SP4446_06 Datasheet, PDF (4/10 Pages) Sipex Corporation – High Output Voltage Boost Regulator LCD Bias Regulator

Inductor Selection
For SP4446, the internal switch will be turned off
only after the inductor current reaches the typical
DC current limit (I_LIM=150mA). However, there is
typically a propagation delay of 200nS between
the time when the current limit is reached and
when the switch is actually turned off. During this
200nS delay, the peak inductor current will in-
crease, exceeding the current limit by a small amount.
The peak inductor current can be estimated by:
I_PK = I_LIM + ^VIN(MAX) • 200nS
L
The larger the input voltage and the lower the
inductor value, the greater the peak current.
In selecting an inductor, the saturation current
specified for the inductor needs to be greater than
the SP4446 peak current to avoid saturating the
inductor, which would result in a loss of efficiency
and could damage the inductor. Choosing an in-
ductor with low DCR decreases power losses and
increase efficiency.
Refer to Table 1 for some suggested low ESR inductors.
MANUF.
PART NUMBER
DCR Current
(Ω) Rating
(mA)
MURATA
LQH32CN100K21 0.44
300
770-436-1300 (10µH)
MURATA
LQH32CN220K21 0.71
250
770-436-1300 (22µH)
TDK
NLFC453232T-100K
847-803-6100 (10µH)
0.273 250
TDK
NLC453232T-100K 0.9
370
847-803-6100 (22µH)
Table 1. Suggested Low ESR inductors
Diode Selection
A schottky diode with a low forward drop and fast
switching speed is ideally used here to achieve
high efficiency. In selecting a Schottky diode, the
current rating of the schottky diode should be
larger than the peak inductor current. Moreover,
the reverse breakdown voltage of the Schottky
diode should be larger than the output voltage.
Capacitor Selection
Ceramic capacitors are recommended for their
APPLICATION INFORMATION
inherently low ESR, which will help produce low
peak to peak output ripple, and reduce high fre-
quency spikes.
For the typical application, a 4.7µF input capacitor
and a 2.2µF output capacitor are sufficient. The
input and output ripple could be further reduced by
increasing the value of the input and output capaci-
tors. Place all the capacitors as close to the SP4446
as possible for layout. For use as a voltage source,
to reduce the output ripple, a small feedforward
capacitor (47pF) across the top feedback resistor
can be used to provide sufficient overdrive for the
error comparator, thus reduce the output ripple.
Refer to Table 2 for some suggested low ESR capacitors.
MANUF.
MURATA
770-436-1300
MURATA
770-436-1300
TDK
847-803-6100
TDK
847-803-6100
PART
NUMBER
CAP/
VOLTAGE
GRM32RR71E 2.2µF/25V
225KC01B
GRM31CR61A 4.7µF/10V
475KA01B
C3225X7R1E 2.2µF/25V
225M
C3216X5R1A 4.7µF/10V
475K
SIZE/
TYPE
1210/X5R
1206/X5R
1206/X7R
1206/X5R
Table 2. Suggested Low ESR Capacitor
Output Voltage Program
In order for the SP4446 to be programmed as a
voltage source, the SP4446 requires 2 feedback
resistors R₁ & R₂ to control the output voltage, as
shown in Figure 1.
VIN
L1
D1
C2
C1
U1
5
1
V_{I N}
SW
SP4446
4
3
SHDN
FB
G ND
1.22V
2
VOUT
R1
R2
Figure 1. Using SP4446 as Voltage Source
Date: 1/2/06
SP4446 High Output Voltage Boost Regualtor, LCD Bias Regulator
4
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