PC87373 datasheet(118/174 Pages) NSC

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9.0 Glue Functions (Continued)

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Revision 1.1

9.2.2

Highest Active Standby Supply Reference

This function generates the REF5V_STBY analog output signal (see specification in Section 13.2.10 on page 155). 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 VSB3 supply voltage and is implemented in the PC87373 device.

One part tracks the VSB5 supply voltage and is implemented by an external resistor connected to the VSB5 power

supply.

Figure 23 shows a simplified diagram of the circuit that generates the REF5V_STBY analog output signal

Figure 23. 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 VA >VB, then VSB3 >VSB5; the switch is closed and the output voltage VREF5V_STBY =VSB3

If VA <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 V

SB5 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 24 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

PC87373

PC87373 数据表(PDF) 118 Page - National Semiconductor (TI)