5

Rev. 7/14/03

SP4446 High Output Voltage Boost Regualtor, LCD Bias Regulator

© Copyright 2003 Sipex Corporation

APPLICATION INFORMAMTION: Continued

The formula and table for the resistor selection

are shown below:

(

- 1

1.22

Table 3. Divider Resistor Selection

12

1M

113K

15

1M

88.7K

18

1M

73.2K

20

1M

64.9K

30

1M

42.2K

Layout Consideration

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

directly effects the input and output ripples. 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.

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

Load Disconnect in Shutdown

When SP4446 was 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 20. When the

SP4446 is active, the voltage set at the emitter of

the transistor exceeds the input voltage, 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 satura-

tion voltage and base current of the PNP transistor.

Fig. 20. Load Disconnect in Shutdown

PIN Diode Driver

The SP4446 can be used as PIN diode driver as

age to be 30V. When input voltage is 3.3V or 5V,

it can supply a maximum current of 3mA and 5mA

to the load.

Fig. 21. Pin Diode Driver