DN412 - 36V 2A Buck Regulator Integrates Power Schottky

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36V 2A Buck Regulator Integrates Power Schottky
Design Note 412
David Ng
Introduction
Everyone wants more power in less space. However, the
task of designing a power supply is easy to describe but
difficult to execute. How does a designer select an optimal
set of components that yields the best possible power
supply in terms of size, cost and performance? Well, it
is easier if the selection is reduced to only a handful of
components.
A Small, Simple Solution
The LT3681 integrates a wide input voltage range, high
performance buck controller, power switch, high side
bootstrapping boost diode and a power Schottky diode.
All of these attributes are crammed into a tiny but thermally efficient 14-pin 3mm × 4mm DFN package. So,
all a designer needs to implement a full-featured buck
converter is to add the output LC filter and a few passives.
For instance, the LT®3681 reduces parts selection to only
a few passive components by integrating all of the power
semiconductors necessary to make a buck converter
into a single package. Don’t think that this high level of
integration limits the usefulness of this part. The LT3681
accepts inputs from 3.6V to 34V, provides excellent line
and load regulation and dynamic response, and offers
a high efficiency solution over a wide load range while
keeping the output ripple low during Burst Mode ® operation. Furthermore, its frequency is adjustable from
300kHz to 2.8MHz, enabling the use of small, low cost
inductors and ceramic capacitors.
The most obvious advantage of having the power Schottky
diode integrated into the LT3681 is space savings, reducing the amount of board space required by the complete
regulator by 15% or more. Moreover, the power Schottky
diode has been optimized for the intended operation of
the LT3681, so there is no need to agonize over finding
the perfect form, fit and function diode for the application.
Figure 1 shows a schematic of the LT3681 producing 5V
at 2A from an input of 6.3V to 34V and Figure 2 shows
the efficiency for a 12V input.
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
95
VIN
OFF ON
20k
BD
RUN/SS
BOOST
VC
4.7μF
RT
VOUT
5V
2A
L
0.47μF 6.8μH
SW
LT3681
DC
DA
330pF
BIAS
PG
GND
f = 800kHz
L: TAYIO YUDEN NP06DZB6R8M
85
80
75
70
65
60
590k
60.4k
90
EFFICIENCY (%)
VIN
6.3V TO
34V
FB
55
200k
22μF
DN412 F01
50
0
0.5
1
1.5
2
LOAD CURRENT (A)
DN412 F02
Figure 1. The LT3681 Integrates All of the Power Semiconductors
Necessary to Make a Simple 2A Buck Converter
03/07/412_conv
Figure 2. The LT3681 Boasts High Efficiency
(12V Input to 5V Output)
Low Ripple and High Efficiency Solution over a
Wide Load Range
The LT3681 switching frequency can be programmed from
300kHz to 2.8MHz by using a resistor tied from the RT
pin to ground. The LT3681 offers low ripple Burst Mode
operation that maintains high efficiency at light loads while
keeping the no load output voltage ripple below 15mVP-P.
During Burst Mode operation, the LT3681 is able to deliver
current in as little as one cycle to the output capacitor
followed by sleep periods where all of the output power
is delivered to the load by the output capacitor. Between
bursts, all circuitry associated with controlling the output
switch is shut down, reducing the input supply current to
only 55μA. As the load current decreases toward no load,
the percentage of time that the LT3681 operates in sleep
mode increases and the average input current is greatly
reduced, so high efficiency is maintained.
Figure 3 shows the low ripple and single cycle burst
inductor current at no load for the 3.3V regulator shown
in Figures 1 and 2. The LT3681 has a very low shutdown
current (less than 1μA), significantly extending battery
life in applications that spend long periods in sleep or
shutdown mode.
For systems that rely on a well-regulated power source,
the LT3681 provides a power good flag that signals when
VOUT reaches 90% of the programmed output voltage.
A resistor and capacitor on the RUN/SS pin programs the
LT3681’s soft-start, reducing the inrush current during
start-up. In applications where the circuit is plugged into a
live input source through long leads, an electrolytic input
capacitor, which has higher ESR than a ceramic capacitor,
is recommended to dampen the overshoot voltage. Refer
to AN88 for further details.
Frequency Foldback Saves Chips
During short circuit, the LT3681 offers cycle-by-cycle
current limit and frequency foldback, which decreases
the switching frequency. This increases the off time,
reducing the RMS current through the power switch and
allowing the inductor current to safely discharge before
the next switching cycle begins.
Conclusion
The robust design, small package and high level of integration of the LT3681 make it an excellent choice for a
wide variety of step-down applications where a compact
footprint and component optimization are critical. The
high input voltage rating, high power switch capability
and excellent package thermal conductivity adds to its
versatility.
INDUCTOR
CURRENT
200mA/DIV
OUTPUT
RIPPLE
10mV/DIV
500μs/DIV
DN412 F03
Figure 3. This LT3681 Design Has Only 15mV of Output
Ripple, Even at No Load Under Burst Mode Operation
Data Sheet Download
http://www.linear.com
For applications help,
call (408) 432-1900, Ext. 2759
Linear Technology Corporation
dn412f_conv LT/TP 0307 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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