5

Date: 1/2/06

SP4446 High Output Voltage Boost Regualtor, LCD Bias Regulator

© Copyright 2006 Sipex Corporation

APPLICATION INFORMATION: Continued

The formula and table for the resistor selection

are shown below:

(

- 1

1.22

12

1M

113K

15

1M

88.7K

18

1M

73.2K

20

1M

64.9K

30

1M

42.2K

Table 3. Divider Resistor Selection

Layout Considerations

Both the input capacitor and the output capacitor

should be placed as close as possible to the IC. This

can reduce the copper trace resistance which di-

rectly effects the input and output ripple. The

feedback resistor network should be kept close to

the FB pin to minimize copper trace connections

that can inject noise into the system. The ground

connectionforthefeedbackresistornetworkshould

connect directly to the GND pin or to an analog

ground plane that is tied directly to the GND pin.

The inductor and the Schottky diode should be

placed as close as possible to the switch pin to

minimize the noise coupling to the other circuits,

especially the feedback network.

PIN Diode Driver

The SP4446 can be used as PIN diode driver as

shown in Figure. 2.

Fig. 2. Pin Diode Driver

3.3V or 5V

R1

1M

C1

4.7uF

30V

R2

42.2K

U1

SP4446

1

2

3

4

5

SHDN

Murata LQH32CN100K21

3 to 5 mA

DS

MBR0530

C2

47pF

1.22V

L1 10uH 0.3A

C3

2.2uF

3.3V or 5V

R1

1M

C1

4.7uF

30V

R2

42.2K

U1

SP4446

1

2

3

4

SW

GND

FB

SHDN

V

IN

Murata LQH32CN100K21

V

IN

3 to 5 mA

DS

MBR0530

V

OUT

C2

47pF

1.22V

L1 10uH 0.3A

C3

2.2uF

V

IN

DS

C1

4.7uF

Q1

MMBT2907A

R1

1M

R3

154K

20V

1.22V

C2

47pF

R2

64.9K

U1

SP4446

1

2

3

4

5

SW

GND

FB

SHDN

V

IN

V

OUT

L1

10uH

C3

2.2uF

When the SP4446 is active, the voltage set at the

emitter of the transistor exceeds the input volt-

age, forcing the transistor into the saturation

region. When the SP4446 is shut down, the input

voltage exceeds the emitter voltage, thus the

transistor becomes inactive and provides high-

impedance isolation between the input and load.

Efficiency will be slightly sacrificed because of

the saturation voltage and base current of the PNP

transistor.

Fig. 3. Load Disconnect in Shutdown

to program the output voltage to be 30V. When

input voltage is 3.3V or 5V, it can supply a

maximum current of 3mA and 5mA to the load.

Load Disconnect in Shutdown

When SP4446 is shut down, the load is still

connected to the input. In applications that re-

quire output isolation during shutdown, an ex-

ternalPNPtransistor(forexample MMBT2907A)

can be added as shown in Figure 3.