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| 型号： | LTC3872ETS8#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | 1 MM HEIGHT, LEAD FREE, PLASTIC, MO-193, TSOT-23, 8 PIN |
| 文件页数： | 6/22页 |
| 文件大小： | 245K |
| 代理商： | LTC3872ETS8#PBF |
LTC3872
14
3872fb
output current. Assuming an efficiency of 90%, this
sense resistor power dissipation represents 1.3% of the
overall input power. In other words, for this application,
the use of VDS sensing would increase the efficiency by
approximately 1.3%.
For more details regarding the various terms in these
equations, please refer to the section Boost Converter:
Power MOSFET Selection.
3. The losses in the inductor are simply the DC input cur-
rentsquaredtimesthewindingresistance.Expressingthis
loss as a function of the output current yields:
PR(WINDING) =
IO(MAX)
1– DMAX
2
RW
4. Losses in the boost diode. The power dissipation in the
boost diode is:
PDIODE = IO(MAX) VD
The boost diode can be a major source of power loss in
a boost converter. For the 3.3V input, 5V output at 7A ex-
ample given above, a Schottky diode with a 0.4V forward
voltage would dissipate 2.8W, which represents 7% of the
input power. Diode losses can become significant at low
output voltages where the forward voltage is a significant
percentage of the output voltage.
5. Other losses, including CIN and CO ESR dissipation and
inductor core losses, generally account for less than 2%
of the total additional loss.
Checking Transient Response
The regulator loop response can be verified by looking at
theloadtransientresponse.Switchingregulatorsgenerally
take several cycles to respond to an instantaneous step
in resistive load current. When the load step occurs, VO
immediately shifts by an amount equal to (
DILOAD)(ESR),
and then CO begins to charge or discharge (depending on
the direction of the load step) as shown in Figure 7. The
regulator feedback loop acts on the resulting error amp
output signal to return VO to its steady-state value. During
this recovery time, VO can be monitored for overshoot or
ringing that would indicate a stability problem.
A second, more severe transient can occur when con-
necting loads with large (>1F) supply bypass capacitors.
The discharged bypass capacitors are effectively put in
parallel with CO, causing a nearly instantaneous drop in
VO. No regulator can deliver enough current to prevent
this problem if the load switch resistance is low and it is
driven quickly. The only solution is to limit the rise time
of the switch drive in order to limit the inrush current
di/dt to the load.
Boost Converter Design Example
Thedesignexamplegivenherewillbeforthecircuitshown
on the front page. The input voltage is 3.3V, and the output
is 5V at a maximum load current of 2A.
1. The duty cycle is:
D
=
VO + VD – VIN
VO + VD
=
5
+ 0.4 – 3.3
5
+ 0.4
= 38.9%
2. An inductor ripple current of 40% of the maximum load
current is chosen, so the peak input current (which is also
the minimum saturation current) is:
IIN(PEAK) = 1+
χ
2
IO(MAX)
1– DMAX
= 1.2
2
1– 0.39
= 3.9A
The inductor ripple current is:
IL = χ
IO(MAX)
1– DMAX
= 0.4
2
1– 0.39
= 1.3A
Figure 7. Load Transient Response for a 3.3V Input,
5V Output Boost Converter Application, 0.1A to 1A Step
IL
500mA/DIV
VOUT
200mV/DIV
AC-COUPLED
20s/DIV
3872 F07
applicaTions inForMaTion
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