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参数资料
| 型号： | LT1956-5EGN |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 570 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 0.150 INCH, PLASTIC, SSOP-16 |
| 文件页数： | 15/24页 |
| 文件大小： | 437K |
| 代理商： | LT1956-5EGN |
22
LT1956/LT1956-5
APPLICATIO S I FOR ATIO
WU
UU
OUTPUT**
–12V, 0.25A
VIN
12V
1956 F14
C2
0.1
F
CC
RC
D1
10MQO60N
R1
44.2k
C1
100
F
16V TANT
C3
2.2
F
50V
D2
FMMD914
L1*
7
H
BOOST
LT1956CGN
VIN
VSW
FB
GND
VC
R2
4.99k
* INCREASE L1 TO 10
H OR 18H FOR HIGHER CURRENT APPLICATIONS.
SEE APPLICATIONS INFORMATION
** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE
AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
+
Figure 14. Positive-to-Negative Converter
INDUCTOR SIZE (
H)
6
0
OUTPUT
RIPPLE
VOLTAGE
(mV
P-P
)
30
90
120
150
18
270
1956 F15
60
12
9
21
15
24
180
210
240
12V TO –12V CONVERTER
OUTPUT CAPACITOR ESR = 0.1
ILOAD = 0.25A
ILOAD = 0.1A
Figure 15. Ripple Voltage on Positive-to-Negative Converter
regulated negative output. A resistor divider to the FB pin,
then provides the proper feedback voltage for the chip.
Inverting regulators differ from buck regulators in the
basic switching network. Current is delivered to the output
as
square waves with a peak-to-peak amplitude much
g
reater than load current. This means that maximum load
current will be significantly less than the LT1956’s 1.5A
maximum switch current, even with large inductor val-
ues. The buck converter in comparison, delivers current
to the output as a triangular wave superimposed on a DC
level equal to load current, and load current can approach
1.5A with large inductors. Output ripple voltage for the
positive- to-negative converter will be much higher than
a buck converter. Ripple current in the output capacitor
will also be much higher. The following equation can be
used to calculate maximum load current for the positive-
to-negative converter:
I
VV
f L
VV
MAX
P
IN
OUT
IN
OUT
IN
OUT
IN
OUT
F
=
+
++
–
()(
)
()( )( )
()(
– . )
(– . )(
)
2
03
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 = 0.37A.
OUTPUT DIVIDER
The resistor connected to VOUT (R2) should be set to
approximately 5k. R1 is calculated from:
R
RVOUT
1
21 22
122
=
(– .
)
.
INDUCTOR VALUE
Unlike buck converters, positive-to-negative converters
cannot use large inductor values to reduce output ripple
voltage. At 500kHz, values larger than 22
H make almost
no change in output ripple. The graph in Figure 15 shows
peak-to-peak output ripple voltage for a 12V to – 12V
converter versus inductor value. The criteria for choosing
the inductor is therefore 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. A com-
promise value is often chosen that reduces output ripple.
As you can see from the graph,
large inductors will not
give arbitrarily low ripple, but
small inductors can give
high ripple.
The difficulty in calculating the minimum inductor size
needed is that you must first know whether the switcher
will be in continuous or discontinuous mode at the critical
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