MAX1259C 데이터 시트보기 (PDF) - Maxim Integrated
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MAX1259C Datasheet PDF : 8 Pages
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MAX1259
Battery Manager
VCCI
+4.25V
+3V
tF
tPF
PF
tR
tREC
BATTERY
CURRENT
Figure 1. Power-Down/Power-Up Conditions
Detailed Description
Battery Switchover and VCCO
Figure 2 shows a typical application for the MAX1259.
CMOS RAM is powered from VCCO. The battery switcho-
ver circuit compares VCC to the VBATT input, and con-
nects VCCO to whichever is higher.
Switchover occurs when VCC equals VBATT as VCC falls,
and when VCC is 60mV greater than VBATT as VCC rises.
This hysteresis prevents repeated, rapid switching if VCC
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 VCCO output can supply the high
instantaneous current, while VCCO 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).
RAM16
RAM2
RAM1
CE15
15, 16
VCC
VCCI
12, 13
VCCO
CE1
CE0
MAX1259
2
VBATT
11
PF
GND
7, 8
NMI
µP
ADDRESS
DECODE
Figure 2. Typical Application Circuit
VCC
3kΩ
6.2kΩ
15, 16
VCCI
12, 13
VCC0
TO RAM
MAX1259
2N3904
0.22F
MAXCAP
2
VBATT
GND
7, 8
NOTE: LARGE VALUE CAPACITORS, SUCH AS A 0.22F MAXCAP,
MAY BE USED FOR SHORT-TERM MEMORY BACKUP.
Figure 3. Using a MAXCAP as a Backup Battery
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