Application Information

TRIPLE OUTPUT TFT BIAS

The circuit in Figure 3 shows how the LM3224 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

FB1

and R

The compensation network of R

optimally stabilize the converter. The inductor also affects

the stability. When operating at 615 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 LM3224 switch; the second stage uses C5, D5,

and D1; the third stage includes C6, D6, and the LM3224

switch; the final stage is C7 and D7. In the first stage, C4

charges to 8V when the LM3224 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

referenced to ground. In the third stage, the 16V at C5

appears across C6 when the switch is closed. When the

switch opens, C6 is referenced to the 8V output minus a

diode drop, which raises the voltage at C6 with respect to

ground to about 24V. Hence, in the fourth stage, C7 is

charged to 24V when the switch is open. From the first stage

20097608

FIGURE 3. Triple Output TFT Bias (615 kHz operation)

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