Datasheet

LT3461/LT3461A
6
3461af
Inductor Selection
The inductors used with the LT3461/LT3461A should
have a saturation current rating of 0.3A or greater. If the
device is used in an application where the input supply will
be hot-plugged, then the saturation current rating should
be equal to or greater than the peak inrush current. For the
LT3461, an inductor value between 10µH and 47µH,
depending upon output voltage, will usually be the best
choice for most designs. For the LT3461A, inductor values
between 4.7µH and 15µH inductor will suffice for most
applications. For best loop stability results, the inductor
value selected should provide a ripple current of 70mA or
more. For a given V
IN
and V
OUT
the inductor value to use
with LT3461A is estimated by the formula:
L (in microhenries) =
DV V
AV
IN OUT
• • • sec
•
1
11
where D
=
+
+
VVV
VV
OUT IN
OUT
1
1
–
Use twice this value for the LT3461.
Capacitor Selection
Low ESR capacitors should be used at the output to
minimize the output voltage ripple. Multilayer ceramic
capacitors using X5R/X7R dielectrics are preferred as they
have a low ESR and maintain capacitance over wide
voltage and temperature range. A 2.2µF output capacitor
is sufficient for most applications using the LT3461, while
a 1µF capacitor is sufficient for most applications using
the LT3461A. High output voltages typically require less
capacitance for loop stability. Always use a capacitor with
sufficient voltage rating.
Either ceramic or solid tantalum capacitors may be used
for the input decoupling capacitor, which should be placed
as close as possible to the LT3461/LT3461A. A 1µF
capacitor is sufficient for most applications.
Phase Lead Capacitor
A small value capacitor can be added across resistor R1
between the output and the FB pin to reduce output
perturbation due to a load step and to improve transient
response. This phase lead capacitor introduces a pole-
zero pair to the feedback that boosts phase margin near
the cross-over frequency. The following formula is useful
to estimate the capacitor value needed:
C
k
R
pF
PL
=
Ω500
2
1•
For an application running 50µA in the feedback divider,
capacitor values from 10pF to 22pF work well.
APPLICATIO S I FOR ATIO
WUU
U
TYPICAL APPLICATIO S
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Figure 4. 5V to 12V with Soft-Start Circuit (LT3461A)
Input Current and Output Voltage
V
IN
5V
L1
10µH
R1
261k
R2
30.1k
C2
1µF
15pF
C1
1µF
V
OUT
12V
70mA
1
2
V
IN
V
OUT
SW
3
4
5
6
FB
SHDN
GND
LT3461A
3461a TA02a
CONTROL
SIGNAL
47nF
47k
C1, C2: TAIYO YUDEN EMK212BJ105
L1: MURATA LQH32CN100K53
I
IN
50mA/DIV
CONTROL
SIGNAL
5V/DIV
1ms/DIV
V
OUT
5V/DIV
3461a TA02b