LTC2908
13
2908fd
APPLICATIONS INFORMATION
RST Output Characteristics
The DC characteristics of the RST pull-up and pull-down
strength are shown in the Typical Performance Character-
istics section. The RST output has a weak internal pull-up
to V
CC
= Max(V1, V2) and a strong pull-down to ground.
The weak pull-up and strong pull-down arrangement allows
this pin to have open-drain behavior while possessing
several other benefi cial characteristics.
The weak pull-up eliminates the need for external pull-up
resistors when the rise time on these pins is not critical.
On the other hand, the open-drain RST behavior allows for
wired-OR connections and can be useful when more than
one signal needs to pull down on the RST line.
As noted in the discussion of power-up and power-down,
the circuits that drive
RST
are powered by V
CC
. During
a fault condition, a V
CC
of at least 0.5V guarantees a
maximum V
OL
= 0.15V at
RST
.
Output Rise and Fall Time Estimation
The following formula estimates the output fall time (90% to
10%) for a particular external load capacitance (C
LOAD
):
t
FALL
≈ 2.2 • R
PD
• C
LOAD
where R
PD
is the on-resistance of the internal pull-
down transistor estimated to be typically 40Ω at room
temperature (25°C) and C
LOAD
is the external load
capacitance on the pin. Assuming a 150pF load capacitance,
the fall time is about 13ns.
The rise time on the RST pin is limited by a weak internal
pull-up current source to V
CC
. The following formula esti-
mates the output rise time (10% to 90%) at the RST pin:
t
RISE
≈ 2.2 • R
PU
• C
LOAD
where R
PU
is the on-resistance of the pull-up transis-
tor. Notice that this pull-up transistor is modeled as a
6μA current source in the Block Diagram as a typical
representation.
The on-resistance as a function of the V
CC
= Max(V1, V2)
voltage (for V
CC
> 1V) at room temperature is estimated
as follows:
R
PU
=
6•10
5
MAX V1, V2
()
–1V
Ω
At V
CC
= 3.3V, R
PU
is about 260k. Using 150pF for load
capacitance, the rise time is 86μs. A smaller external
pull-up resistor may be used if the output needs to pull
up faster and/or to a higher voltage. For example, the rise
time reduces to 3.3μs for a 150pF load capacitance when
using a 10k pull-up resistor.
TYPICAL APPLICATIONS
Six Supply Monitor, 5% Tolerance, 12V, 5V, 3.3V, 2.5V, 1.8V, 1V
RST
R1
2.15M
C1
0.1μF
C2
0.1μF
V1 V2 V3 V4
GND
LTC2908-A1
V
ADJ1
12V
5V
3.3V
2.5V
1.8V
1.0V
2908 TA02
R2
100k
DC/DC
DC/DC
DC/DC
DC/DC
DC/DC
SYSTEM
R3
86.6k
V
ADJ2
R4
100k
