MAX1259CSLASHD datasheet(5/8 Pages) MAXIM

5 / 8 page

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).

Figure 1. Power-Down/Power-Up Conditions

Figure 2. Typical Application Circuit

Figure 3. Using a MAXCAP as a Backup Battery

VCCI

+4.25V

+3V

BATTERY

CURRENT

tPF

tREC

MAX1259

VCCI

VCC

VBATT

VCCO

GND

15, 16

7, 8

12, 13

µP

NMI

ADDRESS

DECODE

RAM1

RAM2

RAM16

CE15

CE1

CE0

TO RAM

12, 13

15, 16

3kΩ

6.2kΩ

2N3904

VCC

VBATT

VCCI

VCC0

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

Battery Manager

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