FAN1851A

PRODUCT SPECIFICATION

2

REV. 2.0.1 6/17/05

Functional Description

The voltage at the supply pin is clamped to +26V by the

internal shunt regulator D3. This shunt regulator also

generates an artificial ground voltage for the noninverting

input of A1 (shown as a +10V source). A1, Q1, and Q2

together act as a current mirror for fault current signals

(which are derived from an external transformer). When a

fault signal is present, the mirrored current charges the

external timing capacitor until its voltage exceeds the latch

trigger threshold (typically 17.5V). When this threshold is

drive the SCR connected to the "SCR Trigger" pin.

Extra Circuitry in the feedback path of A1 works with the

induced by noise in the transformer. If no fault current is

resistor. If fault signals are present at the input of A1 (which

is held at virtual ground, +10V), one of the two current

mirrors in the feedback path of A1 (Q4 and Q5) will become

active, depending on which half-cycle the fault occurs.

better allow fault currents to charge it.

it.

Pin Assignments

SCR Trigger

– Input

+ Input

Ground

Amp Out

1

2

3

4

8

7

6

5

Definition of Terms

Normal Fault:

nal of the hot line and the ground, as shown by the dashed

lines in Figure1.

Figure 1. Normal Fault

Grounded Neutral Fault:

An unintentional electrical path between the load terminal of

the neutral line and the ground, as shown by the dashed lines

in Figure 2.

Figure 2. Grounded Neutral Fault

Hot

GFI

Neutral

Line

Hot

Neutral

Hot

GFI

Neutral

Line

Hot

Neutral