10
LTC1326/LTC1326-2.5
132625fc
Figure 3 contains a simple circuit for 5V systems that can’t
risk the RST output going high in the 3.1V to 4.15V range
(possibly due to very slow rise time on the 5V supply).
Diode D1 powers the LTC1326 while dropping ≈0.6V from
the V
CC5
pin to the V
CC3
pin. This prevents the part’s
internal override circuit from being activated. Without the
override circuit active, the RST pin stays low until V
CC5
reaches V
RT5
≅ 4.725V (See Figure 4).
LTC1326-2.5 Override Functions
The V
CCA
pin, if unused, can be tied to either V
CC3
or V
CC25
.
This is an obvious solution since the trip points for V
CC3
and V
CC25
will always be greater than the trip point for
V
CCA
. Likewise, the V
CC25
, if unused, can be tied to V
CC3
.
V
CC3
must always be used. Tying V
CC3
to V
CC25
and
operating off of a 2.5V supply will result in the continuous
assertion of RST.
Extending ESD Tolerance on the PBR Input Pin
The PBR pin is susceptible to ESD since it may be brought
out to a front panel in normal applications. The ESD
tolerance of this pin can be increased by adding a resistor
in series with the PBR pin. A 10k resistor can increase the
ESD tolerance of the PBR pin to approximately 10kV. The
PBR’s internal pull-up current of 7µA typical means there
is only 70mV (150mV max) dropped across the resistor.
See Figure 5.
APPLICATIO S I FOR ATIO
WUU
U
PUSH-BUTTON
RESET
1
2
3
4
8
7
6
5
V
CC3
V
CC25
V
CCA
GND
PBR
SRST
RST
RST
LTC1326-2.5
3.3V
SYSTEM RESET
2.5V
R2
R1
1326/2.5 F05
ADJUSTABLE SUPPLY
OR DC/DC FEEDBACK
DIVIDER
10k*
*OPTIONAL RESISTOR EXTENDS ESD TOLERANCE OF PBR INPUT TO APPROXIMATELY 10kV
Figure 5. Triple Supply Monitor (3.3V, 2.5V
and Adjustable) with Extended ESD Tolerance
1
2
3
4
8
7
6
5
V
CC3
V
CC5
V
CCA
GND
PBR
SRST
RST
RST
LTC1326
SYSTEM RESET
1326/2.5 F03
5V
0.1µF
4.7k
D1*
*MMBD914 OR EQUIVALENT
V
CC5
(V)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
RST OUTPUT VOLTAGE (V)
1326/2.5 F04
5
4
3
2
1
0
V
CC5
= V
CCA
= 0V TO 5V
4.7k PULL-UP FROM RST TO V
CC5
T
A
= 25°C
Figure 3. LTC1326 Monitoring a Single 5V Supply.
D1 Used to Avoid RST High Near 3.3V to 4V (See
Figure 2).
Figure 4. RST Output Voltage
Characteristics of the Circuit in Figure 3
