Datasheet

MAX697
The MAX697 is nearly identical to the MAX696. The MAX697
lacks the battery-backup feature, so it does not have the
V
BATT
, V
OUT
, or BATT ON pins. This allows the MAX697
to consume less than 250 microamperes, and it allows the
inclusion of RAM write-protection pins. See Figure 2.
Detailed Description
Battery Switchover and V
OUT
(MAX696)
Battery Switchover and V
OUT
(MAX696) The battery-
switchover circuit compares V
CC
to the V
BATT
input, and
connects V
OUT
to whichever is higher. Switchover occurs
when V
CC
is 50mV greater than V
BATT
as V
CC
falls, and
when V
CC
is 70mV more than V
BATT
as V
CC
rises (see
Figure 3). The switchover comparator has 20mV of hys-
teresis to prevent repeated, rapid switching if V
CC
falls
very slowly or remains nearly equal to the battery voltage.
When V
CC
is higher than V
BATT
, V
CC
is internally
switched to V
OUT
with a low-saturation pnp transistor.
V
OUT
has 50mA output current capability. Use an external
pnp pass transistor in parallel with the internal transistor
if the output current requirement at V
OUT
exceeds 50mA
or if a lower V
CC
- V
OUT
voltage differential is desired.
The BATT ON output can directly drive the base of the
external transistor.
It should be noted that the MAX696 need only supply
the average current drawn by the CMOS RAM if there is
adequate filtering. Many RAM data sheets specify a 75mA
maximum supply current, but this peak current spike lasts
only 100ns. A 0.1μF bypass capacitor at V
OUT
supplies
the high instantaneous current, while V
OUT
need only
supply the average load current, which is much less. A
capacitance of 0.1μF or greater must be connected to the
V
OUT
terminal to ensure stability.
A 200Ω MOSFET connects the V
BATT
input to V
OUT
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. When V
CC
equals V
BATT
, the
supply current is typically 12μA. When V
CC
is between 0V
and (V
BATT
- 700mV), the typical supply current is only
600nA (typ), 1μA (max).
Figure 3. MAX696 Battery-Switchover Block Diagram
+
-
+
-
P
700mV
100mV
3V
BATTERY
INPUT
BASE DRIVE
V
CC
0.1µF
TO CMOS
RAM AND
REAL-TIME
CLOCK
V
CC
IN
BATT ON
p-CHANNEL
MOSFET
INTERNAL
SHUTDOWN
SIGNAL WHEN
V
BATT
> V
CC
+ 0.7V
LOW IQ MODE SELECT
V
OUT
V
CC
+5V
V
BATT
MAX696/MAX697 Microprocessor Supervisory Circuits
www.maximintegrated.com
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