LTC3835
13
3835fe
For more information www.linear.com/LTC3835
APPLICATIONS INFORMATION
R
SENSE
Selection For Output Current
R
SENSE
is chosen based on the required output current.
The current comparator has a maximum threshold of
100mV/R
SENSE
and an input common mode range of
SGND to 10V. The current comparator threshold sets the
peak of the inductor current, yielding a maximum average
output current I
MAX
equal to the peak value less half the
peak-to-peak
ripple current, ∆I
L
.
Allowing a margin for variations in the IC and external
component values yields:
R
SENSE
=
I
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to the
internal compensation required to meet stability criterion for
buck regulators operating at greater than 50% duty factor. A
curve is provided to estimate this reduction in peak output
current level depending upon the operating duty factor.
Operating Frequency and Synchronization
The choice of operating frequency, is a trade-off
between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses,
both gate charge loss and transition loss. However, lower
frequency operation requires more inductance for a given
amount of ripple current.
The internal oscillator of the LTC3835 runs at a nominal
400kHz frequency when the PLLLPF pin is left floating
and the PLLIN/MODE pin is a DC low or high
. Pulling the
PLLLPF to INTV
CC
selects 530kHz operation; pulling the
PLLLPF to SGND selects 250kHz operation.
Alternatively, the LTC3835 will phase-lock to a clock
signal applied to the PLLIN/MODE pin with a frequency
between 140kHz and 650kHz (see Phase-Locked Loop
and Frequency Synchronization).
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 ∆I
L
decreases with higher
inductance or frequency and increases with higher V
IN
:
∆I
L
=
1
(f)(L)
V
OUT
1–
V
OUT
V
IN
Accepting larger values of ∆I
L
allows the use of low in-
ductances, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
ripple current is ∆I
L
=0.3(I
MAX
). The maximum ∆I
L
occurs
at the maximum input voltage.
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
10% of the current limit determined by R
SENSE
. Lower
inductor values (higher ∆I
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
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
