LT3483 Linear Technology, LT3483 Datasheet - Page 6

• Manufacturer Part Number: LT3483
• Description: Inverting Micropower DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT3483
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 appro-
priately 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:
where I
tor in the inverting charge pump configuration ranges
from 0.1µF to 0.47µF. Multiply the value predicted by the
above equation for C
needed for the flying capacitor.
Table 2. Recommended Ceramic Capacitor Manufacturers
Setting the Output Voltage
The output voltage is programmed using one feedback
resistor according to the following formula:
Inrush Current
When V
an inrush current will flow through the input inductor and
integrated Schottky diode to charge the flying capacitor.
6
R
C
OUT
1
=
SW
MANUFACTURER
IN
–
=
Taiyo Yuden
10
is increased from ground to operating voltage,
= 0.25A and ∆V
V
–
Murata
OUT
Kemet
AVX
µ
V
OUT
A
L I
•
•
SW
U
∆
OUT
2
V
OUT
U
by 1/10 to determine the value
OUT
= 30mV. The flying capaci-
W
www.avxcorp.com
www.murata.com
www.tyuden.com
www.kemet.com
URL
U
Conditions that increase inrush current include a larger,
more abrupt voltage step at V
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
current should be measured to ensure operation within the
limits of the device.
Board Layout Considerations
As with all switching regulators, careful attention must be
given to the PCB board layout and component placement.
Proper layout of the high frequency switching path is
essential. The voltage signals of the SW and D pins have
sharp rising and falling edges. Minimize the length and
area of all traces connected to the SW and D pins. In
particular, it is desirable to minimize the trace length to
and from the flying capacitor, since current in this capaci-
tor switches directions within a cycle. Always use a
ground plane under the switching regulator to minimize
interplane coupling.
V
GND
OUT
Suggested Layout (SOT-23)
for Inverting Charge Pump
C
OUT
R1
C
FLY
1
2
3
C
IN
IN
, a larger flying capacitor,
L1
6
5
4
3483 AI01
IN
V
SHDN
IN
, inrush
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