16
LTC3736
3736fa
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 LTC3736’s control-
lers runs at a nominal 550kHz frequency when the PLLLPF
pin is left floating and the SYNC/FCB pin is a DC low or
high. Pulling the PLLLPF to V
IN
selects 750kHz operation;
pulling the PLLLPF to GND selects 300kHz operation.
Alternatively, the LTC3736 will phase-lock to a clock signal
applied to the SYNC/FCB pin with a frequency between
250kHz and 850kHz (see Phase-Locked Loop and Fre-
quency Synchronization).
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
V
V
f
L
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
V
V
f
I
V
V
IN
OUT
OSC
RIPPLE
OUT
IN
≥
–
•
•
Burst Mode Operation Considerations
The choice of R
DS(ON)
and inductor value also determines
the load current at which the LTC3736 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
I
V
R
BURST PEAK
SENSE MAX
DS ON
(
)
(
)
(
)
•
=
∆
1
4
The corresponding average current depends on the amount
of ripple current. Lower inductor values (higher I
RIPPLE
)
will reduce the load current at which Burst Mode operation
begins.
The ripple current is normally set so that the inductor
current is continuous during the burst periods. Therefore:
I
RIPPLE
≤
I
BURST(PEAK)
This implies a minimum inductance of:
L
V
V
f
I
V
V
MIN
IN
OUT
OSC
BURST PEAK
OUT
IN
≤
–
•
•
(
)
A smaller value than L
MIN
could be used in the circuit,
although the inductor current will not be continuous
during burst periods, which will result in slightly lower
efficiency. In general, though, it is a good idea to keep
I
RIPPLE
comparable to I
BURST(PEAK)
.
Inductor Core Selection
Once the inductance value is determined, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of ferrite, molyper-
malloy or other cores. Actual core loss is independent of
core size for a fixed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance re-
quires 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!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive
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