Datasheet

LTC1727/LTC1728
11
17278fb
applicaTions inForMaTion
Override Functions (5V Versions Only)
The V
CCA
pin, if unused, can be tied to either V
CC3
or
V
CC5
. This is an obvious solution since the trip points for
V
CC3
and V
CC5
will always be greater than the trip point
for V
CCA
.
The V
CC5
input trip point is disabled if its voltage is
equal to the voltage on V
CC3
±25mV and the voltage on
V
CC5
is less than 4.15V. In this manner, the LTC1727-5/
LTC1728-5 will behave as a 3.3V monitor and V
CC5
reset
capability will be disabled.
The V
CC5
trip point is reenabled when the voltage on
V
CC5
is equal to the voltage on V
CC3
±25mV and the
two inputs are greater than approximately 4.15V. In this
manner, the part can function as a 5V monitor with the
3.3V monitor disabled.
When monitoring either 3.3V or 5V with V
CC3
strapped
to V
CC5
(see Figure 2), the part determines which is the
appropriate range. The part handles this situation as
shown in Figure 3. Above 1V and below V
RT3
, RST is held
low. From V
RT3
to approximately 4.15V the part assumes
3.3V supply monitoring and
RST is deasserted. Above
approximately 4.15V the part operates as a 5V monitor.
In most systems the 5V supply will pass through the
3.1V to 4.15V region in <200ms during power-up, and
the
RST output will behave as desired. Table 1 summa-
rizes the state of
RST at various operating voltages with
V
CC3
= V
CC5
.
Table 1. Override Truth Table (V
CC3
= V
CC5
)
INPUTS (V
CC3
= V
CC5
= V
CC
) RST
0V ≤ V
CC
≤ 1V —
1V ≤ V
CC
≤ V
RT3
0
V
RT3
≤ V
CC
≤ 4.15V 1
4.15V ≤ V
CC
≤ V
RT5
0
V
RT5
≤ V
CC
1
Figure 2. Single Supply Monitor with Others Disabled
Figure 3. RST Voltage vs Supply Voltage
V
CC3
V
CC5
V
CCA
GND
1
5
4
3
2
RST
LTC1728-5
1727/28 F02
TO SYSTEM
RESET
R1
10k
V
CC
3.3V OR 5V
SUPPLY VOLTAGE (V)
0
RST OUTPUT VOLTAGE (V)
3
4
5
4
1727/28 F03
2
1
0
1
2
3
5
V
CC3
= V
CC5
= V
CCA
= 0V TO 5V
10k PULL-UP FROM RST TO V
CC3