Detailed description – Rainbow Electronics MAX16024 User Manual

Page 11

Detailed Description

The

Typical Applications Circuit

shows a typical con-

nection using the MAX16024. OUT powers the SRAM. If
V

is higher than the reset threshold (V

), or if V

lower than V

but higher than V

BATT

, the regulator is

powered from V

. If V

< V

, V

< V

BATT

, and the

regulator is in dropout, the regulator is powered from
BATT (see the

Functional Diagrams

). OUT supplies up

to 100mA from V

Backup-Battery Switchover

In a brownout or power failure, it may be necessary to
preserve the contents of the RAM. With a backup-bat-
tery installed at BATT, the MAX16023/MAX16024 auto-
matically switch the RAM to backup power when V

falls. The MAX16024 has a BATT ON output that goes
high when in battery-backup mode. Three conditions
must be met for these devices to switch to battery
backup mode:

1) V

is lower than the reset threshold.

2) V

is lower than V

BATT

3) The regulator is in dropout (except for the 1.2V out-

put version).

Chip-Enable Signal Gating (MAX16024)

The MAX16024 provides internal gating of CE signals to
prevent erroneous data from being written to CMOS
RAM in the event of a power failure or brownout. During
normal operation, the CE gate enables and passes all
CE transitions. When the reset output asserts, this path
becomes disabled, preventing erroneous data from cor-
rupting the CMOS RAM and CEOUT is pulled up to OUT
through an internal current source. The 1.5ns propaga-
tion delay from CEIN to CEOUT allows the devices to
be used with most µPs and high-speed DSPs.

During normal operation (reset not asserted), CEIN is
connected to CEOUT through a low on-resistance
transmission gate. If CEIN is high when a reset asserts,
CEOUT remains high regardless of any subsequent
transition on CEIN during the reset event.

If CEIN is low when reset asserts, CEOUT is held low for
12µs to allow completion of the read/write operation.
After the 12µs delay expires, CEOUT goes high and
stays high regardless of any subsequent transitions on
CEIN during the reset event. When CEOUT is discon-
nected from CEIN, CEOUT is actively pulled up to OUT.

The propagation delay through the chip-enable circuitry
depends on both the source impedance of the drive to
CEIN and the capacitive loading at CEOUT. Minimize
the capacitive load at CEOUT to minimize propagation
delay, and use a low-output-impedance driver.

MAX16023/MAX16024

Battery-Backup Circuits with

Regulated Output Voltage

______________________________________________________________________________________

RESET THRESHOLD V

CEIN

CEOUT

RESET

IF CEIN GOES HIGH BEFORE RESET ASSERTS,
CEOUT GOES HIGH WITHOUT DELAY AS
CEIN GOES HIGH.

RESET-TO-CEOUT DELAY (12

μs)

Figure 1. Reset and Chip-Enable Timing