DN404 - Dual Monolithic Step-Down Switching Regulator Provides 1.6A Outputs with Reduced EMI and VOUT as Low as 0.8V

advertisement
Dual Monolithic Step-Down Switching Regulator Provides 1.6A
Outputs with Reduced EMI and VOUT as Low as 0.8V
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 efficiency
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
regulators that simplify the lives of system engineers. They
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-phase switching of the two channels maximizes
efficiency and reduces input current ripple and EMI. Both
4.7µH
CDRH4D22/HP-4R7
NSR0230M2T5G
22µF
10V
X5R
52.3k
10k
U1
LT3506AEDHD
1
DFLS230L
2
3
4
4.7µF
25V
X5R
5
6
DFLS230L
7
8
FB1
BOOST1
SW1
VC1
VIN1
PG1
RUN/SS1
VIN1
VIN2
RUN/SS2
VIN2
PG2
SW2
VC2
BOOST2
FB2
16
220pF
15
NSR0230M2T5G
The circuit in Figure 1 generates a 3.3V and a 5V output.
The overall efficiency 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
to 25V) can accept a variety of power sources, from leadacid 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.
100k
39k
14
PG1
13
RUN/SS1
90
12
RUN/SS2
6.8V INPUT
11
10
220pF
PG2
28k
1nF
1nF
9
GND
0.1µF
Typical LT3506A and LT3506 Applications
The primary difference between the LT3506A and the
LT3506 is the switching frequency. 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 VOUT/VIN(MAX) is less than 15%. The
lower switching frequency option usually results in higher
efficiency due to the lower switching and inductor core
losses.
100k
DN404 F01
85
EFFICIENCY (%)
0.1µF
VIN 6.8V
TO 25V
VOUT1
5V
AT 1.6A
the LT3506 and the LT3506A have 0.8V high accuracy
voltage references.
12V INPUT
80
10k
75
3.5µH
CDRH4D22/HP-3R5
31.6k
22µF
10V
X5R
VOUT2
3.3V
AT 1.6A
70
0
500
1000
1500
CURRENT (mA)
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 Efficiency
(Both Outputs Loaded Identically)
The LT3506’s high switching frequencies allow the use of
small, low profile 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 VIN1 pins. It is possible to supply power
via the VIN2 pin from a different source, or it can be simply tied to VOUT1, provided VOUT1 can supply sufficient
current. The efficiency of VOUT2 can be improved when
supplying VIN2 from a separate, lower supply voltage.
Cascading VIN2 to VOUT1 also enables some low duty
cycle applications for VOUT2.
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
D53LC, TYPE A
NSR0230M2T5G
12.4k
VIN
10k
VOUT1
1.8V
AT 1.6A
U1
LT3506EDHD
0.1µF
1
DFLS230L
2
3
4
VIN 3.6V
TO 21V
22µF
6.3V
X5R
4.7µF
25V
X5R
5
6
DFLS230L
7
8
FB1
BOOST1
SW1
VC1
VIN1
PG1
VIN1
RUN/SS1
RUN/SS2
VIN2
VIN2
PG2
SW2
VC2
BOOST2
FB2
16
100k
470pF
20k
15
14
PG1
13
RUN/SS1
12
RUN/SS2
11
10
330pF
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) indicators, RUN/SS
pins and VC pins of the LT3506 and the LT3506A simplify
supply sequencing. The PG pin remains low until the FB
pin is within 10% of the final regulated output voltage.
The easiest way to sequence outputs is to tie the PG1 pin
to the VC2 pin. Then remove the pull-up resistor on PG1.
This enables VOUT1 to come up before VOUT2 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 efficiency.
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, VC, and RUN/SS pins make supply
sequencing simple and straightforward. Two package options (leadless and leaded) and two switching frequency
options allow optimal solutions for most applications.
PG2
24.3k
1nF
1nF
VIN
5V/DIV
9
GND
0.1µF
VOUT1
2V/DIV
100k
DN404 F03
10k
VIN
NSR0230M2T5G
3.3µH
TOKO
D53LC, TYPE A
VOUT2
2V/DIV
4.99k
47µF
6.3V
X5R
VOUT2
1.2V
AT 1.6A
VC2
2V/DIV
20μs/DIV
Figure 3. LT3506 Application Circuit Provides
Dual Outputs of 1.8V at 1.6A and 1.2V at 1.6A
DN404 F04
Figure 4. Start-Up Waveforms of the Circuit
in Figure 1 with Power Sequencing
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2759
www.linear.com
Linear Technology Corporation
dn404f LT/TP 1206 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
Similar pages