Datasheet

LTC3863
17
3863f
For more information www.linear.com/3863
APPLICATIONS INFORMATION
voltage rises across the resistor load. The Miller charge
(the increase in coulombs on the horizontal axis from a to
b while the curve is flat) is specified for a given V
SD
test
voltage, but can be adjusted for different V
SD
voltages by
multiplying by the ratio of the adjusted V
SD
to the curve
specified V
SD
value. A way to estimate the C
MILLER
term
is to take the change in gate charge from points a and b
(or the parameter Q
GD
on a manufacturers data sheet)
and dividing it by the specified V
SD
test voltage, V
SD(TEST)
.
C
MILLER
Q
GD
V
SD(TEST)
The term with C
MILLER
accounts for transition loss, which
is highest at high input voltages. For V
IN
< 20V, the high
current efficiency generally improves with larger MOSFETs,
while for V
IN
> 20V, the transition losses rapidly increase
to the point that the use of a higher R
DS(ON)
device with
lower C
MILLER
actually provides higher efficiency.
Schottky Diode Selection
When the P-channel MOSFET is turned off, a power
Schottky diode is required to function as a commutating
diode to carry the inductor current. The average forward
diode current is independent of duty factor and is de-
scribed as:
I
F(AVG)
= I
OUT
The worst-case condition for diode conduction is a short-
circuit condition where the Schottky must handle the
maximum current as its duty factor approaches 100% (and
the P-channel MOSFETs duty factor approaches 0%). The
diode therefore must be chosen carefully to meet worst-
case voltage and current requirements. A good practice
is to choose a diode that has a forward current rating two
times higher than I
OUT(MAX)
.
Once the average forward diode current is calculated, the
power dissipation can be determined. Refer to the Schottky
diode data sheet for the power dissipation, P
DIODE
, as a
function of average forward current, I
F(AVG)
. P
DIODE
can
also be iteratively determined by the two equations below,
where V
F(IOUT
,
TJ)
is a function of both I
F(AVG)
and junction
temperature T
J
. Note that the thermal resistance, θ
JA(DIODE)
,
given in the data sheet is typical and can be highly layout
dependent. It is therefore important to make sure that the
Schottky diode has adequate heat sinking.
T
J
P
DIODE
θ
JA(DIODE)
P
DIODE
I
F(AVG)
• V
D(IOUT,TJ)
The Schottky diode forward voltage is a function of both
I
F(AVG)
and T
J
, so several iterations may be required to
satisfy both equations. The Schottky forward voltage,
V
D
, should be taken from the Schottky diode data sheet
curve showing instantaneous forward voltage. The forward
voltage will change as a function of both T
J
and I
F(AVG)
.
The nominal forward voltage will also tend to increase as
the reverse breakdown voltage increases. It is therefore
advantageous to select a Schottky diode appropriate to the
input voltage requirements. The diode reverse breakdown
voltage must meet the following condition:
V
R
> V
IN(MAX)
+ |V
OUT
|
C
IN
and C
OUT
Selection
The input and output capacitance, C
IN
/C
OUT
, are required
to filter the square wave current through the P-channel
MOSFET and diode respectively. Use a low ESR capacitor
sized to handle the maximum RMS current:
I
CIN(RMS)
=I
COUT(RMS)
=I
OUT
|V
OUT
|+V
D
V
IN