Datasheet

LT3845A
15
3845afa
APPLICATIONS INFORMATION
Inductor Selection
The critical parameters for selection of an inductor
are minimum inductance value, volt-second product,
saturation current and/or RMS current.
For a given DI
L
, The minimum inductance value is calcu-
lated as follows:
L ≥ V
OUT
•
V
IN(MAX)
– V
OUT
f
SW
• V
IN(MAX)
• DI
L
f
SW
is the switch frequency.
The typical range of values for DI
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 DI
L
= 0.3 • I
OUT(MAX)
yields a
good design compromise between inductor performance
versus inductor size and cost. A value of DI
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 DI
L
require
larger and more costly magnetics. Higher values of DI
L
will increase the peak currents, requiring more filtering
on the input and output of the supply. If DI
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
MIN
> V
OUT
•
2DC
MAX
– 1
DC
MAX
•
R
SENSE
• 8.33
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)
+ DI
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.
MOSFET Selection
The selection criteria of the external N-channel standard
level power MOSFETs include on resistance (R
DS(ON)
),
reverse 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
. In selecting the top MOSFET
balancing the transition losses with the conduction losses
is a good idea while the bottom MOSFET is dominated by
the conduction loss, which is worse during a short-circit
condition or at a very low duty cycle.
Calculate the maximum conduction losses of the
MOSFETs:
P
COND(TOP)
=I
OUT(MAX)
2
•
V
OUT
V
IN
• R
DS(ON)
P
COND(BOT)
=I
OUT(MAX)
2
•
V
IN
– V
OUT
V
IN
• R
DS(ON)