Datasheet

LT3845
15
3845fd
APPLICATIONS INFORMATION
will increase the peak currents, requiring more fi ltering
on the input and output of the supply. If ΔI
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)
+ ΔI
L
/2. The inductor
saturation current specifi cation is the current at which
the inductance, measured at zero current, decreases by
a specifi ed 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 specifi cation is the RMS current at which
the part has a specifi c 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 effi ciency, 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)
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.
In the main MOSFET, transition losses are proportional
to V
IN
2
and can be considerably large in high voltage ap-
plications (V
IN
> 20V). Calculate the maximum transition
losses:
P
TRAN(TOP)
= 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 LT3845 applications.
The total maximum power dissipations of the MOSFET
are:
P
TOP(TOTAL)
= P
COND(MAIN)
+ P
TRAN(MAIN)
P
BOT(TOTAL)
= P
COND(SYNC)
To achieve high supply effi ciency, keep the total power dis-
sipation in each switch 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
(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 max-
imum specifi ed junction temperature, typically 150°C.
Note that when V
IN
is high and f
SW
is high, the transition
losses may dominate. A MOSFET with higher R
DS(ON)
and lower C
RSS
may provide higher effi ciency. MOSFETs
with higher voltage V
DSS
specifi cation usually have higher
R
DS(ON)
and lower C
RSS
.