LTC1985-1.8 데이터 시트보기 (PDF) - Linear Technology
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LTC1985-1.8
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LTC1985-1.8 Datasheet PDF : 8 Pages
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LTC1985-1.8
APPLICATIO S I FOR ATIO
Supply Monitoring
The LTC1985 is a low power, high accuracy triple supply
monitoring circuit with a single 200ms microprocessor
reset output.
All three VCC inputs must be above predetermined thresh-
olds for reset not to be invoked. The LTC1985 will assert
reset during power-up, power-down and brownout condi-
tions on any one or all of the VCC inputs.
3V or 1.8V Power Detect
The LTC1985 is powered from the 3V input pin (VCC3) or
the 1.8V input pin (VCC18), whichever pin has the highest
potential. This ensures the part pulls the RST pin low as
soon as either input pin is ≥1V.
Power-Up
Upon power-up, either the VCC18 or VCC3 pin, can power
the part. This ensures that RST will be low when either
VCC18 or VCC3 reaches 1V. As long as any one of the VCC
inputs is below its predetermined threshold, RST will stay
a logic low. Once all of the VCC inputs rise above their
thresholds, an internal timer is started and RST is driven
high after 200ms.
RST is reasserted whenever any one of the VCC inputs
drops below its predetermined threshold and remains
asserted until 200ms after all of the VCC inputs are above
their thresholds.
Power-Down
On power-down, once any of the VCC inputs drop below its
threshold, RST is held at a logic low. A logic low of 0.3V is
guaranteed until both VCC3 and VCC18 drop below 1V.
ADJUSTABLE R1
SUPPLY
LTC1985-1.8
5
3V
VCC3
4
1.8V
VCC18
3
VCCA
2
R2
GND
1
RST
1985-18 TA06
R3
100k
Override Functions
The VCCA pin, if unused, can be tied to either VCC3 or VCC18.
This is an obvious solution since the trip points for VCC3
and VCC18 will always be greater than the trip point for
VCCA. Likewise, the VCC18, if unused, can be tied to VCC3.
VCC3 must always be used. Tying VCC3 to VCC18 and
operating off of a 1.8V supply will result in the continuous
assertion of RST.
Ensuring RST Valid for Supply Voltages Under 1V
When the supplies drops below 1V the RST output current
sink capability is drastically reduced. The combination of
stray currents and stray capacitance to signals other than
ground can cause the RST output pin to float around. In a
lot of applications this is not a problem since most
microprocessors and other circuits do not operate with
the supply voltage less than 1V. In applications where the
RST output must be valid down to 0V the addition of a pull-
down resistor from RST to ground will ensure RST is held
low. The circuit in Figure 1 shows an application employ-
ing this technique. The value chosen for the pull-down
resistor (R3) is a trade-off between pull-down strength
and loading of the RST pin. If the value of the resistor is too
large the pin may still float and if the resistor value is too
low it may load down the RST as well as burn excess
supply current, a value of 100k is a good compromise.
4
VCC3 = VCC18 = VCCA
3
2
1
0
0
1
2
3
4
VCC3 (V)
1985-18 TA06b
Resistor on RST Output to Ground Ensures RST Valid to VCC = 0V
Figure 1. Typical Application Showing
Resistor on RST Output to Ground
Figure 2. RST Voltage vs VCC3 with
a 100k Resistor on RST to Ground
6
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