Datasheet

LTC3626
13
3626f
APPLICATIONS INFORMATION
A general LTC3626 application circuit is shown on the first
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 capaci-
tor, C
BST
, 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, PGOOD,
average output current monitor and limit, average input
current monitor and limit, and on-die temperature moni-
tor and limit.
Operating Frequency
Selection of the operating 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.
The operating frequency, f, of the LTC3626 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
RT
=
3.2E11
f
where R
RT
is in Ω and f is in Hz.
Connecting the RT pin to INTV
CC
will assert the internal
default frequency f = 2MHz; however, this switching fre-
quency will be more sensitive to process and temperature
variations than using a resistor on RT (see Typical Perfor-
mance Characteristics).
The LTC3626 is not optimized for constant on-time opera-
tion when configured to generate output voltages greater
than 6V. Though output regulation will be maintained under
Figure 1. Switching Frequency vs R
T
this condition, it is possible the operating frequency may be
higher than the programmed value. As a result, for output
voltages greater than 6V, the value of the R
T
resistor may
need adjustment to obtain the desired operating frequency.
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:
∆I
L
=
V
OUT
f • L






1–
V
OUT
V
IN






where ΔI
L
= inductor ripple current, V
IN
= PV
IN
, f = operat-
ing 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
ΔI
L
allows the use of lower value inductors but results
in greater core loss in the inductor, greater ESR loss in
the output capacitor, and larger output ripple. Generally,
highest
efficiency operation is obtained at low operating
frequency with small ripple current.
A reasonable starting point for setting the ripple current is
approximately 1A
P-P
. Note that the largest ripple current
occurs at the highest V
IN
. To guarantee the ripple current
R
T
(kΩ)
0
2000
2500
3500
300 500
3626 F01
1500
1000
100 200
400 600 700
500
0
3000
FREQUENCY (kHz)