Datasheet

Detailed Description
Battery Switchover and V
CCO
Figure 2 shows a typical application for the MAX1259.
CMOS RAM is powered from V
CCO
. The battery switcho-
ver circuit compares V
CC
to the V
BATT
input, and con-
nects V
CCO
to whichever is higher.
Switchover occurs when V
CC
equals V
BATT
as V
CC
falls,
and when V
CC
is 60mV greater than V
BATT
as V
CC
rises.
This hysteresis prevents repeated, rapid switching if V
CC
falls very slowly or remains nearly equal to the battery
voltage. Low-loss switches guarantee an input-to-output
differential of only 200mV, while supplying 250mA from
the primary power supply or 15mA from the battery.
Note: With adequate filtering, the MAX1259 need only
supply the average current drawn by the CMOS RAM.
Many RAM data sheets specify a 75mA maximum sup-
ply current, but this peak current spike lasts only 100ns.
If the sum of the peak currents is greater than 250mA, a
capacitor placed on the V
CCO
output can supply the high
instantaneous current, while V
CCO
need only supply the
average current, which is much less.
The MAX1259 operates with battery voltages from 2.5V to
3.7V. High-value capacitors—either standard electrolytic
or farad-sized, double-layer capacitors—can also be used
for short-term memory backup (Figure 3).
Figure 1. Power-Down/Power-Up Conditions
Figure 2. Typical Application Circuit Figure 3. Using a MAXCAP as a Backup Battery
V
CCI
+4.25V
+3V
PF
BATTERY
CURRENT
t
F
t
PF
t
R
t
REC
MAX1259
V
CCI
V
CC
V
BATT
V
CCO
GND
PF
15, 16
7, 8
12, 13
11
2
µP
NMI
ADDRESS
DECODE
RAM1
RAM2
RAM16
CE15
CE1
CE0
TO RAM
12, 13
15, 16
3kΩ
6.2kΩ
2N3904
2
V
CC
V
BATT
V
CCI
V
CC0
0.22F
MAXCAP
GND
7, 8
NOTE: LARGE VALUE CAPACITORS, SUCH AS A 0.22F MAXCAP,
MAY BE USED FOR SHORT-TERM MEMORY BACKUP.
MAX1259
MAX1259 Battery Manager
www.maximintegrated.com
Maxim Integrated │ 5