Datasheet

LTC3633A-2/LTC3633A-3
12
3633a23f
APPLICATIONS INFORMATION
A general LTC3633A-2 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
T
. Once the
inductor is chosen, the input capacitor, C
IN
, and the out-
put capacitor, 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,
tracking/soft-start, input UVLO, and PGOOD.
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:
R
RT
=
3.2E
11
f
where R
RT
is in  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 re-
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:
ΔI
L
=
V
OUT
f•L
⎛
⎝
⎜
⎞
⎠
⎟
1–
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
Where ∆I
L
= inductor ripple current, f = operating frequency
L = inductor value and V
IN
is the input power supply voltage
applied to the PV
IN
inputs. A trade-off between component
size, effi ciency 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 effi ciency
operation is obtained at low operating frequency with
small ripple current.
0
FREQUENCY (kHz)
1000
2000
3000
5000
200
700600
3633a23 F01
0
6000
4000
100 300 400 500
R
T
RESISTOR (k)
Figure 1. Switching Frequency vs R
T
OPERATION
When running the LTC3633A-2 channels out of phase, the
large current pulses are interleaved, effectively reducing
the amount of time the pulses overlap. Thus, the total
RMS input current is decreased, which both relaxes the
capacitance requirements for the input bypass capacitors
and reduces the voltage noise on the supply line.
One potential disadvantage to this confi guration occurs
when one channel is operating at 50% duty cycle. In this
situation, switching noise can potentially couple from one
channel to the other, resulting in frequency jitter on one
or both channels. This effect can be mitigated with a well
designed board layout.