LINEAR TECHNOLOGY LT3506 Technical data
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Dual Monolithic Step-Down Switching Regulator Provides 1.6A
Outputs with Reduced EMI and V
OUT
Design Note 404
Hua (Walker) Bai
Introduction
Electronic devices are becoming smaller while the power
requirements are increasing to satisfy ever more functionality. To preserve power and manage heat, switching
regulators are desirable because of their high effi ciency
compared to linear regulators.
The LT3506 and the LT3506A are examples of how to
squeeze power out of a supply without overheating the
end product. These are dual 1.6A step-down monolithic
r e g u l a t o r s t h a t s i m p l i f y t h e l i v e s o f s y s t e m e n g i n e e r s . T h e y
eliminate the need for external power switches, thereby
reducing solution size and BOM cost. Their outputs can
be as low as 0.8V, satisfying the needs of the latest DSPs.
Integrated dual output channels reduce the part count,
while anti-ph ase switching of the two channels ma ximizes
effi ciency and reduces input current ripple and EMI. Both
4.7µH
CDRH4D22/HP-4R7
DFLS230L
6.8V
V
IN
TO 25V
DFLS230L
0.1µF
0.1µF
4.7µF
25V
X5R
NSR0230M2T5G
LT3506AEDHD
1
BOOST1
2
SW1
3
V
IN1
4
V
IN1
5
V
IN2
6
V
IN2
7
SW2
8
BOOST2
NSR0230M2T5G
CDRH4D22/HP-3R5
U1
GND
3.5µH
FB1
V
PG1
RUN/SS1
RUN/SS2
PG2
V
FB2
DN404 F01
52.3k
10k
16
220pF
39k
15
C1
14
13
12
11
220pF
28k
10
C2
9
10k
31.6k
100k
100k
22µF
10V
X5R
1nF 1nF
22µF
10V
X5R
the LT3506 and the LT3506A have 0.8V high accuracy
voltage references.
Typical LT3506A and LT3506 Applications
The primary difference between the LT3506A and the
LT3506 is the switching frequenc y. The LT3506A’s switching frequency is 1.1MHz while the LT3506’s is 575kHz.
Higher switching frequency allows smaller components.
For lower output voltages, i.e., less than 3.3V, the LT3506
is recommended if V
lower switching frequenc y option usually results in higher
effi ciency due to the lower switching and inductor core
losses.
The circuit in Figure 1 generates a 3.3V and a 5V output.
The overall effi ciency of the circuit (both channels) is
shown in Figure 2. The circuit shown in Figure 3 generates a 1.2V and a 1.8V from a 3.6V to 21V input. The wide
input voltage of both the LT3506 and the LT3506A (3.6V
V
OUT1
5V
to 25V) can accept a variety of power sources, from lead-
AT 1.6A
acid batteries and 5V rails to unregulated wall adapters
and distributed power supplies.
, LT, LTC, LTM are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
PG1
RUN/SS1
RUN/SS2
PG2
V
OUT2
3.3V
AT 1.6A
as Low as 0.8V
OUT/VIN(MAX)
90
6.8V INPUT
85
12V INPUT
80
EFFICIENCY (%)
75
70
0
500
CURRENT (mA)
1000
is less than 15%. The
1500
DN404 F02
Figure 1. LT3506A Application Circuit Provides
Dual Outputs of 5V at 1.6A and 3.3V at 1.6A
12/06/404
Figure 2. Overall Circuit Effi ciency
(Both Outputs Loaded Identically)
The LT3506’s high switching frequencies allow the use of
small, low profi le surface mount inductors and ceramic
capacitors, resulting in smaller solution size and lower
assembly cost. Furthermore, the low ESR of ceramic
capacitors and high switching frequency results in very
low, predictable output voltage ripple.
The internal supplies of both the LT3506 and LT3506A
are powered from V
via the V
ply tied to V
pin from a different source, or it can be sim-
IN2
OUT1
current. The effi ciency of V
supplying V
Cascading V
IN2
IN2
cycle applications for V
pins. It is possible to supply power
IN1
, provided V
can supply suffi cient
OUT1
can be improved when
OUT2
from a separate, lower supply voltage.
to V
also enables some low duty
OUT1
.
OUT2
The thermally enhanced 16-lead DFN or TSSOP packages have an exposed ground pad on the bottom. This
ground must be soldered to a ground pad on the PCB.
Adding a dozen thermal vias to this pad improves thermal
performance. A higher temperature grade part, I-grade,
is also available.
4.7µH
TOKO
V
DFLS230L
3.6V
V
IN
TO 21V
DFLS230L
V
IN
IN
NSR0230M2T5G
0.1µF
4.7µF
25V
X5R
0.1µF
NSR0230M2T5G
1
BOOST1
2
SW1
3
V
4
V
5
V
6
V
7
SW2
8
BOOST2
D53LC, TYPE A
D53LC, TYPE A
U1
LT3506EDHD
IN1
RUN/SS1
IN1
RUN/SS2
IN2
IN2
GND
3.3µH
TOKO
FB1
V
C1
PG1
PG2
V
C2
FB2
DN404 F03
12.4k
10k
16
470pF
15
14
13
12
11
330pF
10
9
4.99k
20k
24.3k
10k
100k
100k
V
1.8V
22µF
AT 1.6A
6.3V
X5R
PG1
RUN/SS1
RUN/SS2
PG2
1nF 1nF
V
1.2V
47µF
AT 1.6A
6.3V
X5R
OUT1
OUT2
Power Sequencing without Adding Components
Supply sequencing is critical in many systems in order
to prevent possible latch-up and to improve system reliability. Independent PG (Power Good) indic ators, RUN/SS
pins and V
pins of the LT3506 and the LT3506A simplify
C
supply sequencing. The PG pin remains low until the FB
pin is within 10% of the fi nal regulated output voltage.
The easiest way to sequence outputs is to tie the PG1 pin
to the V
This enables V
pin. Then remove the pull-up resistor on PG1.
C2
to come up before V
OUT1
OUT2
as shown
in Figure 4.
2-Phase Switching Eases EMI Concerns
A step-down switching regulator draws current from
its input supply and its input capacitor, resulting in a
large AC current that can generate EMI. The LT3506’s
two regulators are synchronized to a single oscillator
and switch out of phase by 180 degrees. Compared with
two completely independent regulators, the input current ripple of the LT3506 is substantially reduced and
its effective frequency doubled, thereby lowering EMI
and allowing the use of a smaller input capacitor without
reducing effi ciency.
Conclusion
Both the LT3506 and LT3506A step down the output
voltage to as low as 0.8V and provide 1.6A of current
capability per channel. 2-channel anti-phase switching
substantially reduces the input current ripple and eases
EMI concerns. The PG, V
, and RUN/SS pins make supply
C
sequencing simple and straightfor ward. Two pack age options (leadless and leaded) and two switching frequency
options allow optimal solutions for most applications.
V
IN
5V/DIV
V
OUT1
2V/DIV
V
OUT2
2V/DIV
V
C2
2V/DIV
20μs/DIV
DN404 F04
Figure 3. LT3506 Application Circuit Provides
Dual Outputs of 1.8V at 1.6A and 1.2V at 1.6A
Data Sheet Download
www.linear.com
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
Figure 4. Start-Up Waveforms of the Circuit
in Figure 1 with Power Sequencing
For applications help,
call (408) 432-1900, Ext. 2759
dn404f LT/TP 1206 305K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2006
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