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参数资料
| 型号： | LT1766HGN#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 228 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 0.150 INCH, LEAD FREE, PLASTIC, SSOP-16 |
| 文件页数： | 11/28页 |
| 文件大小： | 284K |
| 代理商： | LT1766HGN#PBF |
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19
LT1766/LT1766-5
1766fb
switch off spike will also cause the SW node to go below
ground. The LT1766 has special circuitry inside which
mitigates this problem, but negative voltages over 0.8V
lasting longer than 10ns should be avoided. Note that
100MHz oscilloscopes are barely fast enough to see the
details of the falling edge overshoot in Figure 7.
A second, much lower frequency ringing is seen during
switch off time if load current is low enough to allow the
inductor current to fall to zero during part of the switch off
time (see Figure 8). Switch and diode capacitance reso-
nate with the inductor to form damped ringing at 1MHz to
10 MHz. This ringing is not harmful to the regulator and it
has not been shown to contribute significantly to EMI. Any
attempt to damp it with a resistive snubber will degrade
efficiency.
THERMAL CALCULATIONS
Power dissipation in the LT1766 chip comes from four
sources: switch DC loss, switch AC loss, boost circuit
current, and input quiescent current. The following formu-
las show how to calculate each of these losses. These
formulas assume continuous mode operation, so they
should not be used for calculating efficiency at light load
currents.
Switch loss:
P
RI
V
tI
V
f
SW
SW OUT
OUT
IN
EFF
OUT
IN
=
() ( )
+
()( )( )
2
12
(/ )
APPLICATIO S I FOR ATIO
WU
UU
Boost current loss:
P
VI
V
BOOST
OUT
IN
=
()
2
36
/
Quiescent current loss:
PV
V
Q
IN
OUT
=
()+
()
0 0015
0 003
..
RSW = Switch resistance (≈ 0.3) hot
tEFF = Effective switch current/voltage overlap time
= (tr + tf + tIr + tIf)
tr = (VIN/1.2)ns
tf = (VIN/1.7)ns
tIr = tIf = (IOUT/0.05)ns
f = Switch frequency
Example: with VIN = 40V, VOUT = 5V and IOUT = 1A:
Figure 8. Discontinuous Mode Ringing
1
s/DIV
VIN = 40V
VOUT = 5V
L = 47
H
10V/DIV
SWITCH NODE
VOLTAGE
INDUCTOR
CURRENT
AT IOUT = 0.1A
0.2A/DIV
1766 F08
Figure 7. Switch Node Resonance
50ns/DIV
1766 F07
2V/DIV
SW RISE
SW FALL
P
W
PW
SW
BOOST
Q
= ()( ) ( ) +
()()()( )()
=+
=
= () () =
=+
=
03 1
5
40
97 10
1 2 1 40 200 10
004 0388
043
51 36
40
002
40 0 0015
5 0 003
0 08
2
93
2
.
/
..
.
/
.
(.
)
(.
)
.
Total power dissipation in the IC is given by:
PTOT = PSW + PBOOST + PQ
= 0.43W + 0.02W + 0.08W = 0.53W
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