LT3483/LT3483A
APPLICATIONS INFORMATION
value recommended, as too large of an inductance will
increasetheoutputvoltageripplewithoutprovidingmuch
additional output current.
to 4.7µF. The following formula is useful to estimate the
output capacitor value needed:
2
L •ISW
COUT
=
Table 1. Recommended Inductors
MAX
–VOUT • ∆VOUT
L
I
DCR HEIGHT
DC
where I = 0.25A (LT3483) or I = 0.5A (LT3483A) and
PART
(µH) (mA) (Ω) (mm) MANUFACTURER
SW
SW
∆V =30mV. Theflyingcapacitorintheinvertingcharge
OUT
LQH2MCN4R7M02L 4.7 300 0.84
LQH2MCN6R8M02L 6.8 255 1.0
0.95
1.2
Murata
www.murata.com
pump configuration ranges from 0.1µF to 0.47µF. Multiply
LQH2MCN100M02L 10
225 1.2
the value predicted by the above equation for C
by 1/10
OUT
SDQ12
4.7 1.45 0.40
Cooper Electronics
Tech
www.cooperet.com
to determine the value needed for the flying capacitor.
Coupled
Inductor
10
15
980 0.72
780 1.15
Table 2. Recommended Ceramic Capacitor Manufacturers
LPD3015
Coupled
Inductor
4.7 860 0.52
10 580 1.0
1.4
Coilcraft
www.coilcraft.com
MANUFACTURER
AVX
URL
www.avxcorp.com
www.kemet.com
www.murata.com
www.tyuden.com
Kemet
Capacitor Selection
Murata
Taiyo Yuden
The small size and low ESR of ceramic capacitors make
them ideal for LT3483/LT3483A applications. Use of X5R
and X7R types is recommended because they retain their
capacitance over wider voltage and temperature ranges
than other dielectric types. Always verify the proper volt-
age rating. Table 2 shows a list of several ceramic capaci-
tor manufacturers. Consult the manufacturers for more
detailed information on their entire selection of ceramic
capacitors.
Setting the Output Voltage
The output voltage is programmed using one feedback
resistor according to the following formula:
VOUT
R1= –
10µA
Inrush Current
A 4.7µF ceramic bypass capacitor on the V pin is rec-
IN
When V is increased from ground to operating voltage,
ommended where the distance to the power supply or
battery could be more than a couple inches. Otherwise,
a 1µF is adequate.
IN
an inrush current will flow through the input inductor and
integrated Schottky diode to charge the flying capacitor.
Conditions that increase inrush current include a larger,
A capacitor in parallel with feedback resistor R1 is recom-
mended to reduce the output voltage ripple. Use a 5pF
capacitor for the inverting charge pump, and a 22pF value
for the inverting flyback or other dual inductor configura-
tions. Output voltage ripple can be reduced to 20mV in
some cases using this capacitor in combination with an
appropriately selected output capacitor.
more abrupt voltage step at V , a larger flying capacitor,
IN
and an inductor with a low saturation current.
Whiletheinternaldiodeisdesignedtohandlesuchevents,
the inrush current should not be allowed to exceed 1.5A.
For circuits that use flying capacitors within the recom-
mendedrangeandhaveinputvoltageslessthan5V, inrush
current remains low, posing no hazard to the device. In
The output capacitor is selected based on desired output
voltageripple.Forlowoutputvoltagerippleintheinverting
flyback configuration, use a 4.7µF to 10µF capacitor. The
invertingchargepumputilizesvaluesrangingfrom0.22µF
cases where there are large steps at V , inrush current
IN
should be measured to ensure operation within the limits
of the device.
3483fc
7
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