DN1038 - 42V High Power Density Buck Regulators in a Tiny QFN Package

42V High Power Density Buck Regulators in a Tiny QFN
Package
Design Note 1038
Ying Cheng
Introduction
Power dissipation is a significant problem facing the
designers of DC/DC converters in industrial and automotive applications, where high currents are required,
but space is limited. It is possible to produce a highly
efficient regulator from high performance discrete
components, but expense and solution footprint make
this approach prohibitive. The LT8612/LT8613 high
efficiency buck regulators integrate all the necessary
components into a single IC, significantly reducing
the DC/DC converter size, even when faced with high
step-down ratios. These devices can also be paralleled
to increase the output current capability and spread
the load and heat.
high efficiency power switches allows these switches,
the boost diode, internal compensation and all necessary circuitry to be encapsulated in a tiny 3mm × 6mm
QFN package without overheating. Figure 1 shows a
typical 5V/30W LT8612 converter; its efficiency and
power loss are demonstrated in Figure 2. Even with
an input voltage of 24V, the LT8612 can produce a
30W output with no more than 50°C temperature rise.
At the other end of the load spectrum, the LT8612/
LT8613 uses low ripple Burst Mode ® operation to
enhance efficiency in ultralow load situations.
The minimum on-times of LT8612/LT8613 are as low
as 40ns, allowing for high VIN/VOUT ratios even at high
switching frequencies. High switching frequencies,
up to 2.2MHz, minimize the size and value of the
power inductor and output capacitor. Furthermore,
the inductor can be confidently sized based on the
output load requirement—no need to use a larger
inductor to provide design overhead—a result of the
devices high speed peak-current mode architecture
and robust switch design.
Other desirable features included in these regulators
are ultralow quiescent current to maximize battery
life, and high switching frequency to minimize solution size and avoid noise-sensitive frequency bands.
High Efficient 42V, 6A Regulators in
3mm × 6mm Package
The LT8612/LT8613 is 42V, 6A step-down monolithic
regulator. The low power dissipation of the integrated
VIN
5.8V TO 42V
VIN
10µF
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are
registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
BST
EN/UV
PG
0.1µF
1µF
LT8612
0.1µF
3.3µH
SW
SYNC
BIAS
TR/SS
FB
VOUT
5V
100µF 6A
1210
1M
10pF
INTVCC
RT PGND GND
60.4k
fSW = 700kHz
243k
dn1038 F01
Figure 1. A 5V/30W Step-Down Converter Using the LT8612
12/15/1038
100
VOUT = 5V
fSW = 700kHz
95
7
6
EFFICIENCY
85
5
80
4
75
3
70
60
2
POWER LOSS
65
0
4
3
2
LOAD CURRENT (A)
1
POWER LOSS (W)
EFFICIENCY (%)
90
high current conditions, a multiphase design can
extend the output capabilities of the LT8612/LT8613
regulator. The LT8613 has a built-in rail-to-rail current sense amplifier with monitor and control pins,
enabling accurate input or output average current
regulation. This current loop modulates the internal
switch current limit such that the voltage across the
ISP/ISN pins does not exceed the voltage set by the
ICTRL pin. Another IMON pin is used to monitor the
average current that is measured through the ISP/
ISN pins. This current control feature makes accurate
current sharing among several LT8613s possible
without any additional control circuitry.
8
VIN = 12V 1
VIN = 24V
0
5
6
dn1038 F02
Figure 2. Efficiency and Power Loss of the 5V/30W
LT8612 Step-Down Converter in Figure 1.
Figure 3 shows three LT8613s paralleled for up to
16A output. The top LT8613 is the master regulator
with its output voltage set to 4V, while the other two
LT8613s are slave regulators with their output voltages set a little higher than 4V.
Multiphase Design to Increase Output Current
Capability
Loads more than 6A are not uncommon in automotive and industrial applications. For these relatively
VIN
12V TO 40V
LT8613 BST
MASTER
EN/UV
VIN
10µF
SW
ISP
ISN
BIAS
SYNC
IMON
ICTRL
0.1µF
0.1µF
0.1µF
PH1
V+A
PH2
95.3k
OUT1
SET
OUT2
MOD
OUT3
RT
LT8613
SLAVE
1µF
RT
LT8613
SLAVE
SYNC
IMON
ICTRL
1µF
66.5k
100µF
1M
FB
GND PGND
EN/UV
0.1µF
9mΩ
10pF
VIN
10µF
0.1µF
3.3µH
TR/SS
INTVCC
66.5k
BST
324k
SW
ISP
ISN
BIAS
SYNC
IMON
ICTRL
1M
FB
GND PGND
EN/UV
0.1µF
100µF
10pF
VIN
10µF
9mΩ
TR/SS
INTVCC
66.5k
LTC6909
PH0
V+D
GND
1µF
0.1µF
3.3µH
BST
SW
ISP
ISN
BIAS
316k
0.1µF
3.3µH
9mΩ
100µF
TR/SS
10pF
INTVCC
RT
1M
FB
GND PGND
316k
dn1038 F03
Figure 3. Schematics of 3-Phase LT8613 Design Delivering 16A at 4VOUT
VOUT
4V
16A
95
90
EFFICIENCY (%)
In order to share current among the three LT8613
regulators, the IMON pin of the master LT8613 is
connected to the ICTRL pins of the slave LT8613s.
The three LT8613 regulators are synchronized by
the 3-phase 700kHz clock signals (120° phase shift)
generated from the LTC6909 oscillator. Figure 4 shows
a thermal image of this 3-phase LT8613 design. The
similarity in temperatures of the LT8613s indicates
even current sharing among the three phases. The
efficiency of this design is shown in Figure 5.
85
80
75
VIN = 12V
VOUT = 4V
fSW = 700kHz
70
65
0
2
4
8
10 12
6
LOAD CURRENT (A)
14
16
dn1038 F05
Figure 5. Typical Efficiency of 3-Phase LT8613
Design Shown in Figure 3
If it is desirable to weight the current sharing among
the phases—that is to unbalance the current sharing—simply adjust the values of the sense resistors
across ISP and ISN pins.
Figure 4. Thermal Image of 3-Phase LT8613 Design
Shows Similar Temperatures
Data Sheet Download
www.linear.com/LT8612
Linear Technology Corporation
Conclusion
The LT8612 and LT8613 are fully integrated high power
density monolithic step-down regulators that satisfy
challenging automotive and industrial requirements.
They can be easily paralleled for efficient, high current applications and compact solution footprints.
For applications help,
call (408) 432-1900, Ext. 3206
DN1038 LT 1215 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2015
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