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參數(shù)資料
| 型號： | LTC1871-7 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Quadruple 2-Input Positive-AND Gate 14-SOIC -40 to 85 |
| 中文描述： | 2相升壓型DC / DC控制器 |
| 文件頁數(shù)： | 15/36頁 |
| 文件大?。?/td> | 411K |
| 代理商： | LTC1871-7 |
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15
LTC1871
a lithium-ion battery or a 3.3V logic supply), then sublogic-
level threshold MOSFETs should be used.
Pay close attention to the BV
DSS
specifications for the
MOSFETs relative to the maximum actual switch voltage in
the application. Many logic-level devices are limited to 30V
or less, and the switch node can ring during the turn-off of
the MOSFET due to layout parasitics. Check the switching
waveforms of the MOSFET directly across the drain and
source terminals using the actual PC board layout (not just
on a lab breadboard!) for excessive ringing.
During the switch on-time, the control circuit limits the
maximum voltage drop across the power MOSFET to
about 150mV (at low duty cycle). The peak inductor
current is therefore limited to 150mV/R
DS(ON)
. The rela-
tionship between the maximum load current, duty cycle
and the R
DS(ON)
of the power MOSFET is:
R
V
D
I
DS ON
(
SENSE MAX
MAX
O MAX
(
T
)
(
)
)
–
≤
+
1
1
2
χ
ρ
The V
SENSE(MAX)
term is typically 150mV at low duty
cycle, and is reduced to about 100mV at a duty cycle of
92% due to slope compensation, as shown in Figure 10.
The
ρ
T
term accounts for the temperature coefficient of
the R
DS(ON)
of the MOSFET, which is typically 0.4%/
°
C.
Figure 11 illustrates the variation of normalized R
DS(ON)
over tempera
ture for a typical power MOSFET.
Another method of choosing which power MOSFET to use
is to check what the maximum output current is for a given
R
DS(ON)
, since MOSFET on-resistances are available in
discrete values.
I
V
D
R
O MAX
(
SENSE MAX
MAX
DS ON
(
T
)
(
)
)
–
=
+
1
1
2
χ
ρ
It is worth noting that the 1 – D
MAX
relationship between
I
O(MAX)
and R
DS(ON)
can cause boost converters with a
wide input range to experience a dramatic range of maxi-
mum input and output current. This should be taken into
consideration in applications where it is important to limit
the maximum current drawn from the input supply.
Calculating Power MOSFET Switching and Conduction
Losses and Junction Temperatures
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be
known. This power dissipation is a function of the duty
cycle, the load current and the junction temperature itself
(due to the positive temperature coefficient of its R
DS(ON)
).
As a result, some iterative calculation is normally required
to determine a reasonably accurate value. Since the
con
troller is using the MOSFET as both a switching and a
sensing element, care should be taken to ensure that the
converter is capable of delivering the required load current
over all operating conditions (line voltage and tempera-
ture), and for the worst-case specifications for V
SENSE(MAX)
APPLICATIOU
W
U
U
DUTY CYCLE
0
M
100
150
0.8
1871 F10
50
0
0.2
0.4
0.5
1.0
200
Figure 10. Maximum SENSE Threshold Voltage vs Duty Cycle
JUNCTION TEMPERATURE (
°
C)
–50
ρ
T
1.0
1.5
150
1871 F11
0.5
0
0
50
100
2.0
Figure 11. Normalized R
DS(ON)
vs Temperature
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