LTC3633 LINER [Linear Technology], LTC3633 Datasheet - Page 12

• Manufacturer Part Number: LTC3633
• Description: Dual Channel 3A, 15V Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

LTC3633
APPLICATIONS INFORMATION
A general LTC3633 application circuit is shown on the
fi rst 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
inductor is chosen, the input capacitor, C
put capacitor, C
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
Programming Switching Frequency
Selection of the switching frequency is a trade-off between
effi ciency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves effi ciency 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:
where R
When RT is tied to INTV
default to approximately 2MHz, as set by an internal re-
12
R
RT
=
RT
3.2E
is in Ω and f is in Hz.
6000
5000
4000
3000
2000
1000
f
11
0
Figure 1. Switching Frequency vs R
0
OUT
IN
100
, can be selected. Next, the feedback
UVLO, and PGOOD.
200
CC
R
T
RESISTOR (kΩ)
, the switching frequency will
300
400
500
600
IN
3633 F01
, and the out-
700
T
T
. Once the
sistor. This internal resistor is more sensitive to process
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 specifi cally, the inductor ripple current decreases
with higher inductor value or higher operating frequency
according to the following equation:
Where ΔI
and L = inductor value. A trade-off between component
size, effi ciency and operating frequency can be seen from
this equation. Accepting larger values of ΔI
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 effi ciency
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
ripple current occurs at the highest V
of I
ripple current does not exceed a specifi ed maximum, the
inductance should be chosen according to:
Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core size
for a fi xed 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.
L =
OUT(MAX)
I
L
=
f • I
L
= inductor ripple current, f = operating frequency
V
f • L
V
OUT
OUT
L(MAX)
is not recommended. To guarantee that
1–
V
1–
V
OUT
OUT(MAX)
IN
V
IN(MAX)
V
OUT
. Note that the largest
IN
. Exceeding 60%
L
allows the
3633f