DN69 - Low Parts Count DC/DC Converter Circuit With 3.3V and 5V Outputs

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Low Parts Count DC/DC Converter Circuit
with 3.3V and 5V Outputs - Design Note 69
Ron Vinsant
This Design Note describes a simple low cost dual output
step-down converter circuit based on the LT®1076 five
terminal switching regulator.
Performance
Input voltages can range from 8V to 30V. The load range
on the 5V is 0.05A to 0.5A while the 3.3V load range is
0.1A to 1A. The circuit is self-protecting under no load
conditions; it will “burp” in the same fashion as many
off-line flyback power supplies.
Output voltage regulation is excellent. Over all load
and line conditions, including cross regulation, the
3.3V output varies from 3.25V to 3.27V. The 5V output
varies from 4.81V to 5.19V under the same conditions.
In a typical application of 0.5A on the 3.3V and 0.25A
on the 5V, efficiency is typically 76%. With an input
voltage of 30V and full load, the efficiency drops to
66%. In normal operating regions efficiency is always
better than 70%.
The 5V ripple is less than 75mV and the 3.3V ripple less
than 50mV over all line and load conditions.
This design can help save both parts and cost by the
elimination of a second regulator. Only a few additional
parts are required to make the second output. They
Performance Table
VIN
VOUT, OUTPUT 1 (5V)
VOUT, OUTPUT 2 (3.3V)
8V
30V
At IOUT = 0.4A
4.81
5.07
At IOUT = 1A
3.26
3.26
8V
30V
At IOUT = 0.05A
5.14
5.19
At IOUT = 1A
3.25
3.25
8V
30V
At IOUT = 0.4A
4.81
5.02
At IOUT = 0.1A
3.26
3.27
8V
30V
At IOUT = 0.05A
5.07
5.11
At IOUT = 0.1A
3.26
3.26
04/93/69_conv
are: two resistors, a Schottky diode, a small ceramic
capacitor and a filter electrolytic; only 5 additional
components! The normal single winding inductor has
one small winding added to create the additional output.
The circuit has been built in our lab and has only been
evaluated for room temperature performance. No stability analysis has been done.
Inductor
The inductor is based on a EP-13 ferrite core which is
available from a number of vendors. In our breadboard
we used a Ferroxcube core in 3C81 material gapped
to 6 mils (center gap). The 3.3V winding is 22 turns
of #25 AWG while the 5V winding is 13 turns of #28
AWG. Any magnetics vendor should be able to wind
this device. The inductor has only a 14°C temperature
rise. Coiltronics at (305) 781-8900, or Hurricane Labs
at (801) 635-2003 can supply this inductor off the shelf.
Capacitors
Ripple current in the output capacitors C2 and C3 is
250mARMS total with the input voltage at 30V and
maximum load on the two outputs.
The input capacitor (C1), which undergoes higher stress,
has a ripple current of 830mA maximum at 14V input
and maximum load.
The input and output capacitors have been chosen
primarily for ESR, not for voltage. The 50V rating of
the capacitors is not due to stress from the circuit but
from the fact that the lowest ESR for a particular can
size occurs at 50V, in this series of capacitors, from
this specific manufacturer.
The capacitors in the frequency compensation network
should be at least X7R ceramic, never Z5U, and if
broad temperature operation is expected, polyester or
polycarbonate film caps should be used.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
Layout
In order to achieve proper performance it is important
to lay out the circuit as shown in Figure 1. Use a single
point ground at the output of the converter as shown.
The term “short” indicates that the trace should be as
short as possible between the two points shown. These
traces should have a minimum width of 0.2" in 2 oz.
copper for a length of less than 1.5". Traces longer than
this should be avoided on heavier lines of the schematic.
22Ω
Heat Sinking
Any heat sink of 30°C/W or lower will keep the LT1076
at an acceptable temperature up to a 70°C ambient. See
the LT1076 data sheet for further information.
0.01μF
1N5822
SHORT
SHORT
t
14.3k
SHORT
+
VIN
VSW
LT1076
GND VC
8V
TO
30V
5V, 0.05A TO 0.4A
FB
t
L1 SHORT
86μH
1N5822
330μF
50V
SXE
330μF
50V
SXE
+
330μF
50V
SXE
+
3.3V, 0.1A to 1A
330μF
50V
SXE
1.3k
SHORT
1k
+
+
RETURN
S.P.G.
1μF
2.21k
SHORT
SHORT
–
S.P.G. SINGLE POINT GROUND, (STAR GROUND)
DARK LINES INDICATE HIGH CURRENT PATHS (SEE TEXT)
L1 = HURRICANE LABS HL8685
= COILTRONICS CTX01-11959
ALL ELECTROLYTIC CAPACITORS, UNITED CHEMICON SXE SERIES
Figure 1. 3.3V to 5V Dual Output DC/DC Converter
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn69f_conv LT/GP 0493 190K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1993
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