Datasheet

LTC3864
15
3864f
APPLICATIONS INFORMATION
generated by forcing a constant current out of the gate of a
common-source connected P-MOSFET that is loaded with
a resistor, and then plotting the gate voltage versus time.
The initial slope is the effect of the gate-to-source and
gate-to-drain capacitances. The flat portion of the curve
is the result of the Miller multiplication effect of the drain-
to-gate capacitance as the drain 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
manufacturer’s 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-MOSFET is turned off, a power Schottky diode
is required to function as a commutating diode to carry the
inductor current. The average diode current is therefore
dependent on the P-MOSFET’s duty factor. 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
MOSFET’s duty factor approaches 0%). The diode there-
fore must be chosen carefully to meet worst case voltage
and current requirements. The equation below describes
the continuous or average forward diode current rating
required, where D is the regulator duty factor.
I
F(AVG)
≅I
OUT(MAX)
• 1–D
( )
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
F(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
F
should be taken from the Schottky diode data sheet curve
showing Instantaneous Forward Voltage. The forward
voltage will increase 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.
C
IN
and C
OUT
Selection
The input capacitance C
IN
is required to filter the square
wave current through the P-channel MOSFET. Use a low
ESR capacitor sized to handle the maximum RMS current.
I
CIN(RMS)
≅I
OUT(MAX)
•
V
OUT
V
IN
•
V
IN
V
OUT
–1
The formula has a maximum at V
IN
= 2V
OUT
, where
I
CIN(RMS)
= I
OUT(MAX)
/2. This simple worst-case condition
is commonly used for design because even significant
deviations do not offer much relief. Note that ripple cur-
rent ratings from capacitor manufacturers are often based
on only 2000 hours of life, which makes it advisable to
derate the capacitor.
The selection of C
OUT
is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The ∆V
OUT
is approximately bounded by:
∆V
OUT
≤ ∆I
L
ESR+
1
8 • f • C
OUT



