Datasheet

LTC3860
15
3860fc
APPLICATIONS INFORMATION
Choosing the Inductor and Setting the Current Limit
The inductor value is related to the switching frequency,
which is chosen based on the trade-offs discussed in the
Operation section. The inductor can be sized using the
following equation:
L =
V
OUT
f•ΔI
L
⎛
⎝
⎜
⎞
⎠
⎟
•1−
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
Choosing a larger value of ΔI
L
leads to smaller L, but re-
sults in greater core loss (and higher output voltage ripple
for a given output capacitance and/or ESR). A reasonable
starting point for setting the ripple current is 30% of the
maximum output current, or:
ΔI
L
= 0.3 • I
OUT
The inductor saturation current rating needs to be higher
than the peak inductor current during transient conditions.
If I
OUT
is the maximum rated load current, then the maxi-
mum transient current, I
MAX
, would normally be chosen
to be some factor (e.g., 60%) greater than I
OUT
:
I
MAX
= 1.6 • I
OUT
The minimum saturation current rating should be set to
allow margin due to manufacturing and temperature varia-
tion in the sense resistor or inductor DCR. A reasonable
value would be:
I
SAT
= 2.2 • I
OUT
The programmed current limit must be low enough to
ensure that the inductor never saturates and high enough
to allow increased current during transient conditions and
allow margin for DCR variation.
For example, if:
I
SAT
= 2.2 • I
OUT
and
I
MAX
= 1.6 • I
OUT
A reasonable I
LIMIT
would be:
I
LIMIT
= 2 • I
OUT
If the sensed inductor current exceeds current limit, the
IC will three-state the PWM outputs, reset the soft-start
timer and wait 32768 switching cycles before attempting
to return the output to regulation.
The current limit is programmed using a resistor from the
I
LIM
pin to SGND. The I
LIM
pin sources 20μA to generate
a voltage corresponding to the current limit. The current
sense circuit has a voltage gain of 20 and a zero current
level of 500mV. Therefore, the current limit resistor should
be set using the following equation:
R
ILIM
=
18.5 •I
LIMIT(SET)
•R
SENSE
+ 0.53V
20µA
In multiphase applications only one current limit resistor
should be used per LTC3860. The I
LIM2
pin should be tied
to V
CC
. Internal logic will then cause channel 2 to use the
same current limit levels as channel 1. If an LTC3860 has
a slave and an independent, then both I
LIM
pins must be
independently set to the right voltage.
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. High effi ciency converters generally cannot afford
the core losses found in low cost powdered iron cores,
forcing the use of more expensive ferrite or molypermalloy
cores. Also, core losses decrease as inductance increases.
Unfortunately, increased inductance requires more turns
of wire, larger inductance and larger copper losses.
Ferrite designs have very low core loss and are preferred at
high switching frequencies. However, these core materials
exhibit “hard” saturation, causing an abrupt reduction in the
inductance when the peak current capability is exceeded.
Do not allow the core to saturate!
C
IN
Selection
The input bypass capacitor in an LTC3860 circuit is com-
mon to both channels. The input bypass capacitor needs
to meet these conditions: its ESR must be low enough to
keep the supply drop low as the top MOSFETs turn on, its
RMS current capability must be adequate to withstand the
ripple current at the input, and the capacitance must be
large enough to maintain the input voltage until the input