Datasheet
R2
R3
D2
C
ZCD
PGND
PGND
PGND
L3
VCC
ZCD
D
ZCD
TPS92311
R
ISNS
SWSW
GND
ZCD
VIN
SW
ISNS
SW
GND
COMP
MODE1
DLY
MODE2
NC
NC NC
TPS92311
www.ti.com
SNVS811B –MAY 2012–REVISED MAY 2013
Calculate The Current Sensing Resistor
After the primary inductance and transformer turn ratio is determined, the current sensing resistor, R
ISNS
can be
calculated.
The resistance for R
ISNS
is governed by the output current and transformer turn ratio, the equation in below can
be used.
where
• V
REF
is fixed to 0.14V internally. (14)
Transformer turn ratio, N
P
: N
S
is 3.8 : 1 and I
LED
= 0.35A
(15)
Figure 21. R
ISNS
Resistor Interface
Figure 22. Auxiliary Winding Interface to ZCD
Auxiliary Winding Interface To ZCD
In Figure 22, R2 and R3 forms a resistor divider which sets the thresholds for over voltage protection of V
LED,
V
ZCD-OVP
, and V
ZCD-PEAK
. Before the calculation, we need to set the voltage of the auxiliary winding, V
LAUX
at open
circuit.
For example :
Assume the nominal forward voltage of LED stack (V
LED
) is 21V.
To avoid false triggering ZCD
OVP
voltage threshold at normal operation, select ZCD
OVP
voltage at 1.3 times of the
V
LED
is typical in most applications. In case the transformer leakage is higher, the ZCD
OVP
threshold can be set
to 1.5 times of the V
LED
.
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