Current Mode Switching Supply with Ultralow Inductor DCR Sensing for High Efficiency and High Reliability

Current Mode Switching Supply with Ultralow Inductor DCR
Sensing for High Efficiency and High Reliability
Jian Li, Haoran Wu and Gina Le
Current mode switching supplies have several advantages over voltage-mode switching
supplies: (1) high reliability with fast, cycle-by-cycle current sensing and protection;
(2) simple and reliable loop compensation—stable with all ceramic output capacitors;
(3) easy and accurate current sharing in high current PolyPhase supplies. In high
current applications, power losses in the current sensing component are a concern,
so the resistance of the sense component must be as low as possible. The problem
is that low resistance sensing elements produce reduced signal-to-noise ratios, such
that switching jitter becomes an issue in high current, high density applications.
The LTC3866 solves this problem by making it possible to build reliable current
mode switching supplies with <0.5mΩ current sensing resistance. This single-phase
synchronous buck controller drives all
N-channel power MOSFET switches with
onboard gate drivers. It employs a unique
architecture that enhances the signal-tonoise ratio of the current sense signal,
allowing the use of a very low DC resistance (DCR) power inductor or low value
current sensing resistor to maximize
efficiency in high current applications.
This feature reduces the switching jitter
commonly found in low DCR applications.
The controller has a wide 4.5V–38V input
range, remote output voltage sensing with
accurate 0.5% reference, programmable
and temperature-compensated current
limit when using inductor DCR sensing, short-circuit soft recovery without
overshoot, and chip thermal shutdown.
The LTC3866 facilitates the design of high
efficiency, high power density and high
reliability solutions for telecom systems, industrial and medical instruments,
and DC power distribution systems. The
controller is available in a low thermal
24 | April 2012 : LT Journal of Analog Innovation
Figure 1. LTC3866 current sensing
scheme with ultralow inductor DCR.
High current paths are shown with
thicker lines.
VIN
INTVCC
VIN
BOOST
INDUCTOR
LTC3866
RITEMP
ITEMP
TG
DCR
VOUT
BG
PGND
RS
22.6k
SNSD+
SNS–
RNTC
100k
L
SW
RP
90.9k
SNSA+
R1
R2
C1
C2
SGND
PLACE C1, C2 NEXT TO IC
PLACE R1, R2 NEXT TO INDUCTOR
R1C1 = 5 • R2C2
Figure 2. High efficiency,
1.5V/30A step-down
converter with very low
DCR sensing
100k
0.1µF
FREQ
MODE/PLLIN
RUN
PGOOD
TK/SS
ITEMP
30.1k
220pF
20k
10k
1.5nF
C1
220nF
C2
220nF
4.7µF
EXTVCC
ITH
VFB
220µF
VIN
4.5V TO 20V
LTC3866
VIN
DIFFOUT
INTVCC
DIFFP
BOOST
DIFFN
TG
SNSD+
SW
SNS–
SNSA+
ILIM
0.1µF
0.33µH
DCR = 0.32mΩ
BG
PGND
CLKOUT
SGND
R2
931Ω
R1
4.64k
COUT
470µF
×2
VOUT
1.5V
30A
design features
The LTC3866 employs a unique architecture that
enhances the signal-to-noise ratio of the current sense
signal, allowing the use of a very low DC resistance (DCR)
power inductor or low value current sensing resistor
to maximize efficiency in high current applications.
3
Burst Mode® OPERATION
PULSE-SKIPPING
MODE
CCM
90
EFFICIENCY (%)
80
70
TOP OF BOARD
BOTTOM OF BOARD
60
50
40
INDUCTOR
TOP FET
BOTTOM FET
30
20
VIN = 12V
VOUT = 1.5V
FSW = 400kHz
10
0
0.01
0.1
1
ILOAD (A)
10
100
LTC3866
12V INPUT
1.5V/30A OUTPUT
NO AIRFLOW
Figure 3. Efficiency of the circuit in Figure 2
Figure 4. Thermal test of the circuit in Figure 2
impedance 24-lead 4mm × 4mm QFN and
24-lead exposed pad FE packages.
It is especially well suited to low voltage, high current supplies because of a
unique architecture that enhances the
signal-to-noise ratio of the current sense
circuit. This allows it to operate with
the small sense signals produced by very
low DCR, 1mΩ or less, inductors, which
improve power efficiency in high current
supplies. The improved SNR minimizes
FEATURES
The LTC3866 uses a constant frequency
peak current mode control architecture, guaranteeing cycle-by-cycle
peak current limit and current sharing
between different power supplies.
Figure 5. Switching node jitter comparison at 12V input, 1.5V/25A output
STANDARD DCR SENSING
160ns
jitter due to switching noise, which could
corrupt the signal. The LTC3866 can sense
a DCR value as low as 0.2mΩ with careful PCB layout, though in this extreme
situation, the additional PCB and solder resistance should be considered.
As shown in Figure 1, the LTC3866 comprises two positive sense pins, SNSD+ and
SNSA+, to acquire signals and processes
Figure 6. Short circuit test
LTC3866 ENHANCED DCR SENSING
60ns
VOUT
1V/DIV
VSW
10V/DIV
VSW
10V/DIV
IL
10A/DIV
VIN = 12V
VOUT = 1.5V
ILOAD = 25A
100ns/DIV
VIN = 12V
VOUT = 1.5V
ILOAD = 25A
100ns/DIV
500µs/DIV
April 2012 : LT Journal of Analog Innovation | 25
APPLICATIONS
R2 • C2 = R1 • C1/5. An additional,
optional temperature compensation
circuit guarantees the accurate current
limit over a wide temperature range,
especially important in DCR sensing.
them internally to provide a 14dB (5×) signal-to-noise ratio improvement in response
to low voltage sense signals. The current
limit threshold is still a function of the
inductor peak current and its DCR value,
and can be accurately set from 10mV to
30mV in a 5mV steps with the ILIM pin.
The part-to-part current limit error is only
about 1mV over the full temperature range.
Figure 2 shows a high efficiency,
1.5V/30A step-down converter with
very low DCR sensing. An inductor with DCR = 0.32mΩ is used in this
design to maximize efficiency.
The LTC3866 also features a precise
0.6V reference with a guaranteed limit
of ±0.5% that provides an accurate
output voltage from 0.6V to 3.5V. Its
differential remote VOUT sensing amplifier makes the LTC3866 ideal for low
voltage, high current applications.
The filter time constant, R1 • C1, of the
SNSD+ path should equal the L/DCR of
the output inductor, while the filter at
SNSA+ path should have a bandwidth
five times larger than SNSD+, namely
The efficiency in different operation modes
is shown in Figure 3. The full load efficiency is as high as 90.3% at 12V input
voltage. It is about 1.4% improvement
over the supply using a 1mΩ sense resistor with the same power stage design.
The hot spot (bottom MOSFET) temperature rise is only 39.6°C without any
Figure 7. A high efficiency, 1.5V/80A power supply based on parallel LTC3866s and power blocks
100k
100k
INTVCC1
ITH
ITEMP
PGOOD
VIN
100k
ITH
VFB
PGOOD
ITEMP
INTVCC
PGND
CLKOUT
ILIM
SNSA+
SNS–
SGND
VGATE
TEMP–
GND
CS–
GND
CS+
BG
INTVCC2
+
330µF
+
330µF
GND
10Ω
10Ω
4.75k
10µF
VOUT
VIN
4.7µF
VIN1
VOUT1
VIN2
VOUT2
100µF
PWMH
CMDSH-3
BOOST
TG
100µF
VIN
VIN
SW
26 | April 2012 : LT Journal of Analog Innovation
TEMP+
470µF
INTVCC2
SNSD+
47nF
PWML
ACBEL POWER BLOCK
VRA001-4C1G
2.2Ω
DIFFP
47nF
VOUT2
INTVCC2
EXTVCC
LTC3866EUF
VIN2
VOUT
1.5V
80A
GND
1µF
MODE/PLLIN
FREQ
RUN
TK/SS
100k
DIFFN
0.1µF
INTVCC1
47nF
VOUT1
GND
BG
PGND
CLKOUT
SW
DIFFOUT
CMDSH-3
BOOST
SNSD+
VIN1
PWMH
INTVCC
TG
47nF
VOUT
4.7µF
EXTVCC
10µF
VIN
INTVCC1
DIFFP
ILIM
20k
DIFFN
SNSA+
30.1k
470µF
2.2Ω
LTC3866EUF
SGND
220pF
DIFFOUT
SNS–
5.36k
VFB
MODE/PLLIN
RUN
FREQ
TK/SS
0.1µF
1500pF
VIN
1µF
0.1µF
PWML
TEMP+
VGATE
TEMP–
GND
CS–
GND
CS+
ACBEL POWER BLOCK
VRA001-4C1G
330µF
+
330µF
GND
GND
GND
+
4.75k
design features
2.2Ω
LTspice IV
circuits.linear.com/548
10µF
×2
1µF
VIN
VIN
180µF 12V
×2
MODE/PLLIN
FREQ
20k
RUN
TK/SS
0.1µF
VOUT
500mV/DIV
ILOAD
50A/DIV
DIFFP
BOOST
DIFFN
TG
28.7k
SNSD+
SW
100pF
SNS–
SNSA+
PGND
ILIM
D1: CMDSH-3
M1: BSC024NE2LS
M2: BSC010NE2LS
D1
INTVCC
C1
220nF
4.7µF
VOUT
LTC3866
DIFFOUT
1nF
120k
ITEMP
EXTVCC
ITH
VFB
IL1 & IL2
10A/DIV
PGOOD
L1
1µH
DCR = 1.3mΩ
M1
M2
BG
R1
3.48k
CLKOUT
SGND
100µF
×2
330µF
×2
VOUT
5V
25A
10µs/DIV
Figure 8. Current sharing performance of the
1.5V/80A supply in Figure 7
Figure 9. High efficiency power supply, 12V
input to 5V/25A output
airflow, as shown in Figure 4, where the
ambient temperature is about 23.8°C.
output inductor signal and connect to the
SNSA+ pin. If the RC filter is used, its time
constant, R • C, is set equal to L/DCR of
the output inductor. In these applications, the current limit, VSENSE(MAX), is five
times larger for the specified ILIM, and
the operating voltage range of SNSA+ and
SNS– is 0V to 5.25V. Without using the
internal differential amplifier, the output
voltage of 5V can be generated as shown
in Figure 9. The thermal test shows that
the hot spot (the inductor) temperature
is about 57.3°C at full load without any
airflow, as shown in Figure 10, where
the ambient temperature is 25°C.
The unique design improves the efficiency, as well as the noise sensitivity.
The worst case switching node jitter is
reduced by 60%, as shown in Figure 5,
with a very low 0.32mΩ inductor DCR.
Another unique feature of LTC3866 is
short-circuit soft recovery. The internal
soft recovery circuit guarantees that
there is no overshoot when the power
supply recovers from a short-circuit
condition as shown in Figure 6.
The LTC3866 can be used with a power
block for a more compact design and very
high current. Figure 7 shows a dual-phase,
high efficiency, 1.5V/80A power supply
based on a 2× parallel LTC3866 + power
block scheme. Although the DCR of the
inductor in the power block is only
0.53mΩ, the current sharing performance
is excellent in both DC and transient
conditions, as shown in Figure 8.
In applications where higher value
DCR inductor or RSENSE is used, the LTC3866
can be used like any typical current mode
controller by disabling the SNSD+ pin,
shorting it to ground. An RSENSE resistor or a RC filter can be used to sense the
R3 R2
20k 147k
CONCLUSION
The LTC3866 delivers an outsized set of
features for its small 4mm × 4mm 24-pin
QFN package. The unique, ultralow
DCR current sensing with current mode
control makes the LTC3866 a good fit for
low voltage, high current applications
with high efficiency and high reliability.
Tracking, strong on-chip drivers, multichip
operation and external sync capability
fill out its menu of features. The LTC3866
is ideal for computer and telecom systems, industrial and medical instruments,
and DC power distribution systems. n
Figure 10. Thermal test of the circuit in Figure 9
TOP OF BOARD
INDUCTOR
BOTTOM FET
TOP FET
BOTTOM OF BOARD
LTC3866
13V INPUT
5V/25A OUTPUT
NO AIRFLOW
April 2012 : LT Journal of Analog Innovation | 27
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