LTC2908
W U U
APPLICATIO S I FOR ATIO
U
RST Output Characteristics
The rise time on the RST pin is limited by a weak internal
pull-up current source to VCC. The following formula esti-
mates the output rise time (10% to 90%) at the RST pin:
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 VCC = Max(V1, V2) and a strong pull-down to ground.
tRISE ≈ 2.2 • RPU • CLOAD
where RPU 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 weak pull-up and strong pull-down arrangement
allowsthispintohaveopen-drainbehaviorwhilepossess-
ing several other beneficial characteristics.
The weak pull-up eliminates the need for external pull-up
resistorswhentherisetimeonthesepinsisnotcritical.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.
The on-resistance as a function of the VCC = Max(V1, V2)
voltage (for VCC > 1V) at room temperature is estimated as
follows:
6 •105
RPU
=
Ω
MAX V1,V2 – 1V
(
)
As noted in the discussion of power-up and power-down,
the circuits that drive RST are powered by VCC. During
fault condition, VCC of at least 0.5V guarantees a maxi-
mum VOL = 0.15V at RST.
At VCC = 3.3V, RPU is about 260k. Using 150pF for load
capacitance, the rise time is 86µs. A smaller external pull-
upresistormaybeusediftheoutputneedstopullupfaster
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.
Output Rise and Fall Time Estimation
The following formula estimates the output fall time (90%
to 10%) for a particular external load capacitance (CLOAD):
tFALL ≈ 2.2 • RPD • CLOAD
where RPD is the on-resistance of the internal pull-down
transistor estimated to be typically 40Ω at room tempera-
ture (25°C) and CLOAD is the external load capacitance on
the pin. Assuming a 150pF load capacitance, the fall time
is about 13ns.
sn2908 2908fs
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Linear Systems [ Linear Systems ]
