LT3724
13
3724fd
APPLICATIONS INFORMATION
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 LT3724 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.
Note that when V
IN
is high, the transition losses may
dominate. A MOSFET with higher R
DS(ON)
and lower C
RSS
may provide higher efficiency. MOSFETs with higher volt-
age V
DSS
specification usually have higher R
DS(ON)
and
lower C
RSS
.
Choose the MOSFET V
DSS
specification to exceed the
maximum voltage across the drain to the source of the
MOSFET, which is V
IN(MAX)
plus any additional ringing
on the switch node. Ringing on the switch node can be
greatly reduced with good PCB layout and, if necessary,
an RC snubber.
The internal V
CC
regulator operating range limits the maxi-
mum total MOSFET gate charge, Q
G
, to 90nC. The Q
G
vs
V
GS
specification is typically provided in the MOSFET data
sheet. Use Q
G
at V
GS
of 8V. If V
CC
is back driven from an
external supply, the MOSFET drive current is not sourced
from the internal regulator of the LT3724 and the Q
G
of the
MOSFET is not limited by the IC. However, note that the
MOSFET drive current is supplied by the internal regulator
when the external supply back driving V
CC
is not available
such as during startup or short-circuit.
The manufacturer’s maximum continuous drain current
specification should exceed the peak switch current,
I
OUT(MAX)
+ ∆I
L
/2.
During the supply startup, the gate drive levels are set by
the V
CC
voltage regulator, which is approximately 8V. Once
the supply is up and running, the V
CC
can be back driven
by an auxiliary supply such as V
OUT
. It is important not to
exceed the manufacturer’s maximum V
GS
specification.
A standard level threshold MOSFET typically has a V
GS
maximum of 20V.
Step-Down Converter: Rectifier Selection
The rectifier diode (D1 on the Functional Diagram) in a
buck converter generates a current path for the inductor
current when the main power switch is turned off. The
rectifier is selected based upon the forward voltage, re-
verse voltage and maximum current. A Schottky diode is
recommended. Its low forward voltage yields the lowest
power loss and highest efficiency. The maximum reverse
voltage that the diode will see is V
IN(MAX)
.
In continuous mode operation, the average diode cur-
rent is calculated at maximum output load current and
maximum V
IN
:
I
DIODE(AVG)
=I
OUT(MAX)
V
IN(MAX)
− V
OUT
V
IN(MAX)
To improve efficiency and to provide adequate margin for
short-circuit operation, a diode rated at 1.5 to 2 times the
maximum average diode current, I
DIODE(AVG)
, is recom-
mended.
