Datasheet

LTC3859A
22
3859af
applicaTions inForMaTion
The maximum power loss in R1 is related to duty cycle. For
the buck controllers, the maximum power loss will occur
in continuous mode at the maximum input voltage:
P
LOSS
R1=
(V
IN(MAX)
− V
OUT
) • V
OUT
R1
For the boost controller, the maximum power loss in R1
will occur in continuous mode at V
IN
= 1/2•V
OUT
:
P
LOSS
R1=
(V
OUT(MAX)
−
V
IN
) • V
IN
R1
Ensure that R1 has a power rating higher than this value.
If high efficiency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor, due
to the extra switching losses incurred through R1. However,
DCR sensing eliminates a sense resistor, reduces conduc-
tion losses and provides higher efficiency at heavy loads.
Peak efficiency is about the same with either method.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because
of MOSFET gate charge losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current DI
L
decreases with higher
inductance or frequency. For the buck controllers, DI
L
increases with higher V
IN
:
DI
L
=
1
(f)(L)
V
OUT
1−
V
OUT
V
IN






For the boost controller, the inductor ripple current DI
L
increases with higher V
OUT
:
DI
L
=
1
(f)(L)
V
IN
1−
V
IN
V
OUT






Accepting larger values of DI
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is DI
L
= 0.3(I
MAX
). The maximum
DI
L
occurs at the maximum input voltage for the bucks
and V
IN
= 1/2•V
OUT
for the boost.
The inductor value also has secondary effects. The tran-
sition to Burst Mode operation begins when the average
inductor current required results in a peak current below
25% of the current limit (30% for the boost) determined
by R
SENSE
. Lower inductor values (higher DI
L
) will cause
this to occur at lower load currents, which can cause a dip
in efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to decrease.
Inductor Core Selection
Once the value for L is known, 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 more expensive ferrite or molypermalloy
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 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 con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!