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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)： | 5/20頁 |
| 文件大小： | 261K |
| 代理商： | LTC3872EDDB#TRM |
LTC3872
13
3872fa
Power MOSFET Selection.
3. The losses in the inductor are simply the DC input cur-
rent squared times the winding resistance. Expressing 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 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 signicant at low
output voltages where the forward voltage is a signicant
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 veried by looking at
the load transient response. Switching regulators generally
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 (
ΔILOAD)(ESR),
and then CO begins to charge or discharge (depending on
the direction of the load step) as shown in Figure 6. 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 (>1μF) 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
The design example given here will be for the circuit shown
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
VV
V
VV
OD
IN
OD
=
+
=
+
=
–. – .
.
.%
50 4 3 3
50 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:
I
D
IN PEAK
OMAX
MAX
()
–
.
–
=+
=
1
21
12
2
10
χ
..
.
39
=
A
The inductor ripple current is:
Δ=
=
I
D
A
L
OMAX
MAX
χ
–
.
–.
.
()
1
04
2
10 39
13
And so the inductor value is:
L
V
If
D
V
AkHz
IN MIN
L
MAX
=
Δ
==
()
.
33
1 3
550
039 1..8
μH
The component chosen is a 2.2μH inductor made by
Sumida (part number CEP125-H 1ROMH).
APPLICATIO S I FOR ATIO
WU
U
Figure 6. 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
20
μs/DIV
3872 F06
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