PC8374L datasheet(120/239 Pages) NSC

120 / 239 page

7.0 Glue Functions (Continued)

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120

Revision1.1

7.2.2

Highest Active Standby Supply Reference

This function generates the REF5V_STBY analog output signal (see specification in Section 10.2.11 on page 216). When

the Standby power supply is turned on or off, the REF5V_STBY signal tracks either the VSB3 or VSB5 power supplies, which-

ever has a higher voltage.

The circuit that generates the REF5V_STBY output is composed of two parts:

● One part tracks the V

SB3 supply voltage and is implemented in the PC8374L device.

● One part tracks the V

SB5 supply voltage and is implemented by an external resistor connected to the VSB5 power

supply.

Figure 21 shows a simplified diagram of the circuit that generates the REF5V_STBY analog output signal

Figure 21. REF5_STBY Generation (Simpliﬁed Diagram)

The voltage levels at both sides of switch “S” are measured by the comparator (“Comp”), which controls the state of the

switch as follows:

● If V

A >VB, then VSB3 >VSB5; the switch is closed and the output voltage VREF5V_STBY =VSB3

● If V

A <VB, then VSB3 <VSB5; the switch is open and the output voltage VREF5V_STBY=VSB5, through the external 1

Ω resistor, is connected to the VSB5 power supply

The internal circuit is powered from the VSB3 supply. For VSB3 <VSO, switch “S” is open; therefore, the VREF5V_STBY output

tracks only the VSB5 supply. Figure 22 shows the behavior of the REF5V_STBY output in four cases of VSB3 and VSB5 ramp

up and ramp down.

VSB5

1 K

Ω

VSB3

REF5V_STBY

Comp

0.1-1

µF

PC8374L

Winbond Electronics Corp. Advanced PC Product Center

PC8374L Datasheet(PDF) 120 Page - National Semiconductor (TI)