Datasheet

LTC1727/LTC1728
12
17278fb
applicaTions inForMaTion
Figure 4 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 LTC1728-5 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
≅ 4.675V. (See Figure 5.)
LTC1727-2.5/LTC1728-2.5/LTC1728-1.8/LTC1728-3.3
Override Functions
The V
CCA
pin, if unused, can be tied to either V
CC3
or V
CC25
/
V
CC18
. This is an obvious solution since the trip points for
V
CC3
and V
CC25
/V
CC18
will always be greater than the trip
point for V
CCA
. Likewise, the V
CC25
/V
CC18
, if unused, can
be tied to V
CC3
. V
CC3
must always be used. Tying V
CC3
to
V
CC25
/V
CC18
and operating off of a 2.5V/1.8V supply will
result in the continuous assertion of RST.
Figure 4. LTC1728-5 Monitoring a Single 5V
Supply. D1 Used to Avoid RST High Near 3.1V
to 4V (See Figure 3).
Figure 5. RST Output Voltage
Characteristics of the Circuit in Figure 4
V
CC3
V
CC5
V
CCA
GND
1
5
4
3
2
RST
LTC1728-5
1727/28 F04
TO SYSTEM
RESET
R1
10k
5V
0.1µF
D1
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)
1727/28 F05
5
4
3
2
1
0
V
CC5
= V
CCA
= 0V TO 5V
10k PULL-UP FROM RST TO V
CC5
T
A
= 25°C
Typical applicaTions
Triple Supply Monitor (3.3V, 5V and Adjustable)
5
4
3
2
1
V
CC3
V
CC5
V
CCA
GND
RST
LTC1728-5
3.3V
SYSTEM RESET
*TO PRESERVE THRESHOLD ACCURACY, SET PARALLEL
COMBINATION OF R1 AND R2 ≤ 66.5k
5V
R2*
R1*
1727/28 TA03
ADJUSTABLE SUPPLY
OR DC/DC FEEDBACK
DIVIDER