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参数资料
| 型号： | LTC3785EUF-1#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 650 kHz SWITCHING FREQ-MAX, PQCC24 |
| 封装： | 4 X 4 MM, LEAD FREE, PLASTIC, MO-220WGGD, QFN-24 |
| 文件页数： | 5/20页 |
| 文件大小： | 213K |
| 代理商： | LTC3785EUF-1#PBF |
LTC3785-1
13
37851fa
(switches B and C shown in Figure 1). Important param-
eters for the power MOSFETs are the breakdown voltage
VBR(DSS),thresholdvoltageVGS(TH),on-resistanceRDS(ON),
reverse transfer capacitance CRSS and maximum current
IDS(MAX). The drive voltage is set by the 4.35V VCC supply.
Consequently, logic-level threshold MOSFETs must be used
in LTC3785-1 applications. If the input voltage is expected
to drop below 5V, then sub-logic threshold MOSFETs should
be considered. In order to select the power MOSFETs, the
power dissipated by the device must be known.
For switch A, the maximum power dissipation happens
in boost mode, when it remains on all the time. Its maxi-
mum power dissipation at maximum output current is
given by:
PA(BOOST)
=
VOUT
VIN
IOUT(MAX)
2
ρTRDS(ON)
where
ρT is a normalization factor (unity at 25°C) ac-
counting for the signicant variation in on-resistance with
temperature, typically about 0.4%/°C as shown in Figure 4.
For a maximum junction temperature of 125°C, using a
value
ρT = 1.5 is reasonable.
Switch B operates in buck mode as the synchronous
rectier. Its power dissipation at maximum output current
is given by:
PB(BUCK)
=
VIN –VOUT
VIN
IOUT(MAX)
2 ρTR
DS(ON)
APPLICATIONS INFORMATION
Switch C operates in boost mode as the control switch. Its
power dissipation at maximum current is given by:
PC(BOOST)
=
VOUT –VIN
()V
OUT
VIN
2
IOUT(MAX)
2 ρT
RDS(ON) + k VOUT
3
IOUT(MAX)
VIN
CRSS f
where CRSSisusuallyspeciedbytheMOSFETmanufactur-
ers. The constant k, which accounts for the loss caused by
reverse recovery current, is inversely proportional to the
gate drive current and has an empirical value of 1.0.
For switch D, the maximum power dissipation happens in
boost mode when its duty cycle is higher than 50%. Its
maximum power dissipation at maximum output current
is given by:
PD BOOST
()=
VOUT
VIN
IOUT(MAX)
2 ρTR
DS(ON)
Typically, switch A has the highest power dissipation and
switch B has the lowest power dissipation unless a short
occurs at the output. From a known power dissipated
in the power MOSFET, its junction temperature can be
obtained using the following formula:
TJ = TA + P RTH(JA)
The RTH(JA) to be used in the equation normally includes
the RTH(JC) for the device plus the thermal resistance from
the case to the ambient temperature (RTH(CA)). This value
of TJ can then be compared to the original, assumed value
used in the iterative calculation process.
SCHOTTKY DIODE (D1, D2) SELECTION
Optional Schottky diodes D1 and D2 shown in the Block
Diagram conduct during the dead time between the conduc-
tion of the power MOSFET switches. They are intended to
prevent the body diode of synchronous switches B and D
from turning on and storing charge during the dead time.
In particular, D2 signicantly reduces reverse recovery
current between switch D turn off and switch C turn on,
which improves converter efciency and reduces switch C
voltage stress. In order for D2 to be effective, it must be
located in very close proximity to SWD.
JUNCTION TEMPERATURE (°C)
–50
TNORMALIZED
ON-RESIST
ANCE
1.0
1.5
150
37851 F04
0.5
0
50
100
2.0
Figure 4. Normalized RDS(ON) vs Temperature
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