Datasheet

LTC1726
8
1726fd
For more information www.linear.com/LTC1726
applicaTions inForMaTion
Figure 2. RST Voltage vs Supply Voltage
Figure 1. Single Supply Monitor with Others Disabled
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 1) the part determines which is the ap-
propriate range. The part 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 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 summarizes 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
Supply Monitoring
The LTC1726 is a low power, high accuracy triple supply
monitor and watchdog timer. The watchdog and reset
periods are both adjustable using external capacitors.
All three V
CC
inputs must be above predetermined thresh-
olds for reset not to be asserted. The LTC1726 will assert
reset during power-up, power-down and brownout condi-
tions on any one or all of the V
CC
inputs.
Upon power-up, either the V
CC5
/V
CC25
or V
CC3
pin can
power the drive circuits for the RST pin. This ensures that
RST will be low when either V
CC5
/V
CC25
or V
CC3
reaches
1V. As long as any one of the V
CC
inputs is below its pre-
determined threshold, RST will stay a logic low. Once all of
the V
CC
inputs rise above their thresholds, the adjustable
reset timer is started and RST is released after the reset
time-out period.
On power-down, once any of the
V
CC
inputs drops below
its threshold, RST is held at a logic low. A logic low of
0.3V is guaranteed until both V
CC3
and V
CC5
/V
CC25
drop
below 1V.
3V or 5V/2.5V Power Detect
Since the LTC1726 is a multisupply monitor, it will be
required to assert reset as soon as there is power on any
one of the monitor inputs. Therefore, the part derives its
power from either the V
CC3
or V
CC5
/V
CC25
input, whichever
pin has the greatest potential. This ensures the part pulls
the RST pin low as soon as either input pin is ≥1V. The
adjustable input is excluded from being a potential supply
pin because of its 1V nominal operating range.
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 LTC1726-5 behaves
as a 3.3V monitor and the 5V reset function is disabled.
The
V
CC5
trip point is re-enabled when the voltage on V
CC5
is equal to the voltage on V
CC3
±25mV and the two inputs
V
CC3
V
CC5
V
CCA
GND
6
1
2
3
4
RST
LTC1726-5
1726 F01
TO SYSTEM
RESET
PINS 5, 7 AND 8 NOT SHOWN FOR CLARITY
R1
10k
V
CC
3.3V OR 5V
SUPPLY VOLTAGE (V)
0
RST OUTPUT VOLTAGE (V)
3
4
5
4
1726 F02
2
1
0
1
2
3
5
V
CC3
= V
CC5
= V
CCA
= 0V TO 5V
10k PULL-UP FROM RST TO V
CC3
T
A
= 25°C