Datasheet

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:
1) V
CC
falls below the reset threshold voltage.
2) V
CC
is below V
BATT
.
Table 1 lists the status of the inputs and outputs in battery-
backup mode. The devices do not power up if the only
voltage source is V
BATT
. OUT only powers up from V
CC
at startup.
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
is low and for a minimum of 140ms (t
RP
) after it returns
high. MR has an internal 20kΩ (min) pullup resistor to
V
CC
. This input can be driven from TTL/CMOS logic
outputs or with open-drain/collector outputs. Connect a
normally open momentary switch from MR to GND to cre-
ate a manual-reset function; external debounce circuitry
is not required. When driving MR from long cables, or
when using the device in a noisy environment, 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 watchdog
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 (t
WD
), the
internal watchdog timer runs out and triggers a reset pulse
for the reset timeout period (t
RP
). The 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 period (t
WD
); see Figure 2.
Figure 2. MAX16034/MAX16038 Watchdog Timeout Period and
Reset Active Time
Table 1. Input and Output Status in
Battery-Backup Mode
PIN STATUS
V
CC
Disconnected from OUT
OUT Connected to BATT
BATT
Connected to OUT. Current drawn from the
battery is less than 1µA (at V
BATT
= 2.8V,
excluding I
OUT
) when V
CC
= 0V.
RESET
Asserted
BATTON High state
MR, RESETIN,
CEIN, and WDI
Inputs ignored
CEOUT
Connected to OUT
PFO
Asserted
t
WD
= WATCHDOG TIMEOUT PERIOD
t
RP
= RESET TIMEOUT PERIOD
WDI
RESET
t
WD
t
WD
t
RP
t
RP
MAX16033–MAX16040 Low-Power Battery-Backup
Circuits in Small μDFN Packages
www.maximintegrated.com
Maxim Integrated │ 10