DN518 - 36V Input, Low Output Noise, 5A μModule Regulator for Precision Data Acquisition Systems

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36V Input, Low Output Noise, 5A µModule Regulator for
Precision Data Acquisition Systems – Design Note 518
Jaino Parasseril
Introduction
Low output noise, fast transient response and high
efficiency are just a few of the stringent power supply
demands made by applications featuring high data rate
FPGA I/O channels and high bit count data converters.
The power supply designer faces the difficult task of
meeting all of these requirements with as few components as possible, since no single topology easily
meets all three.
For instance, high performance linear regulators
achieve the required low output noise and fast transient response, but tend to dissipate more power than
a switching topology, resulting in thermal issues.
Switching regulators, on the other hand, are generally
more efficient and run cooler than linear regulators,
but generate significantly more output noise and cannot respond as quickly to transients. Power supply
designers often resort to combining the two topologies,
using a switching regulator to efficiently step down a
relatively high bus voltage, followed by a linear post
regulator to produce a low noise output. Although it is
possible to produce a low noise supply in this way, it
requires careful design to achieve high efficiency and
fast transient response.
An easier way to reap the benefits of both a linear regulator and a switching regulator is to use the LTM®8028,
which achieves low noise, fast transient response and
high efficiency by combining both regulators into a
single part.
Integrated Switching and Linear Regulators
The LTM8028 is a 36VIN, 5A µModule ® regulator that
combines a synchronous switching converter and low
noise linear regulator in a 15mm × 15mm × 4.92mm
BGA package. It operates from an input range of 6V to
36V with an output voltage that can be programmed
between 0.8V and 1.8V. The combination of the two
converters results in tight tolerance of line and load
regulation over the –40°C to 125°C temperature range.
The switching frequency can be adjusted between
200kHz and 1MHz with the RT resistor, or the SYNC
09/13/518
pin can synchronize the internal oscillator to an external
clock. The 5A current limit can be reduced by utilizing the
IMAX pin. The PGOOD pin can be used to detect when
the output voltage is within 10% of the target value.
PCB Trace Voltage Compensation Using SENSEP
The resistance of PCB traces between the µModule
regulator and the load can result in voltage drops
that cause a load regulation error at the point of load.
As the output current increases, the voltage drop
increases accordingly. To eliminate this voltage error,
the LTM8028’s SENSEP pin can be connected directly
to the load point.
Programmable Output Voltage
The output voltage can be digitally programmed in
50mV increments by controlling the LTM8028's 3-state
inputs: VO0, VO1 and VO2. Additionally, the MARGA
pin can be used for output margining via analog control
that adjusts the output voltage by up to ±10%.
DC1738A Highlights the LTM8028 Capabilities
A 1.8V output application is shown in Figure 1. The
LTM8028 comes in a 15mm × 15mm × 4.92mm BGA
package and is featured in the demonstration circuit
DC1738A, shown in Figure 2.
VIN
6V TO 36V
10µF
R1
402k
VIN
RUN
SS
MARGA
SYNC
0.01µF
LTM8028
VOUT
1.8V
5A
VOUT
SENSEP
BKV
PGOOD
IMAX
100µF
RT VOB VO0 V01 V02 GND
133k
470µF
137µF
+
DN518 F01
Figure 1. µModule Regulator Takes a Wide Ranging 6V
to 36V Input and Produces a Low Noise 1.8V Output with
Up to 5A Output Current
L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are
registered trademarks and UltraFast is a trademark of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
Noise Test Comparison Using LTC2185 ADC
When powering high speed analog-to-digital converters
(ADCs), it is important to use a power supply that is as
clean as possible. Any switching spurs that are present
on the power supply rail will translate into AM modulation in the ADC output spectrum. The noise performance
of the LTC ®2185, a 16-bit ADC, was evaluated to see
the difference between using (1) a typical LDO, (2) a
typical switching regulator, and (3) the LTM8028 low
noise μModule regulator. A simplified schematic of the
test is shown in Figure 3, where the DUT is represented
by either of the configurations.
frequency. The sampling process produces 250kHz
spurs at baseband. As a result, the SINAD drops to
71.84dB, around 4dB compared to an LDO. This reduces
the LTC2185 to nearly 12-bit performance. In demanding applications where tenths of dBs are significant,
losing 4dB of SINAD because of a noisy regulator is
unacceptable. In addition to degrading the SINAD of the
ADC, these spurs may land on neighboring channels
or on other signals of interest, making it impossible
to receive meaningful data from those channels. With
the LTM8028, only a few extraneous spurs exist near
the desired frequency and the SINAD performance is
only 0.03dB worse than the LDO baseline. The spurious
content that was very pronounced in the spectrum of the
switching regulator is virtually eliminated. As a result,
there will not be any performance degradation of the
LTC2185 when using a LTM8028 regulator.
Figure 4 shows the FFT plots using the three different
methods of powering the LTC2185 when sampling a
70MHz tone at 100Msps. The LDO provides a clean
power supply, achieving a SINAD of 76.22dB. However,
when powered by a typical 250kHz switching regulator,
there are spurs around the fundamental with an offset
frequency of 250kHz. These are switching regulator spurs that are AM modulated around the carrier
Conclusion
The LTM8028 µModule regulator combines a linear
regulator and a switching regulator to form a DC/DC
converter with minimal power loss, low noise and
UltraFast™ transient response, all in a 15mm × 15mm
× 4.92mm BGA package.
6.5V
VIN
DUT
GND
VOUT
1.8V
VDD
AIN+
OVDD
LTC2185 ADC
AIN–
ENC+
ENC–
GND
DN518 F03
1.8V
0V
Figure 3. Noise Test Schematic Using Different
Supplies to Power 16-Bit LTC2185 ADC
Figure 2. The LTM8028 Makes It Possible to Build a
Minimal Component-Count Regulator That Meets Stringent
Noise, Efficiency and Transient Response Requirements
0
LTC2185 ADC POWERED
BY LTM8028 µMODULE
–20 SINAD = 76.19dB
–30
LTC2185 ADC POWERED
BY TYPICAL SWITCHING
–20 REGULATOR
–30 SINAD = 71.84dB
–10
–40
–50
–60
–70
–80
MAGNITUDE (dBFS)
–10
MAGNITUDE (dBFS)
MAGNITUDE (dBFS)
0
0
LTC2185 ADC POWERED
BY TYPICAL LDO
–20 SINAD = 76.22dB
–30
–10
–40
–50
–60
–70
–80
–40
–50
–60
–70
–80
–90
–90
–90
–100
–100
–100
–110
–120
–110
–110
0
10
20
30
FREQUENCY (MHz)
40
50
–120
0
DN518 F04a
10
20
30
FREQUENCY (MHz)
40
50
DN518 F04b
–120
0
10
20
30
FREQUENCY (MHz)
40
50
DN518 F04c
Figure 4. 32k-Point FFT, fIN = 70.3MHz, –1dBFs, 100Msps, Using CMOS Clock Drive
Data Sheet Download
www.linear.com/LTM8028
Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 3747
dn518 LT/AP 0913 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2013
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