Datasheet

LTC2904/LTC2905
12
29045fd
APPLICATIONS INFORMATION
Output Rise and Fall Time Estimation
The RST and RST outputs have strong pull-down capabil-
ity. 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.
The rise time, on the RST and RST pins is limited by weak
internal pull-up current sources to V
CC
. The following
formula estimates the output rise time (10% to 90%) at
the RST and RST pins:
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:
R
MAX V V V
PU
=Ω
610
12 1
5
•
(,)–
At V
CC
= 3.3V, R
PU
is about 260k. Using 150pF for load
capacitance, the rise time is 86μs. An 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 re-
duces to 3.3μs for a 150pF load capacitance, when using
a 10k pull-up resistor.
RST and RST Output Characteristics
The DC characteristics of the RST and RST pull-up and
pull-down strength are shown in the Typical Performance
Characteristics section. Both RST and RST have 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 allow
these two pins to have open-drain behavior while possess-
ing several other benefi cial characteristics.
The weak pull-ups eliminate 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 confi guration 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 Power-Up and Power-Down sections the
circuits that drive RST and RST are powered by V
CC
. During
fault condition, V
CC
of at least 1V guarantees a maximum
V
OL
= 0.4V at RST. However, at V
CC
= 1V the weak pull-up
current on RST is barely turned on. Therefore, an external
pull-up resistor of no more than 100k is recommended on
the RST pin if the state and pull-up strength of the RST
pin is crucial at very low V
CC
.
Note however, by adding an external pull-up resistor, the
pull-up strength on the RST pin is increased. Therefore,
if it is connected in a wired-OR connection, the pull-down
strength of any single device needs to accommodate this
additional pull-up strength.