AMC22C12 datasheet(21/46 Pages) TI

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7.3.2 Reference Input

The voltage on the REF pin determines the trip threshold of the window comparator. The internal precision

current source forces a 100-μA current through an external resistor connected from the REF pin to GND1. The

resulting voltage across the resistor (VREF) equals the magnitude of the positive and negative trip thresholds,

see Figure 7-1. Place a 100-nF capacitor parallel to the resistor to filter the reference voltage. This capacitor

must be charged by the 100-μA current source during power-up and the charging time may exceed the high-side

blanking time (tHS,BLK). In this case, as shown in Figure 7-2, the comparator may output an incorrect state after

the high-side blanking time has expired until VREF reaches the final value. See the Power-Up and Power-Down

Behavior section for more details on power-up behavior.

tHS, STA + tHS,BLK

OUT

VREF settling time

low

Output not valid

Output valid

VREF

VDD2

VDD1

<1%

OUT not valid

if VIN is in this range

during power-up

Figure 7-2. Output Behavior for Long Settling Times of the Reference Voltage

The voltage on the REF pin also determines the functionality of the negative comparator (Cmp1) and the

hysteresis of the positive comparator (Cmp0) shown in the Functional Block Diagram. If VREF exceeds the

VMSEL threshold defined in the Electrical Characteristics table, Cmp1 is disabled and the hysteresis of Cmp0

is increased from 4 mV (typical) to 25 mV. Positive-comparator mode is intended for voltage-monitoring

applications that require higher input voltages and higher noise immunity.

The reference pin can be driven by an external voltage source to change the comparator thresholds during

operation. However, do not drive VREF dynamically across the VMSEL threshold during normal operation because

doing so changes the hysteresis of the Cmp0 comparator and can lead to unintentional switching of the output.

Figure 7-3 shows a mode selection timing diagram.

tHSEL

Hysteresis

(Cmp0)

Enable

(Cmp1)

VMSEL

tHSEL

enabled

disabled

tEN13

tDIS13

VREF

4 mV

25 mV

4 mV

Mode of operation

window comparator mode

positive-comparator mode

Figure 7-3. Mode Selection

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AMC22C12-Q1

SBASAJ9A – JUNE 2022 – REVISED AUGUST 2022

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