LTC1728 データシートの表示（PDF） - Linear Technology
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LTC1728
Linear Technology
LTC1728 Datasheet PDF : 18 Pages
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LTC1727/LTC1728
APPLICATIONS INFORMATION
Override Functions (5V Versions Only)
The VCCA pin, if unused, can be tied to either VCC3 or
VCC5. This is an obvious solution since the trip points for
VCC3 and VCC5 will always be greater than the trip point
for VCCA.
The VCC5 input trip point is disabled if its voltage is
equal to the voltage on VCC3 ± 25mV and the voltage on
VCC5 is less than 4.15V. In this manner, the LTC1727-5/
LTC1728-5 will behave as a 3.3V monitor and VCC5 reset
capability will be disabled.
The VCC5 trip point is reenabled when the voltage on
VCC5 is equal to the voltage on VCC3 ± 25mV and the
two inputs are greater than approximately 4.15V. In this
manner, the part can function as a 5V monitor with the
3.3V monitor disabled.
When monitoring either 3.3V or 5V with VCC3 strapped
to VCC5 (see Figure 2), the part determines which is the
appropriate range. The part handles this situation as
shown in Figure 3. Above 1V and below VRT3, RST is held
low. From VRT3 to approximately 4.15V the part assumes
3.3V supply monitoring and RST is deasserted. Above
approximately 4.15V the part operates as a 5V monitor.
In most systems the 5V supply will pass through the
3.1V to 4.15V region in <200ms during power-up, and
the RST output will behave as desired. Table 1 summa-
rizes the state of RST at various operating voltages with
VCC3 = VCC5.
Table 1. Override Truth Table (VCC3 = VCC5)
INPUTS (VCC3 = VCC5 = VCC)
RST
0V ≤ VCC ≤ 1V
—
1V ≤ VCC ≤ VRT3
0
VRT3 ≤ VCC ≤ 4.15V
1
4.15V ≤ VCC ≤ VRT5
0
VRT5 ≤ VCC
1
VCC
3.3V OR 5V
LTC1728-5
5 VCC3
4 VCC5
3
VCCA
RST 1
2
GND
R1
10k
TO SYSTEM
RESET
1727/28 F02
Figure 2. Single Supply Monitor with Others Disabled
5
VCC3 = VCC5 = VCCA = 0V TO 5V
10k PULL-UP FROM RST TO VCC3
4
3
2
1
0
0
1
2
3
4
5
SUPPLY VOLTAGE (V)
1727/28 F03
Figure 3. RST Voltage vs Supply Voltage
For more information www.linear.com/LTC1727
17278fd
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