NCP302 datasheet(21/26 Pages) ONSEMI

21 / 26 page

NCP302, NCP303

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Figure 46. Multi−Rail Supply Undervoltage Monitor with Power Good

R1 is Optional CD Pin Pullup

3.3 V Power Supply 2

(I/O Subsystem)

VIN

0 V

VTCD

CD Pin

Note: VTCD 0.675 * VIN

tD2

Reset Output

Input

GND

NCP302L

Series

Reset Output

Input

GND

Reset Output

Input

GND

NCP301

LSN30T1

NCP301

LSN45T1

5.0 V Power Supply 3

(Peripheral Subsystem)

Power Supply 1

(System Core)

*Required for

NCP303

To MCU or

Logic Circuitry

0 V

tD2

Power Supply 1

Power Supply 2

Power Supply 3

NCP302L

RESET Output

This circuit monitors multiple power supply rails for

undervoltage conditions. If any of the three power supplies

are in an undervoltage condition, the NCP302 reset output

will be immediately set to an active low level. All three

power supplies must be above their minimum voltage levels

for the NCP302 reset output to generate a “Power Good”

level (Reset Output = Power Supply 1 or VP).

Optionally, R1 may be added to provide a smaller

effective

CD pin pullup resistance, (RD’), where

RD’ = R1 || RD, with RD (internal CD pin pullup resistance)

approximately equal to 1.0 M

W, and R1 > 5 kW. If R1 << RD,

then R1 also can decrease the reset output delay time (tD2)

variance over the operating temperature range.

The Power Good signal time delay (tD2) can be estimated

by: tD2

≈ RD * CD, with RD in Ohms, and CD in Farads. If

R1 is installed, then RD’ is substituted for RD. RP is added

only if using the NCP303 to replace the NCP302. This

allows the Reset Output to be pulled up to VP, which can be

the Power Supply 1 or an independent power supply rail.

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