參數(shù)資料
型號(hào)： LTC1709EG-9
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： 3 A DUAL SWITCHING CONTROLLER, 360 kHz SWITCHING FREQ-MAX, PDSO36
封裝： 0.209 INCH, PLASTIC, SSOP-36
文件頁(yè)數(shù)： 14/28頁(yè)
文件大?。?/td> 319K
代理商： LTC1709EG-9
14
LTC1709-7
factors for the top and bottom MOSFETs of each output
stage are given by:
MainSwitchDutyCycle
V
V
OUT
IN
=
SynchronousSwitchDutyCycle
V
V
V
IN
OUT
IN
=
–
The MOSFET power dissipations at maximum output
current are given by:
P
V
V
I
R
k V
( )
I
C
f
MAIN
OUT
IN
MAX
2
DS ON
(
IN
MAX
2
RSS
=
(
+
(
)
)( )
+
1
2
2
δ
)
P
V
V
V
I
R
SYNC
IN
OUT
IN
MAX
2
DS ON
(
=
+
(
)
–
)
1
2
δ
where
δ
is the temperature dependency of R
DS(ON)
and k
is a constant inversely related to the gate drive current.
Both MOSFETs have I
2
R losses but the topside N-channel
equation includes an additional term for transition losses,
which peak at the highest input voltage. For V
IN
< 20V the
high current efficiency generally improves with larger
MOSFETs, while for V
IN
> 20V the transition losses rapidly
increase to the point that the use of a higher R
DS(ON)
device
with lower C
RSS
actual provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during a
short-circuit when the synchronous switch is on close to
100% of the period.
The term (1 +
δ
) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs temperature curve, but
δ
= 0.005/
°
C can be used as an approximation for low
voltage MOSFETs. C
RSS
is usually specified in the
MOSFET characteristics. The constant k = 1.7 can be
used to estimate the contributions of the two terms in the
main switch dissipation equation.
The Schottky diodes, D1 and D2 shown in Figure 1
conduct during the dead-time between the conduction of
the two large power MOSFETs. This helps prevent the
body diode of the bottom MOSFET from turning on,
storing charge during the dead-time, and requiring a
reverse recovery period which would reduce efficiency. A
1A to 3A Schottky (depending on output current) diode is
generally a good compromise for both regions of opera-
tion due to the relatively small average current. Larger
diodes result in additional transition losses due to their
larger junction capacitance.
C
IN
and C
OUT
Selection
In continuous mode, the source current of each top
N-channel MOSFET is a square wave of duty cycle V
OUT
/
V
IN
. A low ESR input capacitor sized for the maximum
RMS current must be used. The details of a closed form
equation can be found in Application Note 77. Figure 4
shows the input capacitor ripple current for a 2-phase
configuration with the output voltage fixed and input
voltage varied. The input ripple current is normalized
against the DC output current. The graph can be used in
place of tedious calculations. The minimum input ripple
current can be achieved when the input voltage is twice the
output voltage
In the graph of Figure 4, the 2-phase local maximum input
RMS capacitor currents are reached when:
V
V
k
OUT
IN
=
4
2
1
where k = 1, 2
These worst-case conditions are commonly used for
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design. Always
consult the capacitor manufacturer if there is any
question.
APPLICATIU
W
U
U
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