LT3844
13
3844fc
For more information www.linear.com/LT3844
applicaTions inForMaTion
The typical range of values for ∆I
L
is (0.2 • I
OUT(MAX)
) to
(0.5 • I
OUT(MAX)
), where I
OUT(MAX)
is the maximum load
current of the supply. Using ∆I
L
= 0.3 • I
OUT(MAX)
yields a
good design compromise between inductor performance
versus inductor size and cost. A value of ∆I
L
= 0.3 • I
OUT(MAX)
produces a ±15% of I
OUT(MAX)
ripple current around the DC
output current of the supply. Lower values of ∆I
L
require
larger and more costly magnetics. Higher values of ∆I
L
will increase the peak currents, requiring more filtering
on the input and output of the supply. If ∆I
L
is too high,
the slope compensation circuit is ineffective and current
mode instability may occur at duty cycles greater than
50%. To satisfy slope compensation requirements the
minimum inductance is calculated as follows:
L > V
OUT
•
MAX
−
DC
MAX
•
SENSE
f
Some magnetics vendors specify a volt-second product
in their data sheet. If they do not, consult the magnetics
vendor to make sure the specification is not being exceeded
by your design. The volt-second product is calculated as
follows:
Volt-second (µs) =
IN(MAX)
−
OUT
OUT
V
IN(MAX)
• f
SW
The magnetics vendors specify either the saturation cur-
rent, the RMS current or both. When selecting an inductor
based on inductor saturation current, use the peak cur-
rent through the inductor, I
OUT(MAX)
+ ∆I
L
/2. The inductor
saturation current specification is the current at which
the inductance, measured at zero current, decreases by
a specified amount, typically 30%.
When selecting an inductor based on RMS current rating,
use the average current through the inductor, I
OUT(MAX)
.
The RMS current specification is the RMS current at which
the part has a specific temperature rise, typically 40°C,
above 25°C ambient.
After calculating the minimum inductance value, the
volt-second product, the saturation current and the RMS
current for your design, select an off-the-shelf inductor.
Contact the Application group at Linear Technology for
further support.
For more detailed information on selecting an inductor,
please see the “Inductor Selection” section of Linear
Technology Application Note 44.
Step-Down Converter: MOSFET Selection
The selection criteria of the external N-channel standard
level power MOSFET include on resistance(R
DS(ON)
), re-
verse transfer capacitance (C
RSS
), maximum drain source
voltage (V
DSS
), total gate charge (Q
G
) and maximum
continuous drain current.
For maximum efficiency, minimize R
DS(ON)
and C
RSS
.
Low R
DS(ON)
minimizes conduction losses while low C
RSS
minimizes transition losses. The problem is that R
DS(ON)
is
inversely related to C
RSS
. Balancing the transition losses
with the conduction losses is a good idea in sizing the
MOSFET. Select the MOSFET to balance the two losses.
Calculate the maximum conduction losses of the MOSFET:
P
COND
= (I
OUT(MAX)
)
2
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
(R
DS(ON)
)
Note that R
DS(ON)
has a large positive temperature depen-
dence. The MOSFET manufacturer’s data sheet contains
a curve, R
DS(ON)
vs Temperature.
Calculate the maximum transition losses:
P
TRAN
= (k)(V
IN
)
2
(I
OUT(MAX)
)(C
RSS
)(f
SW
)
where k is a constant inversely related to the gate driver
current, approximated by k = 2 for LT3844 applications.
The total maximum power dissipation of the MOSFET is
the sum of these two loss terms:
P
FET(TOTAL)
= P
COND
+ P
TRAN
To achieve high supply efficiency, keep the P
FET(TOTAL)
to
less than 3% of the total output power. Also, complete
a thermal analysis to ensure that the MOSFET junction
temperature is not exceeded.
T
J
= T
A
+ P
FET(TOTAL)
• θ
JA
where θ
JA
is the package thermal resistance and T
A
is the
ambient temperature. Keep the calculated T
J
below the
maximum specified junction temperature, typically 150°C.
