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參數(shù)資料
| 型號： | LTC3872EDDB#TRM |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 3 X 2 MM, PLASTIC, MO-229WECD-1, DFN-8 |
| 文件頁數(shù)： | 20/20頁 |
| 文件大?。?/td> | 261K |
| 代理商： | LTC3872EDDB#TRM |
LTC3872
9
3872fa
APPLICATIO S I FOR ATIO
WU
U
the maximum drain current (ID(MAX)) and the MOSFET’s
thermal resistances (RTH(JC)andRTH(JA)).Logic-level(4.5V
VGS-RATED) threshold MOSFETs should be used when input
voltage is high, otherwise if low input voltage operation
is expected (e.g., supplying power from a lithium-ion
battery or a 3.3V logic supply), then sublogic-level (2.5V
VGS-RATED) threshold MOSFETs should be used.
Pay close attention to the BVDSS specications 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 270mV, 100mV and 170mV at low duty cycle with
IPRG tied to VIN, GND, or left oating respectively. The
peak inductor current is therefore limited to (270mV,
170mV and 100mV)/RDS(ON) depending on the status of
the IPRG pin.
The relationship between the maximum load current, duty
cycle and the RDS(ON) of the power MOSFET is:
RV
D
I
DS ON
SENSE MAX
MAX
O MAX
T
()
(
)
()
–
≤
+
1
2
χ
ρ
VSENSE(MAX) is the maximum voltage drop across the
power MOSFET. VSENSE(MAX) is typically 270mV, 170mV
and 100mV. It is reduced with increasing duty cycle as
shown in Figure 3. The
ρTtermaccountsforthetemperature
coefcient of the RDS(ON) of the MOSFET, which is typically
0.4%/°C. Figure 4 illustrates the variation of normalized
RDS(ON) over temperature 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 RDS(ON), since MOSFET on-resistances are available
in discrete values.
IV
D
R
O MAX
SENSE MAX
MAX
DS ON
T
()
–
=
+
1
2
χ
ρ
It is worth noting that the 1 – DMAX relationship between
IO(MAX) and RDS(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,thepowerdissipatedbythedevicemustbeknown.
This power dissipation is a function of the duty cycle, the
load current and the junction temperature itself (due to
the positive temperature coefcient of its RDS(ON)). As a
JUNCTION TEMPERATURE (
°C)
–50
ρ
TNORMALIZED
ON
RESISTANCE
1.0
1.5
150
3872 F04
0.5
0
50
100
2.0
Figure 4. Normalized RDS(ON) vs Temperature
Figure 3. Maximum SENSE Threshold Voltage vs Duty Cycle
DUTY CYCLE (%)
1
0
MAXIMUM
CURRENT
SENSE
VOLTAGE
(mV)
50
100
150
200
250
300
20
40
60
80
3872 G03
100
IPRG = HIGH
IPRG = FLOAT
IPRG = LOW
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