SP4446 Datasheet, PDF (5/10 Pages) Sipex Corporation – High Output Voltage Boost Regulator LCD Bias Regulator

APPLICATION INFORMAMTION: Continued
The formula and table for the resistor selection
are shown below:
will be slightly sacrificed because of the satura-
tion voltage and base current of the PNP transistor.
R₁ =( ^VOUT - 1 ) • R₂
1.22
Table 3. Divider Resistor Selection
V_OUT (V)
12
15
18
20
30
R₁ (Ω)
1M
1M
1M
1M
1M
R₂ (Ω)
113K
88.7K
73.2K
64.9K
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
connection for the feedback resistor network should
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.
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-
ternal PNP transistor (for example 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
R3
154K
VIN
L1
10uH
DS ^Q1
MMBT2907A
C1
U1
R1
4.7uF
5
1
VIN
SW
1M
SP4446
4 _SHDN
FB ³
1.22V
GND
R2
64.9K
2
C2
47pF
VOUT
20V
C3
2.2uF
Fig. 20. Load Disconnect in Shutdown
PIN Diode Driver
The SP4446 can be used as PIN diode driver as
shown in Fig. 21. Two Resistors (R₁=1MΩ,
R₂=42.2KΩ) are used to program the output volt-
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.
V_IN
3.3V or 5V
Murata LQH32CN100K21
L1 10uH 0.3A
DS
MBR0530
C1
4.7uF
R1
1M
U1
5
1
V_IN
SW
SP4446
4 _SHDN
FB ³
GND
2
1.22V
R2
42.2K
C2
47pF
30V VOUT
3 to 5 mA
C3
2.2uF
Fig. 21. Pin Diode Driver
Rev. 7/14/03
SP4446 High Output Voltage Boost Regualtor, LCD Bias Regulator
5
© Copyright 2003 Sipex Corporation