lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 13

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
For maximum efficiency, minimize R
R
minimizes transition losses. The problem is that R
is inversely related to C
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:
Note that R
dence. The MOSFET manufacturer’s data sheet contains a
curve, R
Calculate the maximum transition losses:
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:
To achieve high supply efficiency, keep the P
less than 3% of the total output power. Also, complete a
thermal analysis to ensure that the MOSFET junction
temperature is not exceeded.
where θ
ambient temperature. Keep the calculated T
maximum specified junction temperature, typically 150°C.
Note that when V
dominate. A MOSFET with higher R
may provide higher efficiency. MOSFETs with higher volt-
age V
lower C
Choose the MOSFET V
maximum voltage across the drain to the source of the
MOSFET, which is V
the switch node. Ringing on the switch node can be greatly
reduced with good PCB layout and, if necessary, an RC
snubber.
DS(ON)
P
P
T
P
COND
J
TRAN
FET(TOTAL)
= T
DSS
RSS
JA
DS(ON)
minimizes conduction losses while low C
A
= (k)(V
specification usually have higher R
=
is the package thermal resistance and T
+ P
DS(ON)
.
(
I
OUT MAX
FET(TOTAL)
= P
vs Temperature.
IN
(
has a large positive temperature depen-
COND
)
IN
2
U
IN(MAX)
(I
is high, the transition losses may
)
RSS
OUT(MAX)
)
2
+ P
DSS
• θ
⎛
⎜
⎝
U
. Balancing the transition losses
V
TRAN
JA
plus any additional ringing on
V
OUT
IN
specification to exceed the
)(C
⎞
⎟
⎠
(
R
RSS
DS(ON)
W
DS ON
DS(ON)
(
)(f
SW
)
and lower C
)
and C
)
FET(TOTAL)
J
DS(ON)
U
below the
RSS
A
DS(ON)
. Low
is the
and
RSS
RSS
to
The internal V
maximum total MOSFET gate charge, Q
vs V
data sheet. Use Q
an external supply, the MOSFET drive current is not
sourced from the internal regulator of the LT3724 and the
Q
that the MOSFET drive current is supplied by the internal
regulator when the external supply back driving V
available such as during startup or short-circuit.
The manufacturer’s maximum continuous drain current
specification should exceed the peak switch current,
I
During the supply startup, the gate drive levels are set by
the V
Once the supply is up and running, the V
driven by an auxiliary supply such as V
not to exceed the manufacturer’s maximum V
tion. A standard level threshold MOSFET typically has a
V
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 continuous mode operation, the average diode current
is calculated at maximum output load current and maxi-
mum V
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
recommended.
OUT(MAX)
GS
G
I
of the MOSFET is not limited by the IC. However, note
DIODE AVG
GS
maximum of 20V.
CC
specification is typically provided in the MOSFET
IN
voltage regulator, which is approximately 8V.
(
:
+ ΔI
)
L
=
CC
/2.
I
G
OUT MAX
regulator operating range limits the
at V
(
GS
of 8V. If V
)
V
IN MAX
(
V
IN MAX
IN(MAX)
(
)
CC
−
is back driven from
OUT
V
G
)
OUT
, to 90nC. The Q
.
CC
. It is important
DIODE(AVG)
LT3724
GS
can be back
specifica-
CC
13
is not
3724fb
, is
G