Datasheet

LT3483/LT3483A
7
3483fc
APPLICATIONS INFORMATION
value recommended, as too large of an inductance will
increase the output voltage ripple without providing much
additional output current.
Table 1. Recommended Inductors
PART
L
(µH)
MAX
I
DC
(mA)
DCR
(Ω)
HEIGHT
(mm) MANUFACTURER
LQH2MCN4R7M02L
LQH2MCN6R8M02L
LQH2MCN100M02L
4.7
6.8
10
300
255
225
0.84
1.0
1.2
0.95 Murata
www.murata.com
SDQ12
Coupled
Inductor
4.7
10
15
1.45
980
780
0.40
0.72
1.15
1.2 Cooper Electronics
Tech
www.cooperet.com
LPD3015
Coupled
Inductor
4.7
10
860
580
0.52
1.0
1.4 Coilcraft
www.coilcraft.com
Capacitor Selection
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.
A 4.7µF ceramic bypass capacitor on the V
IN
pin is rec-
ommended where the distance to the power supply or
battery could be more than a couple inches. Otherwise,
a 1µF is adequate.
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.
The output capacitor is selected based on desired output
voltage ripple. For low output voltage ripple in the inverting
flyback configuration, use a 4.7µF to 10µF capacitor. The
inverting charge pump utilizes values ranging from 0.22µF
to 4.7µF. The following formula is useful to estimate the
output capacitor value needed:
C
OUT
=
L • I
SW
2
–V
OUT
• ∆V
OUT
where I
SW
= 0.25A (LT3483) or I
SW
= 0.5A (LT3483A) and
∆V
OUT
= 30mV. The flying capacitor in the inverting charge
pump configuration ranges from 0.1µF to 0.47µF. Multiply
the value predicted by the above equation for C
OUT
by 1/10
to determine the value needed for the flying capacitor.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER URL
AVX www.avxcorp.com
Kemet www.kemet.com
Murata www.murata.com
Taiyo Yuden www.tyuden.com
Setting the Output Voltage
The output voltage is programmed using one feedback
resistor according to the following formula:
R1= –
V
OUT
10µA
Inrush Current
When V
IN
is increased from ground to operating voltage,
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,
more abrupt voltage step at V
IN
, a larger flying capacitor,
and an inductor with a low saturation current.
While the internal diode is designed to handle such events,
the inrush current should not be allowed to exceed 1.5A.
For circuits that use flying capacitors within the recom-
mended range and have input voltages less than 5V, inrush
current remains low, posing no hazard to the device. In
cases where there are large steps at V
IN
, inrush current
should be measured to ensure operation within the limits
of the device.