24V, 15A Step-Down Regulator in Sub-1mm Height Package Pushes Monolithic Performance Limits

24V, 15A Step-Down Regulator in Sub-1mm Height Package
Pushes Monolithic Performance Limits
Stephanie Dai
Monolithic switching regulators and switching controllers together dominate the DC/DC
converter market. Generally, there is little overlap in their respective applications.
Controller-based solutions are favored for high performance, higher power applications
where minimal power loss and top thermal performance are priorities. In contrast,
monolithic regulators are favored in lower power applications where compact size
is the main requirement. Controllers typically offer more features than monolithic
solutions, but are at a significant solution-size disadvantage. The light footprint of
monolithic regulators usually comes at a cost of features and increased power loss,
and their reliance on integrated MOSFETs places practical limits on power.
The LTC3613 monolithic regulator
blurs the line drawn between applications for controllers and monolithic
regulators by combining a high performance fully featured controller
with onboard low RDS(ON) MOSFETs.
FEATURES
The LTC3613 can accept an input voltage between 4.5V to 24V and supports
output voltages between 0.6V to 5.5V. The
onboard top and bottom MOSFETs feature low RDS(ON), around 7mΩ and 5mΩ,
respectively, keeping power dissipation
low and allowing the LTC3613 to deliver
up to 15A of adjustable load current.
The LTC3613 features true remote differential output voltage sensing. This allows
for accurate regulation of the output
with maximum load currents and shared
ground planes. This feature is critical for
low output voltage applications, where
even small voltage offsets caused by
parasitic IR drops on board traces can
cost several percentage points in regulation accuracy. Remote differential output
sensing and the LTC3613’s accurate internal
reference combine to offer excellent output regulation accuracy over line, load and
26 | July 2012 : LT Journal of Analog Innovation
temperature: ±0.25% at 25°C, ±0.67% from
0°C to 85°C and 1% from –40°C to 125°C.
frequency is constant over steady state
conditions under line and load. It also
allows the LTC3613 to recover from a large
load step in only a few short cycles. This
architecture yields well balanced current
sharing among multiple LTC3613s, which
can be easily paralleled for high power
applications. It also includes a phaselock loop (PLL) for synchronization to an
The LTC3613 has a low minimum on-time
of 60ns, allowing for high step-down
ratios at high switching frequencies.
Because of its sophisticated controlled ontime, valley current mode architecture, the
on-time is controlled so that the switching
Figure 1. 24V input to 1.2V output using inductor DCR sensing to minimize solution size and cost and to
maximize efficiency
LTspice IV
INTVCC
RPGD
100k
RDIV1
52.3k
PVIN
350kHz
CSS
0.1µF
CITH1
220pF RITH
28k
CIN2
10µF
VOUT
PGOOD
LTC3613
VRNG
RDIV2
10k
circuits.linear.com/560
SVIN
RUN
SENSE–
SENSE+
MODE/PLLIN
EXTVCC
SW
CDCR RDCR
0.1µF 3.09k
L1
0.56µH
VOUT
1.2V
15A
CB 0.1µF
BOOST
TRACK/SS
DB
INTVCC
ITH
INTVCC
CVCC
4.7µF
CITH2 100pF
RT
115k
VIN
CIN1 6V TO 24V
82µF
25V
+
RFB2
20k
RFB1
20k
COUT2
100µF
×2
+
PGND
RT
SGND
VOSNS+
VOSNS–
CIN1: SANYO 25SVPD82M
COUT1: SANYO 2R5TPE330M9
3613 F10
DB: CENTRAL CMDSH-3
L1: VISHAY IHLP4040DZ-056µH
COUT1
330µF
2.5V
×2
design ideas
LTC3608
LTC3609
LTC3610
LTC3611
LTC3613
PV IN(MAX)
18V
32V
24V
32V
24V
I LOAD(MAX)
8A
6A
12A
10A
15A

Frequency Sync
Precise Differential
Output Sensing
±1%
±1%
±1%
±1%
±0.67%
Accurate Current
Sensing
Bottom FET R DS(ON)
Bottom FET R DS(ON)
Bottom FET R DS(ON)
Bottom FET R DS(ON)
R SENSE or DCR sensing
MOSFET R DS(ON)
Top/Bottom
10mΩ/8mΩ
18mΩ/13mΩ
12mΩ/6.5mΩ
15mΩ/9mΩ
7.5mΩ/5.5mΩ
Package
7mm × 8mm × 0.9mm
64-pin
7mm × 8mm × 0.9mm
64-pin
9mm × 9mm × 0.9mm
52-pin
9mm × 9mm × 0.9mm
52-pin
7mm × 9mm × 0.9mm
56-pin
Table 1. High power monolithic regulator family
external clock, especially beneficial for
high current, low output voltage applications where interleaving of paralleled
phases can minimize output voltage ripple.
50%, then the maximum sense voltage
is reduced to about one-fourth of its full
value, limiting the inductor current level
to one-fourth of its maximum value.
The LTC3613 includes several safety and
protection features including overvoltage protection and current-limit foldback. If the output exceeds 7.5% of the
programmed value, then it is considered
an overvoltage (OV) condition, the top
MOSFET is immediately turned off and
the bottom MOSFET is turned on indefinitely until the OV condition is cleared.
A power good output monitor is also
available which flags if the part is outside
a ±7.5% window of the 0.6V reference
voltage. In the case of an output short
circuit, if the output fails by more than
The LTC3613 offers precise control of
the output during start-up and shutdown sequencing though its output
voltage tracking and soft-start features. An external VCC input pin is
also available, allowing for bypassing of its internal LDO for an efficiency
benefit in high power applications.
Figure 2. Efficiency of the regulator in Figure 1
Figure 3. Load transient response of the circuit in
Figure 1
The LTC3613 can be configured to sense
the inductor current through a series
sense resistor, RSENSE , or via an inductor DCR sensing network. The tradeoffs between the two current sensing
schemes are largely matters of cost, power
100
90
EFFICIENCY (%)
80
PULSE-SKIPPING
MODE
VOUT
100mV/DIV
70
60
50
FORCED
CONTINUOUS
MODE
IL
10A/DIV
40
30
20
VIN = 12V
VOUT = 1.2V
10
0
0.1
1
10
LOAD CURRENT (A)
100
ILOAD
10A/DIV
40µs/DIV
LOAD TRANSIENT = 0A TO 15A
VIN = 12V, VOUT = 1.5V
FIGURE 10 CIRCUIT
consumption and accuracy. DCR sensing
owes its increasing popularity to its lower
expense and power loss compared to a
sense resistor scheme. Even so, the tight
tolerances of current-sensing resistors
provide the most accurate current limit.
Figure 1 shows a typical application of
the LTC3613, configured for DCR sensing
in a high step-down solution, 24V to 1.2V,
and synchronized to a 350kHz external
clock. Figure 2 shows the efficiency and
Figure 3 shows transient performance.
CONCLUSION
The LTC3613 offers far more design flexibility than a typical monolithic switching
regulator by including a variety of userprogrammable features such as soft-start,
programmable frequency, external clock
synchronization, adjustable current limit
and selectable light load operating modes.
Its critical safety features such as overvoltage protection and programmable current limit with foldback current limiting
further improve the robustness of the
part. An external VCC input is provided
for high power applications. Its compact
solution size, extensive feature set and
high performance capabilities extend
its range of use compared to traditional
monolithics, making it suitable for an
an expanding range of applications. n
July 2012 : LT Journal of Analog Innovation | 27
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