LT1945
6
1945fa
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:
II
VV
L
ns
PEAK LIM
IN MAX SAT
=+
−
⎛
⎝
⎜
⎞
⎠
⎟
()
100
Where V
SAT
= 0.25V (switch saturation voltage). The current
overshoot will be most evident for regulators with high input
voltages and smaller inductor values. This overshoot can
be benefi cial 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
effi ciency 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 capacitors
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 suffi cient 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
APPLICATIONS INFORMATION
Inductor Selection—Inverting Regulator
The formula below calculates the appropriate inductor value
to be used for an inverting regulator using the LT1945 (or
at least provides a good starting point). This value provides
a good tradeoff in inductor size and system performance.
Pick a standard inductor close to this value (both inductors
should be the same value). A larger value can be used to
slightly increase the available output current, but limit it to
around twice the value calculated below, as too large of an
inductance will increase the output voltage ripple without
providing much additional output current. A smaller value
can be used (especially for systems with output voltages
greater than 12V) to give a smaller physical size. Inductance
can be calculated as:
L
VV
I
t
OUT D
LIM
OFF
=
+
⎛
⎝
⎜
⎞
⎠
⎟
2
where V
D
= 0.4V (Schottky diode voltage), I
LIM
= 350mA
and t
OFF
= 400ns.
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 ap-
plication, 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
VV V
I
t
OUT
IN MIN
D
LIM
OFF
=
−+
()
