MAX917 데이터 시트보기 (PDF) - Maxim Integrated

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MAX917

SOT23, 1.8V, Nanopower, Beyond-the-Rails Comparators With/Without Reference

Maxim Integrated 

SOT23, 1.8V, Nanopower, Beyond-the-Rails

Comparators With/Without Reference

IN+

VTHR

IN-

VHB

VTHF

THRESHOLDS

HYSTERESIS

BAND

OUT

VCC

R3

R1

VIN

R2

VREF

VCC

OUT

VEE

MAX917

MAX919

Figure 2. Threshold Hysteresis Band

Additional Hysteresis (MAX917/MAX919)

The MAX917/MAX919 have a 4mV internal hysteresis

band (VHB). Additional hysteresis can be generated

with three resistors using positive feedback (Figure 3).

Unfortunately, this method also slows hysteresis re-

sponse time. Use the following procedure to calculate

resistor values.

1) Select R3. Leakage current at IN is under 2nA, so

the current through R3 should be at least 0.2µA to

minimize errors caused by leakage current. The cur-

rent through R3 at the trip point is (VREF - VOUT)/R3.

Considering the two possible output states in solving

for R3 yields two formulas: R3 = VREF/IR3 or R3 =

(VCC - VREF)/IR3. Use the smaller of the two resulting

resistor values. For example, when using the

MAX917 (VREF = 1.245V) and VCC = 5V, and if we

choose IR3 = 1µA, then the two resistor values are

1.2MΩ and 3.8MΩ. Choose a 1.2MΩ standard value

for R3.

2) Choose the hysteresis band required (VHB). For this

example, choose 50mV.

3) Calculate R1 according to the following equation:

R1 = R3 (VHB / VCC)

For this example, insert the values

R1 = 1.2MΩ (50mV/5V) = 12kΩ

4) Choose the trip point for VIN rising (VTHR) such that

VTHR > VREF · (R1 + R3)/R3 (VTHF is the trip point for

VIN falling). This is the threshold voltage at which the

comparator switches its output from low to high as

VIN rises above the trip point. For this example,

choose 3V.

5) Calculate R2 as follows:

R2 = 1/[VTHR/(VREF · R1) - (1 / R1) - (1 / R3)]

Figure 3. MAX917/MAX919 Additional Hysteresis

R2 = 1/[3.0V/(1.2V · 12kΩ) - (1 / 12kΩ) -

(1/1.2MΩ)] = 8.05kΩ

For this example, choose an 8.2kΩ standard value.

6) Verify the trip voltages and hysteresis as follows:

VIN rising: VTHR = VREF · R1 [(1 / R1) + (1 / R2)

+ (1 / R3)]

VIN falling: VTHF = VTHR - (R1 · VCC / R3)

Hysteresis = VTHR - VTHF

Additional Hysteresis (MAX918/MAX920)

The MAX918/MAX920 have a 4mV internal hysteresis

band. They have open-drain outputs and require an

external pullup resistor (Figure 4). Additional hysteresis

can be generated using positive feedback, but the for-

mulas differ slightly from those of the MAX917/

MAX919. Use the following procedure to calculate

resistor values.

1) Select R3 according to the formulas R3 = VREF / 1µA

or R3 = (VCC - VREF)/1µA - R4. Use the smaller of

the two resulting resistor values.

2) Choose the hysteresis band required (VHB).

3) Calculate R1 according to the following equation:

R1 = (R3 + R4) (VHB/VCC)

4) Choose the trip point for VIN rising (VTHR) (VTHF is

the trip point for VIN falling). This is the threshold

voltage at which the comparator switches its output

from low to high as VIN rises above the trip point.

5) Calculate R2 as follows:

( )⋅ ⎡

R2 = 1/⎣⎢⎢VTHR/ VREF

R1

⎛

−⎜

1⎞

⎟

⎝ R1⎠

−

1

⎤

⎥

R3 ⎦⎥

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