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LTM4607 数据表(PDF) 18 Page - Linear Technology

部件名 LTM4607
功能描述  High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller
PDF  28 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTM4607 数据表(HTML) 18 Page - Linear Technology

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LTC3780
18
3780fe
APPLICATIONS INFORMATION
Power MOSFET Selection and
Efficiency Considerations
The LTC3780 requires four external N-channel power
MOSFETs, two for the top switches (switch A and D, shown
in Figure 1) and two for the bottom switches (switch B and C
shown in Figure 1). Important parameters for the power
MOSFETs are the breakdown voltage VBR,DSS, threshold
voltage VGS,TH, on-resistance RDS(ON), reverse transfer
capacitance CRSS and maximum current IDS(MAX).
The drive voltage is set by the 6V INTVCC supply. Con-
sequently, logic-level threshold MOSFETs must be used
in LTC3780 applications. If the input voltage is expected
to drop below 5V, then the sub-logic threshold MOSFETs
should be considered.
In order to select the power MOSFETs, the power dis-
sipated 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 maximum power dissipation
at maximum output current is given by:
P
V
V
IR
A BOOST
OUT
IN
OUT MAX
T
DS ON
,(
)
(
)
= ⎛
⎝⎜
⎞
⎠⎟
2
ρ
where
ρT is a normalization factor (unity at 25°C) ac-
counting for the significant variation in on-resistance with
temperature, typically about 0.4%/°C as shown in Figure 9.
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
rectifier. Its power dissipation at maximum output current
is given by:
P
VV
V
IR
BBUCK
IN
OUT
IN
OUT MAX
T
DS ON
,(
)
(
)
–
=
2
ρ
Switch C operates in boost mode as the control switch. Its
power dissipation at maximum current is given by:
P
VV
V
V
IR
kV
I
V
Cf
C BOOST
OUT
IN
OUT
IN
OUT MAX
T
DS ON
OUT
OUT MAX
IN
RSS
,(
)
(
)
()
–
= ()
+
2
2
3
ρ
where CRSSisusuallyspecifiedbytheMOSFETmanufactur-
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 1.7.
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:
P
V
V
V
V
I
D BOOST
IN
OUT
OUT
IN
OUT MAX
,(
)
=
⎛
⎝⎜
⎞
⎠⎟
2
ρTTDS ON
R ()
For the same output voltage and current, 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(JC)). This value
of TJ can then be compared to the original, assumed value
used in the iterative calculation process.
JUNCTION TEMPERATURE (°C)
–50
1.0
1.5
150
3780 F09
0.5
0
0
50
100
2.0
Figure 9. Normalized RDS(ON) vs Temperature



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