9

LT1638/LT1639

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

will flow through capacitor C1, charging it linearly instead

of exponentially. The maximum slope that the circuit will

how fast the input changes the output will never change

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

–

+

–

+

1/4 LT1639

–

+

1/4 LT1639

1/4 LT1639

D1

D2

C1

R5

100k

R6

100k

1638/39 F02

LT1634-1.2V

R3

100k

R4

100k

R2

R1

1k

D3

D4

LT1634-1.2V

FOR R2 = 50k, C1 = 500pF,

MAXIMUM SLOPE = 0.048V/

µs

d

dt

1.2V

(R2)(C1)

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.

Response of Slope Limiting Filter

Figure 2. Lowpass Slope Limiting Filter with 0TC

–

+

1/2 LT1638

C1

1638/39 F01

R2

D2

D1

R1

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

d

dt

(R2)(C1)

d

dt

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

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

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

The application in Figure 3 utilizes the Over-The-Top

capabilities of the LT1638. The 0.2

Ω resistor senses the

load current while the op amp and NPN transistor form a

closed loop making the collector current of Q1

Figure 1. Lowpass Slope Limiting Filter

1638/39 TA02