Cirrus Logic AN368 - Manual (Page 33)

AN368
AN368REV2 33
b. Flyback Mode Operation Using a Dual LED String Synchronizer Circuit
The dual-LED string load is implemented in a series output configuration. The series configuration is illustrated
in Figure 13. In the series configuration, the white LED string is always on. The red LED string is on when FET
Q3 is turned ‘OFF’ and is shorted out when FET Q3 is turned ‘ON’. The Synchronizer Circuit controls the
switching between the two lamp strings. Non-isolated designs can use the SYNC pin to directly drive the
synchronizer FET Q3, eliminating the need for a Synchronizer Circuit.
Figure 14 illustrates the Synchronizer Circuit for a flyback power stage and the series configuration of the dual-
LED load. The voltage of the secondary winding V
sync
provides the CLK signal of the D flip-flop. The CLK
signal triggers the dual-gate signal, which alternates between shorting out and feeding charge to the red LED
string on channel 2. An n-channel MOSFET switches between the two LED output strings.
Figure 13. Series Configuration
C8
Q3
C15
D5
Synchronizer
Circuit
Channel 1 LED
(White)
Channel 2 LED
(Red)
Q5
V
B S T
GD
TX1
R21
D15
GND IGND
Q5
V
B S T
GD
TX1
R21
R3
D6
U2
C10
C8
C15
D5
D
GND
_
Q
V CC
D15
R12
D10
Q3 R2
C16
Channel 1 LED
(White)
Channel 2 LED
(Red)
GND
IGND
I
sync
V
sync
I
D 9
D9
Figure 14. Dual-LED Load Synchronizer