Datasheet

9
LTC1326/LTC1326-2.5
132625fc
During a supply induced reset condition, the ability of the
PBR pin to force a soft reset condition on the SRST pin
is disabled. In other words SRST will remain high. If the
PBR pin is held low, both during and after a supply
induced reset (low RST), the RST pin will remain low until
200ms after the PBR goes high.
Power Detect/Gate Drive
The LTC1326/LTC1326-2.5 for the most part are powered
internally from the V
CC3
pin. The exception is at the gate
drive of the output FET on the RST pin. On the input to this
FET is power detection circuitry used to detect and drive
the gate from either the 3.3V input pin (V
CC3
) or the 5V
input pin (V
CC5
) on the LTC1326 or the 2.5V input pin
(V
CC25
) on the LTC1326-2.5. The gate drive is derived
from the pin with the highest potential. This ensures the
part pulls the RST pin low as soon as either input pin is
≥ 1V.
Dual and Single Supply Monitor Operation
The V
CC3
, V
CC5
and V
CCA
inputs may be individually
disabled by the following override techniques which allow
the LTC1326 or LTC1326-2.5 to be used as a dual or single
supply monitor.
LTC1326 Override Functions
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 part will behave as a
3.3V monitor and the V
CC5
reset 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
LTC1326 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 1), the LTC1326 determines which is the
appropriate range. The LTC1326 handles this situation as
shown in Figure 2. Above 1V and below V
RT3
, RST is held
low. From V
RT3
to approximately 4.15V, the LTC1326
assumes 3.3V supply monitoring and RST is deasserted.
Above approximately 4.15V, the LTC1326 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 summarizes
the state of RST and RST at various operating voltages
with V
CC3
= V
CC5
.
APPLICATIO S I FOR ATIO
WUU
U
Table 1. Override Truth Table (V
CC3
= V
CC5
)
INPUTS (V
CC3
= V
CC5
= V
CC
) RST RST
0V ≤ V
CC
≤ 1V — —
1V ≤ V
CC
≤ V
RT3
01
V
RT3
≤ V
CC
≤ 4.15V 1 0
4.15V ≤ V
CC
≤ V
RT5
01
V
RT5
≤ V
CC
10
Figure 2. RST Voltage vs Supply Voltage
1
2
3
4
8
7
6
5
V
CC3
V
CC5
V
CCA
GND
PBR
SRST
RST
RST
LTC1326
SYSTEM RESET
R2
1326/2.5 F01
R1
ADJUSTABLE
SUPPLY
3.3V OR
5V
4.7k
Figure 1
SUPPLY VOLTAGE (V)
0
RST OUTPUT VOLTAGE (V)
3
4
5
4
1326/2.5 F02
2
1
0
1
2
3
5
V
CC3
= V
CC5
= V
CCA
= 0V TO 5V
4.7k PULL-UP FROM RST TO V
CC3