LT1945
APPLICATIO S I FOR ATIO
For higher output voltages, the formula above will give
large inductance values. For a 2V to 20V converter (typical
LCD bias application), a 47µH inductor is called for with
the above equation, but a 10µH or 22µH inductor could be
used without excessive reduction in maximum output
current.
Inductor Selection—Inverting Charge Pump Regulator
For the inverting regulator, the voltage seen by the internal
power switch is equal to the sum of the absolute value of
the input and output voltages, so that generating high
output voltages from a high input voltage source will often
exceed the 36V maximum switch rating. For instance, a
12V to – 30V converter using the inverting topology would
generate 42V 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 LT1945. 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
Current Limit Overshoot
For the constant off-time control scheme of the LT1945,
the power switch is turned off only after the 350mA current
limit is reached. There is a 100ns delay between the time
when the current limit is reached and when the switch
actually turns off. During this delay, the inductor current
exceeds the current limit by a small amount. The peak
inductor current can be calculated by:
I
PEAK
V
IN(MAX)
−
V
SAT
=
I
LIM
+
100ns
L
Where V
SAT
= 0.25V (switch saturation voltage). The
current overshoot will be most evident for regulators with
6
U
high input voltages and smaller inductor values. This
overshoot can be beneficial as it helps increase the amount
of available output current for smaller inductor values.
This will be the peak current seen by the inductor (and the
diode) during normal operation. For designs using small
inductance values (especially at input voltages greater
than 5V), the current limit overshoot can be quite high.
Although it is internally current limited to 350mA, the
power switch of the LT1945 can handle larger currents
without problem, but the overall efficiency will suffer. Best
results will be obtained when I
PEAK
is kept below 700mA
for the LT1945.
Capacitor Selection
Low ESR (Equivalent Series Resistance) capacitors should
be used at the output to minimize the output ripple voltage.
X5R or X7R multilayer ceramic capacitors are the best
choice, as they have a very low ESR and are available in
very small packages. Y5V ceramics are not recommended.
Their small size makes them a good companion to the
LT1945’s MS10 package. Solid tantalum capacitors (like
the AVX TPS, Sprague 593D families) or OS-CON capaci-
tors can be used, but they will occupy more board area
than a ceramic and will have a higher ESR. Always use a
capacitor with a sufficient voltage rating.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the LT1945. A 4.7µF input capacitor is suffi-
cient for most applications. Table 2 shows a list of several
capacitor manufacturers. Consult the manufacturers for
more detailed information and for their entire selection of
related parts.
Table 2. Recommended Capacitors
CAPACITOR TYPE
Ceramic
VENDOR
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
AVX
(803) 448-9411
www.avxcorp.com
Murata
(714) 852-2001
www.murata.com
Ceramic
Ceramic
1945f
W
U
U
LINER [ LINEAR TECHNOLOGY ]
