LTC2908
APPLICATIO S I FOR ATIO
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 possess-
ing several other beneficial 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 configuration 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
fault condition, V
CC
of at least 0.5V guarantees a maxi-
mum 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 tempera-
ture (25°C) and C
LOAD
is the external load capacitance on
the pin. Assuming a 150pF load capacitance, the fall time
is about 13ns.
U
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 transistor.
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:
6 • 10
5
R
PU
=
Ω
MAX
(
V1, V2
)
– 1V
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.
sn2908 2908fs
W
U U
11
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
