Datasheet

LTC3863
15
3863f
For more information www.linear.com/3863
APPLICATIONS INFORMATION
The duty factor increases with increasing V
OUT
and de-
creasing V
IN
. For a given V
OUT
, the maximum duty factor
occurs at minimum V
IN
.
A typical starting point for selecting an inductor is to choose
the inductance such that the maximum peak-to-peak in-
ductor ripple current, ∆I
L(MAX)
, is set to 40% ~ 50% of the
inductor average current, I
L(AVG)
, at maximum load current.
Since ∆I
L(MAX)
occurs at maximum V
IN
in continuous mode,
the inductance is calculated at maximum V
IN
:
L =
V
IN(MAX)
2
• |V
OUT
|+V
D
( )
0.4•I
OUT(MAX)
• f • V
IN(MAX)
+| V
OUT
|+V
D
( )
2
The inductance can be further adjusted to achieve specific
design optimization of efficiency, output ripple, component
size and loop response.
Once the inductance value has been determined, the type
of inductor must be selected. Core loss is independent of
core size for a given inductor value, but it is very depen-
dent on the inductance selected. As inductance increases,
core losses decrease. Unfortunately, increased inductance
requires more turns of wire and therefore, copper losses
will increase.
High efficiency converters generally cannot tolerate the
core loss of low cost powdered iron cores, forcing the use
of more expensive ferrite materials. Ferrite designs have
very low core loss and are preferred at high switching
frequencies, so design goals can concentrate on cop-
per loss and preventing saturation. Ferrite core material
saturates hard, which means that inductance collapses
abruptly when the peak design current is exceeded. This
will result in an abrupt increase in inductor ripple current
and output voltage ripple. Do not allow the core to saturate!
A variety of inductors are available from manufacturers
such as Sumida, Panasonic, Coiltronics, Coilcraft, Toko,
Vishay, Pulse and Würth.
Current Sensing and Current Limit Programming
The LTC3863 senses the inductor current through a cur-
rent sense resistor, R
SENSE
, placed across the V
IN
and
SENSE pins. The voltage across the resistor, V
SENSE
, is
proportional to inductor current and in normal operation is
compared to the peak inductor current setpoint. An inductor
current limit condition is detected when V
SENSE
exceeds
95mV. When the current limit threshold is exceeded, the
P-channel MOSFET is immediately turned off by pulling
the GATE voltage to V
IN
regardless of the controller input.
The peak inductor current limit is equal to:
I
L(PEAK)
≅
95mV
R
SENSE






This inductor current limit would translate to an output
current limit based on the inductor ripple and duty factor:
I
OUT(LIMIT)
=
95mV
R
SENSE
–
∆I
L
2






• 1–D
( )
The SENSE pin is a high impedance input with a maximum
leakage of ±2µA. Since the LTC3863 is a peak current
mode controller, noise on the SENSE pin can create pulse
width jitter. Careful attention must be paid to the layout of
R
SENSE
. To ensure the integrity of the current sense signal,
V
SENSE
, the traces from V
IN
and SENSE pins should be
short and run together as a differential pair and Kelvin
(4-wire) connected across R
SENSE
(Figure 3).
Figure 3. Inductor Current Sensing
V
IN
R
SENSE
LTC3863
V
IN
SENSE
R
F
MP
OPTIONAL
FILTERING
3863 F03
C
F