Datasheet

LTC3604
10
3604f
APPLICATIONS INFORMATION
A general LTC3604 application circuit is shown on the fi rst
page of this data sheet. External component selection is
largely driven by the load requirement and begins with the
selection of the inductor L. Once the inductor is chosen, the
input capacitor, C
IN
, the output capacitor, C
OUT
, the internal
regulator capacitor, C
INTVCC
, and the boost capacitor, C
BOOST
,
can be selected. Next, the feedback resistors are selected to
set the desired output voltage. Finally, the remaining option-
al external components can be selected for functions such
as external loop compensation, track/soft-start, externally
programmed oscillator frequency and PGOOD.
Operating Frequency
Selection of the operating 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.
The operating frequency, f
O
, of the LTC3604 is determined
by an external resistor that is connected between the RT
pin and ground. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator and can be calculated
by using the following equation:
R
E
f
RT
O
=
32 11.
where R
RT
is in  and f
O
is in Hz.
Connecting the RT pin to INTV
CC
will default the converter
to f
O
= 2MHz; however, this switching frequency will be
more sensitive to process and temperature variations than
when using a resistor on RT (see Typical Performance
Characteristics).
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
V
fL
V
V
L
OUT OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
•
–1
where ΔI
L
= inductor ripple current, f = operating frequency
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
L
allows the
use of lower value inductors but results in greater core
loss in the inductor, greater ESR loss in the output capaci-
tor, and larger output ripple. Generally, highest effi ciency
operation is obtained at low operating frequency with
small ripple current.
A reasonable starting point for setting the ripple current is
about 40% of I
OUT(MAX)
. Note that the largest ripple current
occurs at the highest V
IN
. To guarantee the ripple current
does not exceed a specifi ed maximum the inductance
should be chosen according to:
L
V
fI
V
V
OUT
LMAX
OUT
IN MAX
=
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
•
–
() ()
Δ
1
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 loss
decreases. Unfortunately, increased inductance requires
more turns of wire leading to increased copper loss.
Ferrite designs exhibit very low core loss and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core materials saturate “hard,” meaning the induc-
tance collapses abruptly when the peak design current is
R
T
(k)
0
0
FREQUENCY (kHz)
1000
2000
3000
4000
6000
100
200 300 400
3604 F01
500 600
5000
Figure 1. Switching Frequency vs R
T