Datasheet

13
LT1533
APPLICATIONS INFORMATION
WUU
U
Output ripple current at maximum input (DC = 34.7%) is:
∆I
H kHz
mA
OUT
=
−
()
=
12 1 2 34 7
800 50
92
•.%
•µ
The maximum inductor current is:
ImA
mA
mA
LMAX
=+=150
92
2
196
Primary inductance should be greater than:
L
H
H
PRI
==
5 800
36
309
2
•
.
µ
µ
The magnetizing ripple current is approximately:
∆I
H kHz
mA
MAG
=
+
=
12 0 5
3 6 309 50
225
.
.• •µ
Peak switch current is:
I
SW(PEAK)
= 3.6 • 196mA + 225mA = 930mA
which is less than the 1A maximum switch current.
Note that you can discern your magnetizing ripple by
looking at the reflected inductance ripple and subtracting
the switch current ripple.
∆I
MAG
= N • ∆I
L
– ∆I
SW
With knowledge of turns ratio and primary inductance
along with volt/sec requirements (to prevent saturation)
the transformer can be designed.
Transformers are available from Coiltronics for some
standard applications. Figure 5 lists them. Variations are
available from Coiltronics at 561-241-7876. Also, see
Linear Technology’s Application Notes AN19, AN44 and
AN70 for further information about magnetics.
Capacitors
Correct choice of input and output capacitors can be very
important to low noise switcher performance. Push-pull
topologies and other low noise topologies will in general
have continuous currents which reduce the requirements
Figure 5. Transformers for Typical Applications
for capacitance. However, noise depends more on the ESR
of the capacitors.
Input capacitors must also withstand surges that occur
during the switching of some types of loads. Some solid
tantalum capacitors can fail under these surge conditions.
Design Note 95 offers more information but the following
is a brief summary of capacitor types and attributes.
Aluminum Electrolytic: Low cost and higher voltage but in
general don’t use with this part because of high ESR and
poor high frequency performance.
Specialty Polymer Aluminum: Panasonic has come out
with their series CD capacitors. While they are only avail-
able for voltages below 16V, they have very low ESR and
good surge capability.
Solid Tantalum: Small size and low impedance. Typically
available for voltages below 50V. Possible problem with
surge currents (AVX TPS line addresses this issue).
OS-CON: Lower impedance than aluminum but only avail-
able for 25V or less. Form factor may be a problem.
Sometimes their very low ESR can cause loop stability
problems.
2
3
PRIMARY A
PRIMARY B
SECTION A
SECTION B
= TIED TOGETHER
 *=HIGH TURNS RATIO VERSION OF CTX-02-13716-X1.
 ACCOMMODATES LOW SUPPLY VOLTAGES OR
 HIGH DROPOUT REGULATORS
1533 F05
12
10
4
5
9
7
A
2
3
PRIMARY A
PRIMARY B
SECTION A
TIE OUTPUT
COMMON TO
THIS POINT
SECTION B
12
10
4
5
9
7
B
NOMINAL
INPUT
VOLTAGE
5V
5V
5V
5V
5V
5V

OUTPUT
POWER
1.5W
3.0W
1.5W
3.0W
1.5W
10W

CONNECTION
DIAGRAM
A
A
B
B
A
A
COILTRONICS
PART
NUMBER
CTX02-13716-X1
CTX02-13665-X1
CTX02-13713-X1
CTX02-13664-X1
CTX02-13834-X3*
CTX02-13949-X1
NOMINAL
OUTPUT VOLTAGE
AFTER LINEAR
REGULATOR
12V
12V
±15V
±15V
12V
12V
