FLS3217 FAIRCHILD [Fairchild Semiconductor], FLS3217 Datasheet - Page 10

• Manufacturer Part Number: FLS3217
• Description: Single-Stage PFC Primary-Side-Regulation Offline LED Driver with Integrated Power MOSFET
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2012 Fairchild Semiconductor Corporation
FLS3217 / FLS3247 • Rev. 1.0.0
BCM Control
The end of secondary diode conduction time is possibly
over a switching period set by linear frequency control.
In this case, FLS3217 and FLS3247 don’t allow CCM
and the operation mode changes from DCM to BCM.
Therefore, FLS3217 and FLS3247 eliminate sub-
harmonic distortion in CCM.
Short-LED Protection
In the event of a short LED condition, the switching
MOSFET and secondary diode are usually stressed by
the
FLS3247 change OCP level in short-LED condition.
When V
0.2V from 0.7V, as shown in Figure 17. Power is limited
and external component current stress is relieved.
Figure 20 shows operational waveforms at short-LED
condition. Output voltage is quickly lowered to 0V after
the LED-short event. Then the reflected auxiliary voltage
is also 0V, making V
limits the primary-side current and V
down in between UVLO hysteresis.
75% V
60% V
V
V
V
V
O.nom
O
O
O
Figure 18. Primary and Secondary Current
high-power
=
=
O.nom
=
O.nom
+
-
S
V
Primary
OCP
Current
is lower than 0.4V, the OCP level lowers to
Figure 19. Internal OCP Block
At V
At V
V
V
OCP
OCP
t
4
3
S
S
Secondary
DIS
5
3
= 0.2V.
= 0.7V.
Current
nVo
< 0.4V,
> 0.6V,
Lm
current.
t
t
n
DIS
DIS
Lm
3
4
n
t
V
S
Lm
O
3
5
V
less than 0.4V. 0.2V OCP level
O
4
3
t
5
3
However,
LEB
t
DD
“hiccups” up and
FLS3217
1 CS
4
VS
and
Open LED Protection
FLS3217 and FLS3247 protect external components,
such as diode and capacitor, at secondary side in open-
LED condition. During switch-off, the V
charged up to the auxiliary winding voltage, which is
applied as the reflected output voltage. Because the V
voltage has output voltage information, the internal
voltage comparator on the VDD pin can trigger output
over-voltage protection (OVP), as shown in Figure 21.
When at least one LED is open-circuited, output load
impedance becomes very high and the output capacitor
is quickly charged up to V
is shutdown and V
the open-LED condition is removed, shown in Figure 22.
VDD
V
10
V
DD_OFF
DD_ON
Figure 20. Waveforms at Short-LED Condition
0.2V
V
V
V
V
DD
CS
IN
DD
LED Short !
2
Good
Figure 21. Internal OVP Block
V
OVP
DD
R
S
block goes into “Hiccup Mode” until
Q
OVP
V
Internal
+
-
DD
x Ns / Na. Then switching
Bias
Shutdown Gate Driver
Good
DD
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capacitor is
DD