Low-Power Battery Backup

Circuits in Small µDFN Packages

10

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Backup Battery Switchover

To preserve the contents of the RAM in a brownout or

power failure, the MAX16033–MAX16040 automatically

switch to back up the battery installed at BATT when

the following two conditions are met:

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

tery-backup mode. The devices do not power-up if the

Manual-Reset Input

(MAX16033/MAX16037 Only)

Many µP-based products require manual-reset capabil-

ity, allowing the user or external logic circuitry to initiate

a reset. For the MAX16033/MAX16037, a logic-low on

MR asserts RESET. RESET remains asserted while MR

high. MR has an internal 20kΩ (min) pullup resistor to

outputs or with open-drain/collector outputs. Connect a

normally-open momentary switch from MR to GND to

create a manual-reset function; external debounce cir-

cuitry is not required. When driving MR from long

cables or when using the device in a noisy environ-

ment, connect a 0.1µF capacitor from MR to GND to

provide additional noise immunity.

Watchdog Input

(MAX16034/MAX16038 Only)

The watchdog monitors µP activity through the watch-

dog input (WDI). RESET asserts when the µP fails to

toggle WDI. Connect WDI to a bus line or µP I/O line. A

change of state (high to low, low to high, or a minimum

100ns pulse) resets the watchdog timer. If WDI remains

high or low for longer than the watchdog timeout period

internal watchdog timer clears whenever reset is

asserted or whenever WDI sees a rising or falling edge.

If WDI remains in either a high or low state, a reset

pulse periodically asserts after every watchdog timeout

Table 1. Input and Output Status in

Battery-Backup Mode

PIN

STATUS

Disconnected from OUT

OUT

Connected to BATT

BATT

Connected to OUT. Current drawn from the

RESET

Asserted

BATTON

High state

MR, RESETIN,

CEIN, and WDI

Inputs ignored

CEOUT

Connected to OUT

PFO

Asserted

WDI

RESET

Figure 2. MAX16034/MAX16038 Watchdog Timeout Period and

Reset Active Time