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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： | 18/28页 |
| 文件大小： | 284K |
| 代理商： | LT1766HGN#PBF |
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25
LT1766/LT1766-5
1766fb
IP = Maximum rated switch current
VIN = Minimum input voltage
VOUT = Output voltage
VF = Catch diode forward voltage
0.3 = Switch voltage drop at 1.5A
Example: with VIN(MIN) = 5.5V, VOUT = 12V, L = 18H,
VF = 0.63V, IP = 1.5A: IMAX = 0.280A.
OUTPUT DIVIDER
Refer to Applications Information Feedback Pin Functions
to calculate R1 and R2 for the (negative) output voltage
(from Table 1).
Output current where continuous mode is needed:
I
VI
VV
V
CONT
IN
P
IN
OUT
IN
OUT
F
>
++
+
() ( )
()(
)
22
4
Minimum inductor discontinuous mode:
L
VI
fI
MIN
OUT
P
=
2
()(
)
()( )
Minimum inductor continuous mode:
L
VV
fV
V
I
VV
V
MIN
IN
OUT
IN
OUT
P
OUT
F
IN
=
++
+
()(
)
()(
)
–
()
21
For a 40V to –12V converter using the LT1766 with peak
switch current of 1.5A and a catch diode of 0.63V:
IA
CONT >
++
+
=
() ( . )
()(
.
)
.
40
1 5
4 4012 40120 63
0 573
22
For a load current of 0.25A, this says that discontinuous
mode can be used and the minimum inductor needed is
found from:
LH
MIN ==
212 0 25
200 10
1 5
13 3
32
()( .
)
(
)( . )
.
In practice, the inductor should be increased by about
30% over the calculated minimum to handle losses and
variations in value. This suggests a minimum inductor of
18
H for this application.
Ripple Current in the Input and Output Capacitors
Positive-to-negative converters have high ripple current in
the input capacitor. For long capacitor lifetime, the RMS
value of this current must be less than the high frequency
ripple current rating of the capacitor. The following for-
mula will give an
approximate value for RMS ripple cur-
rent.
This formula assumes continuous mode and large
inductor value. Small inductors will give somewhat higher
ripple current, especially in discontinuous mode. The
exact formulas are very complex and appear in Application
Note 44, pages 29 and 30. For our purposes here a fudge
factor (ff) is used. The value for ff is about 1.2 for higher
APPLICATIO S I FOR ATIO
WU
UU
OUTPUT**
–12V, 0.25A
INPUT
5.5V TO
48V
1766 F15
C2
0.33
F
CC
RC
D1
10MQO60N
R1
44.2k
C1
100
F
25V
TANT
C3
2.2
F
100V
CER
D2
1N4148W
L1*
18
H
CF
BOOST
LT1766
VIN
VSW
FB
GND
VC
R2
4.99k
* INCREASE L1 TO 30
H OR 60H FOR HIGHER CURRENT APPLICATIONS.
SEE APPLICATIONS INFORMATION
** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE
AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
FOR V
IN > 44V AND VOUT = –12V, ADDITIONAL VOLTAGE DROP IN THE
PATH OF D2 IS REQUIRED TO ENSURE BOOST PIN MAXIMUM RATING IS
NOT EXCEEDED. SEE APPLICATIONS INFORMATION (BOOST PIN VOLTAGE)
+
Figure 15. Positive-to-Negative Converter
Inductor Value
The criteria for choosing the inductor is typically based on
ensuring that peak switch current rating is not exceeded.
This gives the lowest value of inductance that can be used,
but in some cases (lower output load currents) it may give
a value that creates unnecessarily high output ripple
voltage.
The difficulty in calculating the minimum inductor size
needed is that you must first decide whether the switcher
will be in continuous or discontinuous mode at the critical
point where switch current reaches 1.5A. The first step is
to use the following formula to calculate the load current
above which the switcher must use continuous mode. If
your load current is less than this, use the discontinuous
mode formula to calculate minimum inductor needed. If
load current is higher, use the continuous mode formula.
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