MAX817 데이터 시트보기 (PDF) - Maxim Integrated

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MAX817

+5V Microprocessor Supervisory Circuits

Maxim Integrated 

+5V Microprocessor Supervisory Circuits

Backup-Battery Switchover

In a brownout or power failure, it may be necessary to

preserve the contents of RAM. With a backup battery

installed at BATT, the MAX817/MAX818/MAX819 auto-

matically switch RAM to backup power when VCC falls.

These devices require two conditions before switching

to battery-backup mode: 1) VCC must be below the

reset threshold, and 2) VCC must be below VBATT.

Table 1 lists the status of the inputs and outputs in bat-

tery-backup mode.

As long as VCC exceeds the reset threshold, OUT con-

nects to VCC through a 5Ω PMOS power switch. Once

VCC falls below the reset threshold, VCC or VBATT

(whichever is higher) switches to OUT. When VCC falls

below VRST and VBATT, BATT switches to OUT through

an 80Ω switch.

Table 1. Input and Output Status in

Battery-Backup Mode

SIGNAL

VCC

VOUT

VBATT

V R ESET

VWDI

V C E OUT

V C E IN

STATUS

Disconnected from VOUT.

Connected to VBATT through an internal 80Ω

PMOS switch.

Connected to VOUT. Current drawn from

the battery is less than 1µA, as long as

VCC < VBATT - 0.2V.

Logic low

Watchdog timer is disabled.

Logic high. The open-circuit voltage is equal

to VOUT.

High impedance

V1

ADDITIONAL SUPPLY RESET VOLTAGE

V2

VCC

( ) V2 (RESET) = 1.25

R1 + R2

R2

R1

MAX819

RESET

PFI

MR

R2

PFO

RESET

µP

When VCC exceeds the reset threshold, it is connected to

the substrate, regardless of the voltage applied to BATT

(Figure 10). During this time, the diode (D1) between

BATT and the substrate will conduct current from BATT

to VCC if VBATT is 0.6V greater than VCC. When BATT

connects to OUT, backup mode is activated and the

internal circuitry is powered from the battery (Table 1).

When VCC is just below VBATT, the current draw from

BATT is typically 6µA. When VCC drops to more than 1V

below VBATT, the internal switchover comparator shuts

off and the supply current falls to less than 1µA.

__________Applications Information

The MAX817/MAX818/MAX819 are protected for typical

short-circuit conditions of 10sec or less. Shorting OUT

to ground for longer than 10sec destroys the device.

Decouple VCC, OUT, and BATT to ground by placing

0.1µF capacitors as close to the device as possible.

BATT

VCC

SW1

SW2

D1 D2

SW3

SW4

SUBSTRATE

MAX817

D3

MAX818

MAX819

OUT

CONDITION

SW1/SW2 SW3/SW4

VCC > Reset Threshold

Open

VCC < Reset Threshold and

VCC > VBATT

VCC < Reset Threshold and

VCC < VBATT

Open

Closed

RESET THRESHOLD = 4.65V IN MAX81_L

RESET THRESHOLD = 4.4V IN MAX81_M

Closed

Closed

Open

Figure 9. Monitoring an Additional Supply by Connecting

PFO to MR.

Figure 10. Backup-Battery-Switchover Block Diagram

12 ______________________________________________________________________________________

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