FAN1851A PRODUCT SPECIFICATION
2REV. 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 th e latch
trigger threshold (typically 17.5V). When this threshold is
exceeded, the latch engages and Q3 turns off, allowing I2 to
drive the SCR connected to the "SCR Trigger" pin.
Extra Circuitry in the feedback path of A1 works with the
switched current source I1 to remove any charge on CT
induced by noise in the transformer. If no fault current is
present, then I1 discharges CT with a current equal to 3 ITH,
where ITH is the value of current set by the external RSET
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.
This action will raise the voltage at VS, switching I1 to a
value equal to ITH, and reducing the discharge rate of CT to
better allow fault currents to charge it.
Notice that ITH discharges CT during both half-cycles of the
line, while IF only char ges CT during the half-cycle in which
IF exits the "- Input" pin (since Q1 will only carry fault cur-
rent in one direction). Thus, during one half-cycle, IF-ITH
charge s CT, while during the oth er half-cyc le ITH discharges
it.
Pin Assignments
SCR Trigger
– Input
+ Input
Ground
+VS
CT
RSET
Amp Out
1
2
3
4
8
7
6
5
Definition of Terms
Normal Fault:
An unintentional electrical path, RB, between the load termi-
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
RLOAD RB
Neutral
RG
Hot
GFI
Neutral
Line
Hot
RLOAD
RIN
Neutral
RG