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LTC2908

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 VCC = 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 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 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 VCC. During

a fault condition, a VCC of at least 0.5V guarantees a

maximum VOL = 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 (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

temperature (25°C) and CLOAD 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 VCC. The following formula esti-

mates the output rise time (10% to 90%) at the RST pin:

tRISE ≈ 2.2 • RPU • CLOAD

where RPU 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 VCC = Max(V1, V2)

voltage (for VCC > 1V) at room temperature is estimated

as follows:

PU =

6• 10

MAX V1,V2

() –1V

At VCC = 3.3V, RPU 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

2.15M

0.1μF

V2 V3 V4

GND

LTC2908-A1

VADJ1

12V

3.3V

2.5V

1.8V

1.0V

2908 TA02

100k

DC/DC

SYSTEM

86.6k

VADJ2

100k

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