DN387 - Cascadable 7A Point-of-Load Monolithic Buck Converter

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Cascadable 7A Point-of-Load Monolithic Buck Converter
Design Note 387
Peter Guan
Introduction
Easy-to-use and compact point-of-load power supplies
are necessary in systems with widely distributed, high
current, low voltage loads. The LTC® 3415 provides a
compact, simple and versatile solution. It includes a pair
of integrated complementary power MOSFETs (32mΩ
top and 25mΩ bottom) and requires no external sense
resistor. A complete design requires an inductor and
input/output capacitors, and that’s it. The result is a fast,
constant frequency, 7A current mode DC/DC switching
regulator.
Features
The overall solution is extremely compact since the
LTC3415’s 5mm × 7mm QFN package footprint is small
and its high operating frequency of 1.5MHz allows the
use of small low-profile surface mount inductors and
ceramic capacitors. For loads higher than 7A, multiple
LTC3415s can be cascaded to share the load while running mutually anti-phase, which reduces overall ripple
at both the input and the output.
Other features include:
• Spread spectrum operation to reduce system noise
• Output tracking for controlled VOUT ramp-up and
ramp-down
• Output margining for easy system stress testing
Operation
The LTC3415 offers several operating modes to optimize
efficiency and noise reduction: Burst Mode operation,
pulse-skipping mode or forced continuous mode. The
mode is set by tying the Mode pin to SVIN, SVIN/2 or SGND,
respectively. Burst Mode operation offers high efficiency
at light load by shutting off the internal power MOSFETs
as well as most of the internal circuitry between pulses.
Forced continuous mode maintains a constant switching
frequency throughout the entire load range, making it
easier to filter switching noise for sensitive applications.
Pulse-skipping mode allows constant frequency operation
until the inductor current reaches zero, at which point it
goes into discontinuous operation and finally it will skip
cycles. Pulse-skipping mode offers low output voltage
ripple while offering efficiency levels between Burst Mode
operation and forced continuous mode.
, LTC, LT and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
0.1µF
47µF
6.3V
3x
SGND
38
1
2
3
4
10pF
5
6
®
• Burst Mode operation to lower quiescent current and
boost efficiency during light loads
7
8
9
• Low shutdown current of less than 1µA
• 100% duty-cycle for low drop out operation
• Phase-lock-loop to allow frequency synchronization of
±50% of nominal frequency
1Ω
VIN
MODE
10
11
12
37
CLKOUT RUN
NC
35
PVIN
34
SVIN
33
ITHM
32
ITH
NC
SGND
TRACK
PLLLPF
VFB
PVIN
PVIN
PVIN
PVIN
SW
SW
LTC3415EUHF
5mm × 7mm QFN
SW
SW
SW
SW
SW
SW
MODE
PGND (39)
PGOOD
CLKIN
BSEL
PHMODE
MGN
PGND PGND PGND PGND PGND PGND PGND
13
14
0.2µH
• Internal or external ITH compensation for ease of use
or loop optimization, respectively
36
PVIN
15
16
17
18
31
15k
30
SVIN
29
28
27
26
22pF
30.5k
25
24
23
22
10k
21
20
19
100µF, 6.3V
2x
DN387 F01
Figure 1. 3.3V to 1.8V/7A Application
04/06/387
SVIN
VOUT
1.8V/7A
Figure 1 shows an application of the LTC3415 in a 3.3V
to 1.8V/7A step-down converter configuration. Figure 2
shows its efficiency and power loss vs load current in
Burst Mode operation. Efficiency reaches as high as 92%.
Figure 3 shows its fast transient response to a 5A load
step. As shown, VOUT recovers in 10µs with a dip of less
than 100mV. Frequency can be changed easily from its
nominal 1.5MHz to 1MHz or 2MHz by simply strapping
the PLLLPF pin to SGND or SVIN, respectively. Or if a
particular frequency is desired, an external clock can
be used to synchronize the operating frequency from
750KHz to 2.25MHz with the internal phase-lock-loop.
Spread spectrum operation is available for EMI-sensitive
applications by tying the CLKIN pin to SVIN.
For applications that require controlled output voltage
tracking between various outputs in order to prevent
excessive current draw or even latch-up during turn-on
and turn-off, the LTC3415 has a Track pin that allows
the user to program how its output voltage ramps dur95
Greater than 7A Outputs
By stacking multiple LTC3415s together, more output
power is attained without increasing the number of input
and output capacitors. Operating multiple LTC3415s out
of phase not only allows accurate current sharing, but it
also reduces the overall voltage ripple at both the input
and the output, thus allowing fewer capacitors. Figure
5 shows an efficiency curve of the LTC3415 in 1-phase,
2-phase, 3-phase, 4-phase and 6-phase operation.
Conclusion
With its many operational features and compact total
solution size, the LTC3415 is an ideal fit for today’s pointof-load power supplies. It allows for accurate, compact,
efficient and scalable power supplies with advanced
features, including tracking and margining.
10000
EFFICIENCY
90
85
VOUT2 =
3.3V/7A
500mV/DIV
1000
80
75
100
70
65
POWER LOSS
POWER LOSS (mW)
EFFICIENCY (%)
ing start-up and shutdown. Figure 4 shows the output
waveforms of two LTC3415s in track mode.
VOUT1 =
1.8V/14A
500mV/DIV
10
60
VIN = 3.3V
VOUT = 1.8V
BURST MODE
55
50
10
100
1000
LOAD CURRENT (mA)
DN387 F04
500µs/DIV
1
10000
Figure 4. Output Tracking of Two LTC3415s
DN387 F02
Figure 2. Efficiency and Power Loss of
3.3V to 1.8V/7A Application in Figure 1
100
95
IINDUCTOR
5A/DIV
EFFICIENCY (%)
90
VOUT = 1.8V
100mV/DIV
AC COUPLED
85
80
1*
2* 3* 4* 6*
75
70
65
60
40µs/DIV
VIN = 3.3V
L = 0.2µH
COUT = 2 × 100µF
DN387 F03
IOUT STEP
0A TO 5A
5A/DIV
Figure 3. VOUT Transient Response to a 0A to 5A
Load Step of the Circuit Shown in Figure 1
55
50
* PHASE OPERATION
1
10
LOAD CURRENT (A)
100
DN387 G05
Figure 5. Efficiency vs Load Current of
LTC3415s in Multiphase Operation
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2759
www.linear.com
Linear Technology Corporation
dn387f LT/TP 0406 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
●
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