Datasheet

LTC3633
12
3633fc
For more information www.linear.com/LTC3633
applicaTions inForMaTion
A general LTC3633 application circuit is shown on the
first page of this data sheet. External component selection
is largely driven by the load requirement and switching
frequency. Component selection typically begins with the
selection of the inductor L and resistor R
T
. Once the inductor
is chosen, the input capacitor, C
IN
, and the output capaci-
tor, C
OUT
, can be selected. Next, the feedback resistors
are selected to set the desired output voltage. Finally, the
remaining optional external components can be selected
for functions such as external loop compensation, track/
soft-start, V
IN
UVLO, and PGOOD.
Programming Switching Frequency
Selection of the switching frequency is a trade-off between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
Connecting a resistor from the RT pin to SGND programs
the switching frequency (f) between 500kHz and 4MHz
according to the following formula:
R
RT
=
3.2E
11
f
where R
RT
is in Ω and f is in Hz.
When RT is tied to INTV
CC
, the switching frequency will
default to approximately 2MHz, as set by an internal
resistor. This internal resistor is more sensitive to pro-
cess and temperature variations than an external resistor
(see Typical Performance Characteristics) and is best used
for applications where switching frequency accuracy is
not critical.
Inductor Selection
For a given input and output voltage, the inductor value and
operating frequency determine the inductor ripple current.
More specifically, the inductor ripple current decreases
with higher inductor value or higher operating frequency
according to the following equation:
ΔI
L
=
V
OUT
f • L






1–
V
OUT
V
IN






Where ∆I
L
= inductor ripple current, f = operating frequency
and L = inductor value. A trade-off between component
size, efficiency and operating frequency can be seen from
this equation. Accepting larger values of ∆I
L
allows the
use of lower value inductors but results in greater inductor
core loss, greater ESR loss in the output capacitor, and
larger output voltage ripple. Generally, highest efficiency
operation is obtained at low operating frequency with
small ripple current.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. Note that the largest
ripple current occurs at the highest V
IN
. Exceeding 60%
of I
OUT(MAX)
is not recommended. To guarantee that
ripple current does not exceed a specified maximum, the
inductance should be chosen according to:
L =
V
OUT
f • ΔI
L(MAX)








1–
V
OUT
V
IN(MAX)








Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core size
for a fixed inductor value, but is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire, leading to increased DCR
and copper loss.
0
FREQUENCY (kHz)
1000
2000
3000
5000
200
700600
3633 F01
0
6000
4000
100 300 400 500
R
T
RESISTOR (kΩ)
Figure 1. Switching Frequency vs R
T