FSFR1700HSL FAIRCHILD [Fairchild Semiconductor], FSFR1700HSL Datasheet - Page 11

• Manufacturer Part Number: FSFR1700HSL
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2011 Fairchild Semiconductor Corporation
FSFR1800 / FSFR1700-HS • Rev.1.0.0
C
transformer or switching transition. Generally, hundreds
of pico-farad to tens of nano-farad is adequate,
depending on the quantity of noise.
The start and stop input-voltage can be calculated as:
7. Simple Remote-On/Off: The power stage can be
shutdown with optional Auto-Restart Mode, as shown in
Figure 26.
To configure an external protection with Auto-Restart
Mode, an opto-coupler and the LS pin are used. When
the voltage on the LS pin is pulled below V
IC stops during the status holds. However, the opto-
coupler stops pulling down and the IC can perform the
auto-restart operation itself.
8. Current-Sensing Methods: FSFR-HS series employs
negative voltage sensing to detect the drain current of
MOSFET, which allows a low-noise resistive sensing
using a filter with low time-constant and capacitive
sensing method.
V
V
Filter
dc
dc
8.1 Resistive Sensing Method: The IC can sense
drain current as a negative voltage, as shown in Figure
27 and Figure 28. Half-wave sensing allows low power
dissipation in the sensing resistor; while full-wave
sensing has less switching noise in the sensing signal.
For a time constant range for the filter, 3/100~1/10 of
the operating frequency is reasonable.


link
link
can be used to reduce some noise induced from
,
,
START
STOP
Figure 26. External Protection Circuits


Figure 25. Half-Wave Sensing
V
V
dc
LINE

link
,

STOP
R
1

R

I
2
LINE
R
2

[
V
R
1
]
[
V
]
LINE
(2.5V), the
(6)
(7)
11
8.2 Capacitive Sensing Method: The drain current
can be sensed using an additional capacitor parallel
with the resonant capacitor, as shown in Figure 29.
During the low-side switch turn on, the current, i
through C
scale-down of i
Generally, 1/100~1/1000 is adequate for the ratio of C
against C
generated by switching transition. Several hundreds of
ohm to a few of kilo-ohms can be normally used.
V
V
SENSE
sense

can be estimated as;
I
r
V
Cr
V
. R
B
CS
Figure 27. Half-Wave Sensing
CS
Figure 28. Full-Wave Sensing
, makes V
pk
R
D
sense
CS
CS
C
Cr
is used as a damper for reducing noise
p
Control
Control
B
SG
SG
by the impedance ratio of C
IC
IC

R
I
DS
sense
I
PG
DS
SENSE
[V
PG
R
Cr
Np
sense
]
Cr
across R
Np
V
I
V
I
DS
DS
CS
CS
Ns
Ns
Ns
Ns
SENSE
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. The i
r
and C
CB
(8)
CB
B
is
B
.