DN488 - Step-Down uModule Regulator Produces 15A Output from Inputs Down to 1.5V

Step-Down µModule Regulator Produces 15A Output from
Inputs Down to 1.5V—No Bias Supply Required
1.5V to 5.5V Input, 0.8V to 5V Output from a 15mm × 15mm × 4.32mm LGA Package
Design Note 488
Alan Chern and Jason Sekanina
15A High Efficiency Output from a Low Input Voltage
The LTM®4611 is a switch mode, step-down DC/DC µModule ®
regulator in a compact 15mm × 15mm × 4.32mm LGA surface
mount package. The switching controller, MOSFETs, inductor
and supporting components are housed in the package. With
a built-in differential remote sense amplifier, the LTM4611 can
tightly regulate its output voltage from 0.8V to within 300mV of
VIN and deliver 15A output efficiently from 1.5V to 5.5V input.
VIN
1.8V TO
5.5V
CSS
0.1µF
680µF
18mΩ ESR
2× 47µF
RUN
SGND
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03/11/488
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GND
VOSNS+
VOSNS–
RSET
69kΩ
Figure 1. 1.8VIN to 5.5VIN to 1.5VOUT
with 15A Output Load Current
95
90
EFFICIENCY (%)
85
80
75
70
1.8V TO 1.5V
2.5V TO 1.5V
3.3V TO 1.5V
5V TO 1.5V
65
60
0
2
10 12
8
4
6
OUTPUT CURRENT (A)
VOUT
1.5V
15A
100µF
×4
DIFFVOUT
*OPTIONAL
55
m.linear.com/4611
VFB
LTM4611
VOUT_LCL
DN488 F01
Input and Output Ripple
Scan this code with your
smart phone or click to view
informative videos.
CFF
47pF
CP *
Efficiency is exceptional, even down to the lowest input voltages, as shown in Figure 2.
L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered
trademarks and LTPowerCAD is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
PGOOD
VOUT
MODE_PLLIN
Only a handful of components are needed to create a complete
point-of-load (POL) solution with the LTM4611 (see Figure 1).
The CSS capacitor provides smooth start-up on the output
and limits the input surge current during power-up. CFF and
CP set the loop-compensation for fast transient response
and good stability. The output voltage, 1.5V, is set by a single
resistor, RSET.
Output capacitors should have low ESR to meet output voltage ripple and transient requirements. A mixture of low ESR
polymer and/or ceramic capacitors is sufficient for producing low output ripple with minimal noise and spiking. Output
capacitors are chosen to optimize transient load response and
loop stability to meet the application load-step requirements by
using the Excel-based LTpowerCAD™ design tool. (Table 5 of the
LTM4611 data sheet provides guidance for applications with 7.5A
VIN
TRACK/SS
14
16
DN488 F02
Figure 2. Efficiency of
Figure 1 Circuit
video.linear.com/56
SWITCH
50mV/DIV
VIN
RIPPLE
50mV/DIV
VOUT
RIPPLE
20mV/DIV
1µs/DIV
DN488 F03
dissipation with or without air flow. Figure 5 shows the top
view thermal imaging of the LTM4611 at a power loss of 3.5W
with no air flow, when converting 5V to 1.5V.
Internal self-heating of the LTM4611 remains quite low even at a
low 1.8V input voltage due to its micropower bias generator that
enables strong gate drive for its power MOSFETs. Figure 6 shows
a power loss of 3.2W with hot spots slightly changed from their
positions with a 5V input—the nominal surface temperature is
60°C. Watch the associated videos to see the test set-up and
watch 200 LFM of air flow cool the unit by 10°C.
Figure 3. 5VIN to 1.5VOUT at 15A Output Load
SWITCH
20mV/DIV
VIN
RIPPLE
50mV/DIV
VOUT
RIPPLE
20mV/DIV
1µs/DIV
DN488 F04
Figure 4. 1.8VIN to 1.5VOUT
at 15A Output Load
Figure 5. 5VIN to 1.5VOUT at 15A Output Load.
3.5W Power Loss with 0LFM and 65°C Surface
Temperature Hot Spot
video.linear.com/57
load-steps and 1µs transition times.) For this design example,
four 100µF ceramic capacitors are used. Figures 3 and 4 show
input and output ripple at 15A load with 20MHz bandwidth-limit.
View the associated videos to see the test methodology, as well
as ripple waveforms without bandwidth limiting.
For this design, the choice of input capacitors is critical due
to the low input voltage range. Long input traces can cause
voltage drops, which could nuisance-trip the µModule regulator’s undervoltage lockout (UVLO) detection circuitry. Input
ripple, typically a non-issue with higher input voltages, may fall
a significant percentage below nominal—close to UVLO—at
lower input voltages. In this case, input voltage ripple should
be addressed since input filter oscillations can occur due to
poor damping under heavy load current. This design uses a
large 680µF POSCAP and two 47µF ceramic capacitors to compensate for meter-long input cables used during bench testing.
Thermally Enhanced Packaging
The device’s LGA packaging allows heat sinking from both
the top and bottom, facilitating the use of a metal chassis or
a BGA heat sink. This form factor promotes excellent thermal
Data Sheet Download
www.linear.com/4611
Linear Technology Corporation
Figure 6. 1.8VIN to 1.5VOUT at 15A
Output Load. 3.2W Power Loss
with 0LFM and 65°C Surface
Temperature Hot Spot
video.linear.com/55
Conclusion
The LTM4611 is a step-down µModule regulator that easily
fits into POL applications needing high output current from
low voltage inputs—from 1.5V to 5.5V. Efficiency and thermal
performance remain high across the entire input voltage range,
simplifying electrical, mechanical and system design in data
storage, RAID, ATCA, and many other applications.
For applications help,
call (408) 432-1900, Ext. 2593
dn488f LT/AP 0311 226K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2011
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