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| 型号： | LTC1871IMS#TRPBF-1 |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | LEAD FREE, PLASTIC, MSOP-10 |
| 文件页数： | 13/36页 |
| 文件大小： | 354K |
| 代理商： | LTC1871IMS#TRPBF-1 |
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20
LTC1871-1
18711fa
resistor value would be 10m
Ω, and the power dissi-
pated in this resistor would be 514mW at maximum
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 appli-
cation, 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
current squared times the winding resistance. Express-
ing this loss as a function of the output current yields:
P
I
D
R
R WINDING
OMAX
MAX
W
()
–
=
1
2
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
example given above, a Schottky diode with a 0.4V
forward voltage would dissipate 2.8W, which repre-
sents 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
the load transient response. Switching regulators gener-
ally take several cycles to respond to an instantaneous
step in resistive load current. When the load step occurs,
VOimmediatelyshiftsbyanamountequalto(ΔILOAD)(ESR),
and then CO begins to charge or discharge (depending on
the direction of the load step) as shown in Figure 13. 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 connect-
ing loads with large (> 1
μF) 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 prob-
lem 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
The design example given here will be for the circuit shown
in Figure 1. The input voltage is 3.3V, and the output is 5V
at a maximum load current of 7A (10A peak).
1. The duty cycle is:
D
VV
V
VV
OD
IN
OD
=
+
=
+
=
–. – .
.
.%
50 4 3 3
50 4
38 9
2. Pulse-skip operation is chosen so the MODE/SYNC pin
is shorted to INTVCC.
3. The operating frequency is chosen to be 300kHz to
reduce the size of the inductor. From Figure 5, the
resistor from the FREQ pin to ground is 80k.
4. 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:
I
D
IN PEAK
OMAX
MAX
()
–
.
–
=+
=
1
21
12
7
10
χ
..
.
39
13 8
=
A
APPLICATIO S I FOR ATIO
WU
UU
IOUT
2A/DIV
VOUT (AC)
100mV/DIV
Figure 13. Load Transient Response for a 3.3V Input,
5V Output Boost Converter Application, 0.7A to 7A Step
VIN = 3.3V
VOUT = 5V
MODE/SYNC = INTVCC
(PULSE-SKIP MODE)
100
μs/DIV
1871 F13
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