參數(shù)資料
型號(hào)： LT3800
廠商： Linear Technology Corporation
元件分類： 基準(zhǔn)電壓源/電流源
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： 抗輻射高效，5安培開關(guān)穩(wěn)壓器
文件頁(yè)數(shù)： 13/24頁(yè)
文件大?。?/td> 240K
代理商： LT3800
LT3724
13
3724f
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
I
V
V
R
(
COND
OUT MAX
(
OUT
IN
DS ON
(
=
(
)
)
)
)
2
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
maximum 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
specifica-
tion. 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 current
is calculated at maximum output load current and maxi-
mum V
IN
:
I
I
V
V
V
DIODE AVG
OUT MAX
(
IN MAX
(
OUT
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
recommended.
APPLICATIOU
W
U
U
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