshold
V
CC
> V
BATT
V
CC
< Reset Threshold
V
CC
< V
BATT
SW1/SW2
open
open
SW3/SW4
closed
closed
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 35µA. 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 1µA.
closed
open
ASM690A/802A/805L Reset Threshold = 4.65V
ASM692A /ASM802M Reset Threshold = 4.4V
µP Power Supply Supervisor With Battery Backup Switch
Notice: The information in this document is subject to change without notice
4 of 13
October 2003
rev 1.0
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
ASM690A / 692A
ASM802L / 802M
ASM805L
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
ASM690A
ASM802L
ASM805L
ASM692A
ASM802M
MAXIMUM Battery Voltage
4.80
4.80
4.80
4.55
4.55
+5V
V
CC
D1
V
BATT
V
OUT
To
SRAM
+
RESET
(RESET)
To µP
0.1F
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 - SuperCap™
Capacitor storage, with very high values of capacitance, can
be used as a back-up power source instead of batteries.
SuperCap
™
are capacitors with capacities in the fractional
farad range. A 0.1 farad SuperCap™ 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 SuperCap™, 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
D1
D2
ASM692A
ASM802M
GND
Figure 4: Capacitor as a backup power source
+5V
V
CC
V
OUT
To
SRAM
V
BATT
100K
+
RESET
(RESET)
To µP
0.1F
ASM692A
ASM802M
GND
Figure 5: Capacitor as a backup power source
Voltage clamped to 0.5V below V
CC
µP Power Supply Supervisor With Battery Backup Switch
Notice: The information in this document is subject to change without notice
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