PLM393LVDR datasheet(17/29 Pages) TI1

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1 / 2 R 4

C1 I n2

A 2

(R IIR )

R + R IIR

R I IR

LM393LV-Q1

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Typical Applications (continued)

10.2.2.1 Design Requirements

The square-wave period is determined by the RC time constant of the capacitor C1 and resistor R4. The

maximum frequency is limited by propagation delay of the device and the capacitance load at the output. The low

input bias current allows a lower capacitor value and larger resistor value combination for a given oscillator

frequency, which may help to reduce BOM cost and board space. R4 should be over several kilo-ohms to

minimize loading the output.

10.2.2.2 Detailed Design Procedure

The oscillation frequency is determined by the resistor and capacitor values. The following calculation provides

details of the steps.

Figure 11. Square-Wave Oscillator Timing Thresholds

First consider the output of Figure Figure 10 as high, which indicates the inverted input VC is lower than the

noninverting input (VA). This causes the C1 to be charged through R4, and the voltage VC increases until it is

equal to the noninverting input. The value of VA at the point is calculated by Equation 7.

(7)

if R1 = R2= R3, then VA1 = 2 VCC/ 3

At this time the comparator output trips pulling down the output to the negative rail. The value of VAat this point is

calculated by Equation 8.

(8)

if R1 = R2 = R3, then VA2 = VCC/3

The C1 now discharges though the R4, and the voltage VCC decreases until it reaches VA2. At this point, the

output switches back to the starting state. The oscillation period equals to the time duration from for C1 from

2VCC/3 to VCC / 3 then back to 2VCC/3, which is given by R4C1 × ln 2 for each trip. Therefore, the total time

duration is calculated as 2 R4C1 × ln 2.

The oscillation frequency can be obtained by Equation 9:

(9)

10.2.2.3 Application Curve

Figure 12 shows the simulated results of an oscillator using the following component values:

•

R1 = R2 = R3 = R4 = 100 kΩ

•

C1 = 100 pF, CL = 20 pF

•

V+ = 5 V, V– = GND

•

Cstray (not shown) from VA TO GND = 10 pF

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