LTC1727 datasheet(10/16 Pages) LINER

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LTC1727/LTC1728

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.

APPLICATIO S I FOR ATIO

Figure 3. RST Voltage vs Supply Voltage

Figure 2. Single Supply Monitor with Others 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 summarizes

the state of RST at various operating voltages with

VCC3 = VCC5.

Table 1. Override Truth Table (VCC3 = VCC5)

INPUTS (VCC3 = VCC5 = VCC)

RST

≤ VCC ≤ 1V

—

≤ VCC ≤ VRT3

VRT3 ≤ VCC ≤ 4.15V

4.15V

≤ VCC ≤ VRT5

VRT5 ≤ VCC

VCC3

VCC5

VCCA

GND

RST

LTC1728-5

1727/28 F02

TO SYSTEM

RESET

10k

VCC

3.3V OR 5V

SUPPLY VOLTAGE (V)

1727/28 F03

VCC3 = VCC5 = VCCA = 0V TO 5V

10k PULL-UP FROM RST TO VCC3

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