lt3845a Linear Technology Corporation, lt3845a Datasheet - Page 15

• Manufacturer Part Number: lt3845a
• Description: Lt3845a - High Voltage Synchronous Current Mode Step-down Controller With Adjustable Operating Frequency
• Manufacturer: Linear Technology Corporation

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
lated as follows:
f
The typical range of values for DI
(0.5 • I
current of the supply. Using DI
good design compromise between inductor performance
versus inductor size and cost. A value of DI
produces a ±15% of I
output current of the supply. Lower values of DI
larger and more costly magnetics. Higher values of DI
will increase the peak currents, requiring more filtering
on the input and output of the supply. If DI
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:
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
saturation current specification is the current at which
the inductance, measured at zero current, decreases by
a specified amount, typically 30%.
SW
L ≥ V
L
is the switch frequency.
MIN
OUT(MAX)
OUT
> V
OUT
•
f
L
SW
, The minimum inductance value is calcu-
V
), where I
•
IN(MAX)
2DC
• V
DC
OUT(MAX)
IN(MAX)
MAX
MAX
– V
OUT(MAX)
– 1
OUT(MAX)
OUT
• DI
ripple current around the DC
•
L
R
L
= 0.3 • I
SENSE
L
is (0.2 • I
is the maximum load
+ DI
f
SW
• 8.33
OUT(MAX)
L
L
/2. The inductor
= 0.3 • I
L
OUT(MAX)
is too high,
OUT(MAX)
L
yields a
require
) to
L
When selecting an inductor based on RMS current rating,
use the average current through the inductor, I
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
reverse transfer capacitance (C
source voltage (V
continuous drain current.
For maximum efficiency, minimize R
Low R
minimizes transition losses. The problem is that R
is inversely related to C
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
P
COND(TOP)
COND(BOT)
DS(ON)
minimizes conduction losses while low C
=I
=I
DSS
OUT(MAX)
OUT(MAX)
), total gate charge (Q
RSS
2
2
. In selecting the top MOSFET
•
•
V
V
V
IN
OUT
IN
– V
V
RSS
IN
•R
OUT
), maximum drain
DS(ON)
DS(ON)
LT3845A
G
•R
) and maximum
DS(ON)
and C
OUT(MAX)
DS(ON)
15
DS(ON)
3845afa
RSS
RSS
),
.
.