Rainbow-electronics MAX16021 Manual do Utilizador descarregar pdf (Página 17)

MAX16016/MAX16020/MAX16021

Low-Power µP Supervisory Circuits with

Battery-Backup Circuit and Chip-Enable Gating

______________________________________________________________________________________ 17

MAX16020L

CEIN

GND

CMOS RAM

BATTON

BATT

0.1µF

1µF

CEOUT

OUT

RESET

ADDRESS

DECODE

µP

A0–A15

RESET

Figure 5. BATTON Driving an External Pass Transistor

Adding Hysteresis to PFI

The power-fail comparators have a typical input hys-

teresis of V

PFI-HYS

. This is sufficient for most applica-

tions where a power-supply line is being monitored

through an external voltage-divider (see the

Monitoring

an Additional Supply

section). Figure 4 shows how to

add hysteresis to the power-fail comparator. Select the

ratio of R1 and R2 so that PFI sees V

PFT

when V

falls

to the desired trip point (V

TRIP

). Resistor R3 adds hys-

teresis. R3 is typically an order of magnitude greater

than R1 or R2. R3 should be larger than 50kΩ to pre-

vent it from loading down PFO. Capacitor C1 adds

additional noise rejection.

Battery-On Indicator (Push-Pull Version)

BATTON goes high when in battery-backup mode. Use

BATTON to indicate battery-switchover status or to sup-

ply base drive to an external pass transistor for higher

current applications (Figure 5).

Operation Without a Backup Power Source

The MAX16016/MAX16020/MAX16021 provide a bat-

tery-backup function. If a backup power source is not

used, connect BATT to GND and OUT to V

MAX16016L

MAX16020L

MAX16021L

PFO

GND

PFI

0.1µF

C1*

PFO

+5V

TRIP

0.1µF

+5V

TO µP

R1+R2

TRIP

= V

PFT

(—)

*OPTIONAL

= (V

PFT

+ V

PFT_HYS

) x

(–

–

)

- V

PFT

= R1 x

(

—

)

+ V

PFT

WHERE V

PFT

IS THE POWER-FAIL THRESHOLD VOLTAGE

Figure 4. Adding Hysteresis to the Power-Fail Comparator

1 2 ... 12 13 14 15 16 17 18 19 20 21

MAX16016/MAX16020/MAX16021 1

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