16
LTC3776
3776fa
Operating Frequency and Synchronization
The choice of operating frequency, f
OSC
, is a trade-off
between efficiency and component size. Low frequency
operation improves efficiency by reducing MOSFET switch-
ing losses, both gate charge loss and transition loss.
However, lower frequency operation requires more induc-
tance for a given amount of ripple current.
The internal oscillator for each of the LTC3776’s control-
lers runs at a nominal 550kHz frequency when the PLLLPF
pin is left floating and the SYNC/SSEN pin is tied to GND.
Pulling the PLLLPF to V
IN
selects 750kHz operation;
pulling the PLLLPF to GND selects 300kHz operation.
Alternatively, the LTC3776 will phase-lock to a clock signal
applied to the SYNC/SSEN pin with a frequency between
250kHz and 850kHz (see Phase-Locked Loop and Fre-
quency Synchronization).
When spread spectrum operation is enabled (SYNC/
SSEN = V
IN
), the frequency of the LTC3776 is randomly
varied over the range of frequencies between 450kHz and
580kHz. In this case, a capacitor (1nF to 4.7nF) should be
connected between the FREQ pin and SGND to smooth
out the changes in frequency. This not only provides a
smoother frequency spectrum but also ensures that the
switching regulator remains stable by preventing abrupt
changes in frequency. A value of 2200pF is suitable in
most applications.
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency f
OSC
directly determine the
inductor’s peak-to-peak ripple current:
I
V
V
VV
fL
RIPPLE
OUT
IN
IN OUT
OSC
=
⎛
⎝
⎜
⎞
⎠
⎟
–
•
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
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 input voltage. To guarantee
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
L
VV
fI
V
V
IN OUT
OSC RIPPLE
OUT
IN
≥
–
•
•
Inductor Core Selection
Once the inductance value is determined, the type of
inductor must be selected. Actual core loss is independent
of core size for a fixed inductor value, but it is very
dependent on inductance selected. As inductance in-
creases, core losses go down. Unfortunately, increased
inductance requires more turns of wire and therefore
copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy
materials are small and don’t radiate much energy, but
generally cost more than powdered iron core inductors
with similar characteristics. The choice of which style
inductor to use mainly depends on the price vs size
requirements and any radiated field/EMI requirements.
New designs for surface mount inductors are available
from Coiltronics, Coilcraft, Toko and Sumida.
Schottky Diode Selection (Optional)
The Schottky diodes D1 and D2 in Figure 16 conduct
current during the dead time between the conduction of
the power MOSFETs . This prevents the body diode of the
bottom N-channel MOSFET from turning on and storing
charge during the dead time, which could cost as much as
1% in efficiency. A 1A Schottky diode is generally a good
APPLICATIO S I FOR ATIO
WUUU
Kool Mμ is a registered trademark of Magnetics, Inc.
