Datasheet

LTC692/LTC693
11
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If battery connections are made through long wires, a
10Ω to 100Ω series resistor and a 0.1μF capacitor are
recommended to prevent any overshoot beyond V
CC
due
to the lead inductance (Figure 4).
Table 1 shows the state of each pin during battery backup.
When the battery switchover section is not used, connect
V
BATT
to GND and V
OUT
to V
CC
.
battery backed up CMOS RAM. CE OUT can also be used
to drive the Store or Write input of an EEPROM, EAROM
or NOVRAM to achieve similar protection. Figure 5 shows
the timing diagram of CE IN and CE OUT.
CE IN can be derived from the microprocessor’s address
decoder output. Figure 6 shows a typical nonvolatile CMOS
RAM application.
Memory protection can also be achieved with the LTC692
by using RESET as shown in Figure 7.
APPLICATIONS INFORMATION
3.9M
0.1μF
V
BATT
LTC692
LTC693
GND
692_3 • F04
10Ω
Figure 4. 10Ω/0.1μF Combination Eliminates Inductive
Overshoot and Prevents Spurious Resets During Battery
Replacement
Table 1. Input and Output Status in Battery Backup Mode
SIGNAL STATUS
V
CC
C2 monitors V
CC
for active switchover
V
OUT
V
OUT
is connected to V
BATT
through an internal PMOS switch
V
BATT
The supply current is 1μA maximum
BATT ON Logic high. The open-circuit output voltage is equal to V
OUT
PFI Power failure input is ignored
PFO Logic low
RESET Logic low
RESET Logic high. The open-circuit output voltage is equal to V
OUT
LOW LINE Logic low
WDI Watchdog input is ignored
WDO Logic high. The open-circuit output voltage is equal to V
OUT
CE IN Chip Enable input is ignored
CE OUT Logic high. The open-circuit output voltage is equal to V
OUT
OSC IN OSC IN is ignored
OSC SEL OSC SEL is ignored
Memory Protection
The LTC693 includes memory protection circuitry which
ensures the integrity of the data in memory by preventing
write operations when V
CC
is at an invalid level. Two ad-
ditional pins, CE IN and CE OUT, control the Chip Enable
or Write inputs of CMOS RAM. When V
CC
is 5V, CE OUT
follows CE IN with a typical propagation delay of 20ns.
When V
CC
falls below the reset voltage threshold or V
BATT
,
CE OUT is forced high, independent of CE IN. CE OUT is
an alternative signal to drive the CE, CS, or Write input of
V
CC
V1
CE IN
V
OUT
= V
BATT
CE OUT
V
OUT
= V
BATT
V2
V1 = RESET VOLTAGE THRESHOLD
V2 = RESET VOLTAGE THRESHOLD +
RESET THRESHOLD HYSTERESIS
692_3 • F05
Figure 5. Timing Diagram for CE IN and CE OUT