LT3463/LT3463A
APPLICATIO S I FOR ATIO
output voltages from a high input voltage source will often
exceed the 50V maximum switch rating. For instance, a
12V to – 40V converter using the inverting topology would
generate 52V on the SW pin, exceeding its maximum
rating. For this application, an inverting charge pump is
the best topology.
The formula below calculates the approximate inductor
value to be used for an inverting charge pump regulator
using the LT3463. As for the boost inductor selection, a
larger or smaller value can be used. For designs with
varying V
IN
such as battery powered applications, use the
minimum V
IN
value in the equation below.
L
=
V
OUT
−
V
IN
(
MIN
)
+
V
D
I
LIM
t
OFF
Capacitor Selection
The small size and low ESR of ceramic capacitors makes
them ideal for LT3463 applications. Use only X5R and X7R
types because they retain their capacitance over wider
voltage and temperature ranges than other ceramic types.
A 1µF input capacitor and a 0.22µF or 0.47µF output
capacitor are sufficient for most applications. Table 2
shows a list of several ceramic capacitor manufacturers.
Consult the manufacturers for more detailed information
on their entire selection of ceramic capacitors. For appli-
cations needing very low output voltage ripple, larger
output capacitor values can be used.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER
AVX
Kemet
Murata
Taiyo Yuden
PHONE
843-448-9411
408-986-0424
814-237-1431
408-573-4150
URL
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
Inrush Current
When V
IN
is increased from ground to operating voltage
while the output capacitor is discharged, an inrush current
will flow through the inductor and integrated Schottky
diode into the output capacitor. Conditions that increase
6
U
inrush current include a larger more abrupt voltage step at
V
IN
, a larger output capacitor tied to the outputs, 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 amp.
For circuits that use output capacitor values within the
recommended range and have input voltages of less than
5V, inrush current remains low, posing no hazard to the
device. In cases where there are large steps at V
IN
and/or
a large capacitor is used at the outputs, inrush current
should be measured to ensure safe operation.
Setting the Output Voltages
The output voltages are programmed using two feedback
resistors. As shown in Figure 1, resistors R1 and R2
program the positive output voltage (for Switcher 1), and
resistors R3 and R4 program the negative output voltage
(for Switcher 2) according to the following formulas:
W
U
U
R2
V
OUT 1
=
1.25V
1
+
R1
R4
V
OUT 2
=
–1.25V
R3
R1 and R3 are typically 1% resistors with values in the
range of 50k to 250k.
Board Layout Considerations
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To maximize efficiency, switch rise and fall times are made
as short as possible. To prevent electromagnetic interfer-
ence (EMI) problems, proper layout of the high frequency
switching path is essential. The voltage signal of the SW
pin has sharp rising and falling edges. Minimize the length
and area of all traces connected to the SW pin and always
use a ground plane under the switching regulator to
minimize interplane coupling. In addition, the ground
connection for the feedback resistor R1 should be tied
directly to the GND pin and not shared with any other
component, ensuring a clean, noise-free connection.
3463f
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
