參數(shù)資料
型號(hào)： LTC1708-PG
廠商： Linear Technology Corporation
英文描述： Dual Adjustable 5-Bit VID High Efficiency, 2-Phase Current Mode Synchronous Buck Regulator Controller
中文描述： 可調(diào)式雙5位VID高效率，2相電流模式同步降壓穩(wěn)壓控制器
文件頁(yè)數(shù)： 15/32頁(yè)
文件大?。?/td> 358K
代理商： LTC1708-PG
15
LTC1708-PG
APPLICATIU
then, sub-logic level threshold MOSFETs (V
GS(TH)
< 3V)
should be used. Pay close attention to the BV
DSS
specifi-
cation for the MOSFETs as well; most of the logic level
MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
DS(ON)
, reverse transfer capacitance C
RSS
,
input voltage and maximum output current. When the
LTC1708 is operating in continuous mode the duty cycles
for the top and bottom MOSFETs are given by:
W
U
U
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
DS ON
(
IN
MAX
RSS
=
(
)
)(
+
(
)
+
) (
)( )
2
2
1
δ
)
P
V
V
V
I
R
SYNC
IN
OUT
IN
MAX
DS ON
(
=
(
)
+
(
)
–
)
2
1
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 while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. 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
actually 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 diode D1 shown in Figure 1 conducts during
the dead-time between the conduction of the two power
MOSFETs. This prevents the body diode of the bottom
MOSFET from turning on, storing charge during the dead-
time and requiring a reverse recovery period that could
cost as much as 3% in efficiency at high V
IN
. A 1A to 3A
Schottky is generally a good compromise for both regions
of operation 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
The selection of C
IN
is simplified by the multiphase archi-
tecture and its impact on the worst-case RMS current
drawn through the input network (battery/fuse/capacitor).
It can be shown that the worst case RMS current occurs
when only one controller is operating. The controller with
the highest (V
OUT
)(I
OUT
) product needs to be used in the
formula below to determine the maximum RMS current
requirement. Increasing the output current, drawn from
the other out-of-phase controller, will actually decrease
the input RMS ripple current from this maximum value
(see Figure 4). The out-of-phase technique typically re-
duces the input capacitor’s RMS ripple current by a factor
of 30% to 70% when compared to a single phase power
supply solution.
The type of input capacitor, value and ESR rating have
efficiency effects that need to be considered in the selec-
tion process. The capacitance value chosen should be
sufficient to store adequate charge to keep high peak
battery currents down. 20
μ
F to 40
μ
F is usually sufficient
for a 25W output supply operating at 200kHz. The ESR of
the capacitor is important for capacitor power dissipation
as well as overall battery efficiency. All of the power (RMS
ripple current ESR) not only heats up the capacitor but
wastes power from the battery.
Medium voltage (20V to 35V) ceramic, tantalum, OS-CON
and switcher-rated electrolytic capacitors can be used as
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LTC1709EG7 制造商:Linear Technology 功能描述:
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