6

LT1684

differential input is amplitude normalized to

±1.25V (refer-

enced to the OUT pin). This signal is used to drive the

active filter/amplifier. Filter resistor values must be chosen

to limit the maximum current load on this pin to less than

2mA. The output is current limit protected to a typical value

of

±4.5mA.

IN A (Pin 14): PWM Positive Input. Input is isolated from

digital source by ~100pF series capacitor. A 10k resistor

should be connected to the IN A pin in series with the

isolation capacitor for transient protection. The PWM

receiver implements a diode forward drop of input hyster-

esis (relative to IN B). This hysteresis and internal signal

limiting assure common mode glitch rejection with isola-

tion capacitor mismatches up to 2:1. For maximum perfor-

mance, however, effort should be made to match the two

PWM input isolation capacitors. Pin IN A is differentially

clamped to pin IN B through back-to back isolation-base

diodes. This results in a high impedance differential input

±100mV beyond the input thresholds. 5k internal input

resistors yield a 10k (nominal) differential overdrive im-

pedance.

LT1684 Block Diagram

5k

10k

IN A

100pF

+

5k

10k

IN B

100pF

–

PWM

INPUT

5k

AMPIN

20pF

(RING RETURN)

100pF

FILTER

ELEMENTS

COMP1

COMP2

RING

OUTPUT

15k

CURRENT

LIMIT

OUT

14V

14V

1684 BD

1.25V

–1.25V

PI FU CTIO S