LINEAR TECHNOLOGY LTM4611 Technical data

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
tightl y regulate its ou tput voltage fr om 0.8V to within 300 mV of
VIN and deliver 15A output efficiently from 1.5V to 5.5V input.

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, R

SET

.

Efficiency is exceptional, even down to the lowest input volt­ages, as shown in Figure 2.

®

Input and Output Ripple

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 st ability to mee t the applicat ion load-step r equirements

by
using the E xcel-based LTpowerCAD™ des ign tool. (Table 5 of the
LTM4611 data sheet prov ides guidance for a pplications wi th 7.5 A

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.

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V

IN

1.8V TO

5.5V

680µF

18mΩ ESR

2× 47µF

95

90

85

80

75

70

EFFICIENCY (%)

65

60

55

C

SS

0.1µF

TRACK/SS

RUN

MODE_PLLIN

Figure 1. 1.8V

V

IN

LTM4611

V

DIFFV

SGND GND

to 5.5VIN to 1.5V

IN

PGOOD

V

OUT

V

OUT_LCL

OUT

V

OSNS

V

OSNS

FB

+

–

CP*

with 15A Output Load Current

1.8V TO 1.5V

2.5V TO 1.5V

3.3V TO 1.5V
5V TO 1.5V

10 12 16

0

2 64

8 14

OUTPUT CURRENT (A)

Figure 2. Efficiency of
Figure 1 Circuit

video.linear.com/56

DN488 F01

*OPTIONAL

OUT

DN488 F02

C
47pF

R

SET

69kΩ

V

OUT

1.5V

FF

15A

100µF
×4

03/11/488

SWITCH

V

RIPPLE

V

OUT

RIPPLE

dissipation with or without air flow. Figure 5 shows the top
view thermal imaging of the LTM4611 at a power loss of 3.5W

50mV/DIV

with no air flow, when converting 5V to 1.5V.

Internal self-heating of the LTM4611 remains quite low even at a

IN

50mV/DIV

low 1.8V input voltage due to its micropower bias generator that
enables s trong gate driv e for its power MOSFE Ts. Figure 6 show s

20mV/DIV

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

1µs/DIV

DN488 F03

watch 200 LFM of air flow cool the unit by 10°C.

Figure 3. 5VIN to 1.5V

SWITCH

V

IN

RIPPLE

V

OUT

RIPPLE

Figure 4. 1.8V

1µs/DIV

to 1.5V

IN

at 15A Output Load

OUT

DN488 F04

OUT

20mV/DIV

50mV/DIV

20mV/DIV

at 15A Output Load

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 out put ripple at 15A load with 2 0MHz 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, t ypically a non -issue with higher in put voltages, m ay 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 6 80µF POSCAP an d two 47µF ceramic cap acitors to com­pensat e for meter-long input cables u sed during bench tes ting.

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

Figure 5. 5VIN to 1.5V

at 15A Output Load.

OUT

3.5W Power Loss with 0LFM and 65°C Surface
Temperature Hot Spot

Figure 6. 1.8VIN to 1.5V

OUT

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
perf ormance remain hi gh across the entir e input voltage r ange,
simplifying electrical, mechanical and system design in data
storage, RAID, ATCA, and many other applications.

Data Sheet Download

www.linear.com/4611

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dn488f LT/AP 0311 226K • PRINTED IN THE USA

 LINEAR TECHNOLOGY CORPORATION 2011