LM2622_04 NSC [National Semiconductor], LM2622_04 Datasheet - Page 12

• Manufacturer Part Number: LM2622_04
• Description: 600kHz/1.3MHz Step-up PWM DC/DC Converter
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
TRIPLE OUTPUT TFT BIAS
The circuit in Figure 3 shows how the LM2622 can be
configured to provide outputs of 8V, −8V, and 23V, conve-
nient 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
and R
C
compensation network of R
stabilize the converter. The inductor also affects the stability.
When operating at 600 kHz, a 10uH inductor is recom-
FB
is placed across R
FB2
by:
FB1
to act as a pseudo soft-start. The
C
and C
FIGURE 3. Triple Output TFT Bias (600 kHz operation)
C
are chosen to optimally
FB1
12
mended 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 refer-
enced to the 8V output, the voltage at C5 is 16V when
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