Time

26

46

66

86

106

126

000

001

010

011

100

101

110

111

146

110

101

100

011

010

001

000

24

44

64

84

104

124

144

Voltage trip points do not

include diode drops

TPS92411, TPS92411P

www.ti.com

SLUSBQ6 – OCTOBER 2013

TYPICAL OPERATION (120-VAC APPLICATION)

For the 120-V application shown in Figure 10 the highest efficiency is obtained by using a high-voltage total LED

stack to reduce losses in the linear regulator FET. The best current sharing efficiency between stacks can be

achieved by using the lowest voltage stack at the bottom and making each stack voltage above 2 times the

voltage of the stack below it. In this example 20-V LEDs are used. This effectively gives the lowest stack a total

of 20 V, the middle stack a total of 40 V, and the upper stack a total of 80 V. The RSNS resistor is used to set a

low voltage point so that when the VS pin voltage falls below this threshold (either from the AC line falling or a

higher voltage stack switch above it turning OFF) the TPS92411 switch turns ON and bypasses the LEDs. During

the ON-time, the LEDs are supplied current from the capacitor. The RSET voltage is used to set a threshold to

detect when the input voltage crosses this threshold it turns OFF the switch and allows the LEDs to conduct

current from the line and charge the bypass capacitor.

120-VAC Application Stack Setting Example

•

•

•

•

Switching order as the rectified AC line voltage increases is shown in Table 1. Figure 8 illustrates when each

switch turns ON or OFF.

STACK

TOP 80-V

MIDDLE 40-V

BOTTOM 20-V

0

0

0

0

0

1

0

1

0

0

1

1

1

0

0

1

0

1

1

1

0

1

1

1

(1)

0 denotes switch ON and LEDs bypassed and supplied by the capacitor.

(2)

1 denotes switch OFF and LEDs conducting from the line, capacitor charging up.

Figure 8. Switching Order on Rectified 120-VAC Waveform

The linear regulator in Figure 10 generates a current sense RMS voltage of approximately 2.3 V. The linear

regulator RMS current is equal to the input current drawn from the AC line. For example, for a 11.5-W input

power system the input current should be approximately 0.095 A and a 24-

Ω resistor should be chosen for RCS.

Copyright © 2013, Texas Instruments Incorporated

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