Cirrus Logic AN364 - Manual (Page 11)

AN364
AN364REV3 11
Step 13) Circuit Adjustments
Circuit adjustments are required after the transformer has been designed and constructed. Recalculate
R
FBGAIN
using Equation 12.
Flyback gain FB
Gain
is an internal constant that is programmable by R
FBGAIN.
FB
Gain
is used in the flyback algorithm to control switching period TT. The range of FB
Gain
is limited to:
which has the following implications:
If FB
Gain
is close to 1, the duty cycle will be low, which will cause a low reflected voltage V
Reflected
. In this
situation, a high I
PK(FB)
current is required to reach full brightness, and the only advantage would be to lower
the FET breakdown voltage. If the value of FB
Gain
were in the range of 2 to 2.5, then V
Reflected
would be in the
range of V
BST
to (1.5  V
BST
), resulting in a duty cycle that is between 50% and 60%. This setting causes a
lower I
PK(FB)
, requiring a higher FET breakdown voltage.
The duration of T1 is determined by the time it takes the primary current to reach I
PK(FB)
, and the duration of
T2 is dependent on the time it takes the secondary current to reach 0. To achieve the best full power efficiency,
adjust switching period TT, which will start a new switching cycle after the flyback FET drain voltage has
reached the valley (see Figure 5). Once the flyback transformer is built, R
FBGAIN
can be adjusted to change
TT to allow it to cycle at the valley. Adjusting R
FBGAIN
changes the switching frequency slightly.
Although R
FBGAIN
could be used to control the load current, its primary purpose is to control the valley-
switching range. It is recommended to adjust the load current by changing primary current sense resistor
R
Sense
, which is connected to the FBSENSE pin of the IC and then use R
FBGAIN
to control valley switching.
Step 14) Recalculate R
Sense
The flyback primary current is controlled by comparing the voltage across R
Sense
at pin FBSENSE to an
internal threshold of 1.4V. To guarantee the rated LED current under worst-case conditions, when the LED
string has maximum voltage, the V
BST
is at its minimum point and R
Sense
is at its highest tolerance. Adjust
R
Sense
to obtain the nominal LED current, then adjust R
FBGAIN
to hit the valley.
FB
Gain
TT
fb
T2
fb
------------ = [Eq. 16]
1 FB
Gain
2.5     [Eq. 17]
T2
fb
TT
fb
2.5 T  2
fb
   0 V
Reflected
1.5 V
BST
       15.6k  R
FBGAIN
62.5k           [Eq. 18]
First Valley
Flyback FET
Voltage
V
CLAMP
V
BST
V
Reflected
Figure 5. Switching Waveform of Flyback FET Drain