PC87373 datasheet(117/174 Pages) NSC

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9.0 Glue Functions (Continued)

117

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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 VDD3 >VDD5; the switch is closed, and the output voltage VREF5V =VDD3.

If VA <VB, then VDD3 <VDD5; the switch is open, and the output voltage VREF5V =VDD5, through the external 1KΩ

resistor, is connected to the VDD5 power supply.

The internal circuit is powered from the VDD3 supply. For VDD3 <VSO, switch “S” is open; therefore, the VREF5V output tracks

only the VDD5 supply. Figure 22 shows REF5 output in four cases of VDD3 and VDD5 ramp up and ramp down.

Figure 22. REF5 as a Function of VDD3 and VDD5

Table 44 defines the DC characteristics of the REF5 output, relative to the VDD3 supply. For the AC characteristics, see High-

est Active Main and Standby Supply Reference on page 170.

Table 44. REF5V DC Characteristics

Symbol

Parameter

Conditions

Min

Max

Unit

VSO

Switch Open, VDD3 Voltage Range

0 < VDD5 < 5.5V

1.51

1. Not tested. Guaranteed by characterization.

VTRK

Output Voltage Tracking of VDD3 Voltage

1.5V < VDD3,

VDD5 <VDD3 + 150 mV

− 150

+ 150

a. VDD3 Ramps Up Before and Ramps Down After VDD5

VSO

3.3V

VDD3

VDD5

REF5V

b. VDD3 Ramps Up After and Ramps Down Before VDD5

VSO

3.3V

VDD3

VDD5

REF5V

VDD3

REF5V

VDD3

VDD5

PC87373 Datasheet(PDF) 117 Page - National Semiconductor (TI)

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