LT1638CS8 datasheet(11/16 Pages) LINER

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LT1638/LT1639

16389fd

APPLICATIONS INFORMATION

TYPICAL APPLICATIONS

With 1.2MHz bandwidth, Over-The-Top capability, re-

verse-battery protection and rail-to-rail input and output

features, the LT1638/LT1639 are ideal candidates for

general purpose applications.

The lowpass slope limiting filter in Figure 1 limits the

maximum dV/dT (not frequency) that it passes. When the

input signal differs from the output by one forward diode

drop, D1 or D2 will turn on. With a diode on, the voltage

across R2 will be constant and a fixed current, VDIODE/R2,

will flow through capacitor C1, charging it linearly instead

of exponentially. The maximum slope that the circuit will

pass is equal to VDIODE divided by (R2)(C1). No matter

how fast the input changes the output will never change

any faster than the dV/dT set by the diodes and (R2)(C).

nonlinear common mode rejection. If the op amp is oper-

ating inverting there is no common mode induced distor-

tion. If the op amp is operating in the PNP input stage

(input is not within 0.8V of V+), the CMRR is very good,

typically 98dB. When the LT1638 switches between input

stages there is significant nonlinearity in the CMRR. Lower

load resistance increases the output crossover distortion,

but has no effect on the input stage transition distortion.

For lowest distortion the LT1638/LT1639 should be oper-

ated single supply, with the output always sourcing

current and with the input voltage swing between ground

and (V+ – 0.8V). See the Typical Performance Character-

istics curves.

Gain

The open-loop gain is almost independent of load when

the output is sourcing current. This optimizes perfor-

mance in single supply applications where the load is

returned to ground. The typical performance curve of

Open-Loop Gain for various loads shows the details.

VOUT

VIN

Response of Slope Limiting Filter

Figure 2. Lowpass Slope Limiting Filter with 0 TC

Figure 1. Lowpass Slope Limiting Filter

1638/39 TA02

A modification of this application is shown in Figure 2

using references instead of diodes to set the maximum

slope. By using references, the slope is independent of

temperature. A scope photo shows a 1VP-P, 2kHz input

signal with a 2V pulse added to the sine wave; the circuit

passes the 2kHz signal but limits the slope of the pulse.

–

1/2 LT1638

VOUT

1638/39 F01

VIN

FOR R1 = 10k, R2 = 100k, C1 = 1000pF

VOUT(MAX) =

(R2)(C1)

VOUT(MAX) = 0.006V/μs

–

1/4 LT1639

–

1/4 LT1639

VCC

VOUT

VEE

VIN

100k

1638/39 F02

LT1634-1.2V

100k

LT1634-1.2V

FOR R2 = 50k, C1 = 500pF,

MAXIMUM SLOPE = 0.048V/

μs

VOUT =

1.2V

(R2)(C1)

D1 TO D4 = IN4148

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