Datasheet
:
26.1
26.1V
RR
OUT
2FB1FB
LM2622
R
FB1
40.2k
L
10uH
C
C
3.9nF
D1
R
C
5.1k
C
OUT1
10uF
R
FB2
7.5k
SHDN
3
V
IN
6
SW
5
FSLCT
7
FB
2
GND
4
V
C
1
C1
4.7uF
D3
C2
0.1uF
D2
-8V
8V
V
IN
= 2.7V - 3.3V
C
FB1
0.1uF
C
IN
10uF
C
OUT2
10uF
D6
C6
1uF
D5
C4
1uF
C5
1uF
D4
D7
C7
1uF
23V
C
FB2
*
LM2622
www.ti.com
SNVS068E –MAY 2000–REVISED MARCH 2013
Application Information
Figure 19. Triple Output TFT Bias (600 kHz operation)
TRIPLE OUTPUT TFT BIAS
The circuit in Figure 19 shows how the LM2622 can be configured to provide outputs of 8V, −8V, and 23V,
convenient for biasing TFT displays. The 8V output is regulated, while the −8V and 23V outputs are unregulated.
The 8V output is generated by a typical boost topology. The basic operation of the boost converter is described
in the OPERATION section. The output voltage is set with R
FB1
and R
FB2
by:
(20)
C
FB
is placed across R
FB1
to act as a pseudo soft-start. The compensation network of R
C
and C
C
are chosen to
optimally stabilize the converter. The inductor also affects the stability. When operating at 600 kHz, a 10uH
inductor is recommended to insure the converter is stable at duty cycles greater than 50%. Refer to the
COMPENSATION section for more information.
The -8V output is derived from a diode inverter. During the second cycle, when the transistor is open, D2
conducts and C1 charges to 8V minus a diode drop (≊0.4V if using a Schottky). When the transistor opens in the
first cycle, D3 conducts and C1's polarity is reversed with respect to the output at C2, producing -8V.
The 23V output is realized with a series of capacitor charge pumps. It consists of four stages: the first stage
includes C4, D4, and the LM2622 switch; the second stage uses C5, D5, and D1; the third stage includes C6,
D6, and the LM2622 switch; the final stage is C7 and D7. In the first stage, C4 charges to 8V when the LM2622
switch is closed, which causes D5 to conduct when the switch is open. In the second stage, the voltage across
C5 is VC4 + VD1 - VD5 = VC4 ≊ 8V when the switch is open. However, because C5 is referenced to the 8V
Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM2622