Texas Instruments | TPS65311-Q1 BUCK1 Controller DCR Current Sensing | Application notes | Texas Instruments TPS65311-Q1 BUCK1 Controller DCR Current Sensing Application notes

Texas Instruments TPS65311-Q1 BUCK1 Controller DCR Current Sensing Application notes
Application Report
SLVA791 – September 2016
TPS65311-Q1 BUCK1 Controller DCR Current Sensing
Krishnamurthy Hegde .......................................................................................................... MSA-ASP
ABSTRACT
This application report provides guidelines and recommendations for implementing BUCK1 controller
current sense using resistance of the inductor (DCR current-sensing method) for TPS65311-Q1 and
TPS65310A-Q1 devices. This method was verified on the TPS65311-Q1 device. However, the BUCK1
controller is identical between TPS65310A-Q1 and TPS65311-Q1 devices and the results should be
applicable to TPS65310A-Q1 device also.
1
Introduction
BUCK1 controller in the TPS65310A-Q1 and TPS65311-Q1 devices operate using constant frequency
peak current mode control. Peak current-mode control regulates the peak current through the inductor
such that the output voltage VBUCK1 is maintained to its set value. The error between the feedback
voltage VSENSE1 and the internal reference produces an error signal at the output of the error amplifier
(COMP1), which serves as target for the peak inductor current. At S1–S2, the current through the inductor
is sensed as a differential voltage and compared with this target during each cycle. For applications which
require precise current sensing, typically an external shunt resistor is used. But, the external sense
resistor could be bulky and also for better efficiency, the resistance of the inductor (DCR) can be used to
sense the inductor current. Figure 1 shows the typical block diagram for both current sensing methods.
Sense Resistor
VINPROT
L
RS
GU
HS
DCR Sensing
L
PWM
Logic
RL
PH
Gate
Drivers
VBUCK1
GL
LS
RDCR
CDCR
Current
Comparator
S1
VS1-S2,INT
VSLOPE
Current
Sensing
VS1-S2, EXT
S2
Slope
Compensation
R1
VSENSE1
gm
Error
Amp
Current Loop
(Inner Loop)
C2
R2
COMP1
R3
C1
Voltage Loop (Outer Loop)
Copyright © 2016, Texas Instruments Incorporated
Figure 1. BUCK1 Block Diagram With Current Sensing Using Inductor Resistance-RL
or External Sense Resistor-RS
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1
Sense Resistor Method
2
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Sense Resistor Method
As specified in the TPS65311-Q1 datasheet (section: Output Inductor, Sense Resistor and Capacitor
Selection for the BUCK1 Controller), an external resistor-RS, is used to sense the current through the
inductor. The current sense resistor pins, S1 and S2, are fed into an internal differential amplifier which
supports the range of VBUCK1 voltages. The sense resistor, RS, must be chosen so that the maximum
forward peak current in the inductor generates a voltage of 75 mV across the sense pins. This specified
typical value is for low duty cycles only. At typical duty-cycle conditions around 28% (assuming 3.3-V
output and 12-V input), 50 mV is a more reasonable value, considering tolerances and mismatches. Use
Figure 2 (Reduction of Current-Limit vs Duty Cycle) of the TPS65311-Q1 datasheet to estimate the
maximum sense voltage across the sense resistor for different duty cycles.
50 mV
RS =
Im ax_peak
(1)
Optimal slope compensation which is adaptive to changes in input voltage and duty cycle allows stable
operation at all conditions. For the optimal performance of the slope compensation circuit, empirical
Equation 2 must be followed while choosing the inductor and the sense resistor.
L = 410 ´ Rs
where
•
•
3
L = inductor in µH
RS = external sense resistor in Ω
(2)
DCR Sensing Method
The current sense pins, S1 and S2, are high-impedance pins with low leakage across the VBUCK1
operating range. This allows current sensing using the DC resistance, RL or DCR of the inductor, for better
efficiency and this eliminates the need of additional sense resistor to save cost and PCB space. For DCR
sensing method, a RC network, RDCR and CDCR, is used in parallel to the inductor. The voltage across CDCR
is sensed using S1 and S2 pins as shown in Figure 1.
When RDCR and CDCR are selected in such a way that the RC time constant is equal to the ratio of
inductance and series resistance of the inductor, the voltage across CDCR will be directly proportional to the
inductor current (IL), as expressed in Equation 3 and Equation 4.
L
RDCR u CDCR
(3)
RL
V_CDCR = IL u RL
where
•
•
•
•
RDCR, CDCR= external RC network used in parallel to the inductor
L= inductor in µH
RL = Inductor DC resistance
V_CDCR= voltage across sense capacitor
(4)
A 10-µH inductor with the specification in Table 1 was used for these measurements.
Table 1. Inductor Specification
DCR (mΩ)
Part Number
MSS1260T-103ML
2
10 ±20%
Isat (A)
Irms (A)
Typical
Maximum
SRF Typical
(MHz)
10% Drop
20% Drop
30% Drop
20°C Rise
40°C Rise
21.5
23.9
22
6
6.92
7.48
3
4
Inductance (µH)
TPS65311-Q1 BUCK1 Controller DCR Current Sensing
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Temperature Dependency of Inductor Series Resistance
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Based on Equation 3, with L= 10 µH, RL= 21.5 mΩ, and by assuming CDCR = 220 nF, the result is:
RDCR = 2.11 KΩ
For this experiment, RDCR= 2 KΩ was used as it was the closest available resistor value when compared
with 2.11 KΩ.
Different combinations of CDCR and RDCR can also work, but this experiment uses CDCR = 220 nF.
4
Temperature Dependency of Inductor Series Resistance
Inductor series resistance (DCR) is a resistance of copper coil that varies with temperature.
Temperature dependency of inductor series resistance is given in Equation 5:
RL (T) = RL_25C ´ éë1 + TC_Copper ´
(T
- 25 )ùû
where
•
•
•
5
T= temperature of the inductor
RL_25C = inductor series resistance at room temperature (25°C)
TC_Copper = temperature coefficient of copper that is equal to 0.00393
(5)
Measurement Procedure
Since the resistance of the inductor (DCR) varies with temperature, measurements were taken across
temperature to check the effect of temperature on the DCR current-sensing method. BUCK1 overcurrent
detection causes device reset and RESN is pulled low. DC load current at which the device detects
BUCK1 overcurrent (OC) is noted and recorded as the overcurrent detection threshold. However, actual
inductor-peak current will be higher than this average DC current and can be estimated based on the
differential voltage measured across the current sense resistor, RS, (when using the Sense Resistor
Method) or across CDCR (when using DCR Sensing Method). Since measuring actual inductor current with
a current probe requires the inductor to be lifted off the board, this causes accuracy issues and difficulty in
measuring the inductor current across temperature.
The scope of this measurement is to check the basic functionality and feasibility of the DCR Sensing
Method for this device. These measurements are not intended for optimization of the DCR Sensing
Method circuit, nor a reference design for a production application. Any adaption by customers must be
validated in customers' own design and environment.
5.1
Test Conditions
The test conditions are as follows:
• Device Input Supply voltage, VBAT = 12 V (bench-top power supply with 4-A current rating)
• BUCK1 Output Voltage, VBUCK1 = 3.35 V
• Load current on VBUCK1 = Electronic source meter with 3.1 A maximum DC-load current capability
• Case 1, Sense Resistor Method:
– Reference measurement using 22-mΩ current-sense resistor
– TPS65311-Q1 EVM with 22-mΩ current-sense resistor for BUCK1 current sensing
• Case 2, DCR Sensing Method:
– Measurement using DCR current sense using RDCR (2 kΩ), CDCR (220 nF), and 10 µH inductor with
21.5 mΩ DCR
TPS65311 EVM was modified by shorting 22-mΩ current-sense resistor and placing RDCR (2 kΩ) and CDCR
(220 nF) across the inductor. Voltage across the sense capacitor was measured using a differential probe.
Voltage spikes were observed during the measurements, which could be due to the board modifications
that had to be made, such as cutting the trace and blue wiring. However, to demonstrate feasibility, the
spikes should be acceptable.
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Measurement Procedure
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Current-sense resistor
was removed, traces
were cut.
RDCR and CDCR were
introduced.
Figure 2. TPS65311-EVM Top View
Figure 3. TPS65311-EVM Bottom View
Since the load-current source had a maximum current of 3.1 A, two parallel current-source meters were
used in the case where the load current required to detect the overcurrent exceeded 3.1 A (due to
limitations of load current source).
Using the DCR Sensing Method, there was a negative offset of approximately 10 mV, which can be
adjusted by a slightly different combination of RDCR and CDCR to make the baseline close to 0 mV. This
offset could be due to the slight mismatch in the RC time constants according to Equation 4 or due to the
blue wiring or measurement setup.
5.2
Measurement Results
All measurement plots taken at room temperature (except Figure 12). The following is the legend for
Figure 4 through Figure 12:
• Blue trace: BUCK1 output Voltage
• Green trace: SW1 voltage
• Red trace: Differential voltage measured across sense resistor(Case-1) or Differential voltage
measured across CDCR (Case-2)
• Pink trace: Load current on BUCK1
Around 10 mV offset
compared to Figure 4
Figure 4. Case 1: Measurement With No Load Condition
4
Figure 5. Case 2: Measurement With No Load Condition
TPS65311-Q1 BUCK1 Controller DCR Current Sensing
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Measurement Procedure
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Figure 6. Case 1: Measurement With DC load — 1 A
Figure 7. Case 2: Measurement with DC load — 1 A
Figure 8. Case 1: Measurement With DC load — 2 A
Figure 9. Case 2: Measurement With DC load — 2 A
Device detects OC at around 68 mV
Device detects OC at around 68 mV
Just before OC detection, 0.5 A load current through 2nd
source-meter is not shown
Just before OC Detection
Figure 10. Case-1: Measurement With DC load — 2.6 A
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Figure 11. Case 2: Measurement With 3.2 A (2.70 A and
0.5 A) Load
TPS65311-Q1 BUCK1 Controller DCR Current Sensing
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5
Measurement Procedure
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Large inductor ripple current
compared to room
temperature
Just before OC detection
Figure 12. Case 2: Measurement With 1.8 A Load at +125°C
Table 2. Temperature Measurement Using the Different Current Sensing Methods
Ambient Temperature
5.3
DC Load Current When Device Detects OC (A)
Case-1, Sense Resistor Method
Case-2, DCR Sensing Method
–40C
2.65
4.0
–25C
2.66
3.9
0C
2.66
3.7
+25C
2.65
3.4
+50C
2.64
3.1
+75C
2.63
2.6
+100C
2.63
2.2
+125C
2.62
1.8
Estimation of Peak Inductor Current at Which Device Has Detected Overcurrent (OC)
Based on Figure 2 of the TPS65311-Q1 datasheet, for Vin = 12 V and Vout = 3.3 V, the current limit
voltage (across S1-S2) of approximately 68 mV is almost the same value as observed in the
measurement plots. This estimation is done to validate the measurement results by back calculating the
average DC current at which device detected the overcurrent threshold.
5.3.1
Case 1, Sense Resistor Method
Based on Figure 10, inductor peak current is calculated as:
68 mV
IL_max
3.1 A
22 m:
50 mV
IL_min
2.27 A
22 m:
(6)
(7)
With Inductor min and max currents the DC load current can be back calculated as follows:
IL_max IL_min
I_DC
2.69 A
2
6
TPS65311-Q1 BUCK1 Controller DCR Current Sensing
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(8)
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Measurement Procedure
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Calculated DC current is very close to the actual DC load current (2.6 A) at which device detected
overcurrent threshold. Thus, the calculation correlates and the current is fairly constant across
temperature.
5.3.2
Case 2, DCR Sensing Method
As shown in Figure 5, there is an approximately –10 mV offset, which must be accounted for while doing
back calculation of peak inductor current based on measured sense voltage. Based on Figure 11, the
measured peak voltage was 68 mV and 78 mV must be used for peak-inductor current (IL_max)
calculation. Similarly for IL_min calculation, 56 mV was taken instead of measured value of around 46 mV.
(In Figure 11, there is an undershoot due to measurement issue and hence cursor measured value
(46mV) was taken for IL min calculation.)
78 mV
IL_max
3.36 A
21.5 m:
(9)
56 mV
IL_min
2.6 A
21.5 m:
(10)
With Inductor min and max currents the DC load current is calculated by:
IL_max IL_min
I_DC
3.15 A
2
(11)
Considering the tolerances, calculated DC current is close to the actual DC load current (3.2 A) and the
calculation looks correlated.
Inductor resistance across temperature can be calculated according to Equation 5 and then used to
calculate the peak inductor current at which OC is detected. This will be an approximation as there are
additional tolerances due to actual inductor resistance, variation of sense capacitor (CDCR) across
temperature, measurement accuracy, and other variations.
Table 3. Calculation of Peak Inductor Current at OC Detection Point for DCR Sensing Method
Ambient Temperature
5.3.3
Case-2, DCR Sensing Method
Inductor Resistance (ohm) (RL)
(Calculated Based on Equation Equation 5)
Calculated/Estimated peak Inductor Current
at OC detection point (78 mV / RL)
–40C
0.0160
4.87
–25C
0.0173
4.52
0C
0.0194
4.02
+25C
0.0215
3.63
+50C
0.0236
3.30
+75C
0.0257
3.03
+100C
0.0278
2.80
+125C
0.0299
2.60
Measurements Summary
Figure 13 provides the measurement plots (based on Table 2 and Table 3) for both methods across
temperature. It is evident from the results that the sense resistor method has less variation across
temperature and the DCR current sense method has considerable variation across temperature. Also,
estimated peak current (assuming that peak current is proportional to average DC current) is correlating
with the measured DC current between 0°C to 50°C and beyond this temperature range there is a wider
difference between calculated and the measured values.
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Conclusion
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5
Current (A)
4
3
2
Estimated Peak Inductor Current, Case-2
Measured DC load current, Case-1
Measured DC load current, Case-2
1
-50
-25
0
25
50
75
Ambient Temperature (qC)
100
125
D001
Figure 13. Comparison of Case-1 and Case-2 Measurement Results Across Temperature
6
Conclusion
•
•
•
•
•
•
As expected, DCR Sensing Method has a wide variation across temperature. To accommodate for
these variations, an inductor with large current rating and tighter DCR tolerance must be used. Since it
needs an additional two components (RDCR, CDCR), accuracy of current sense also depends on
accuracy of these two components.
With DCR current-sensing method, BUCK1 controller efficiency can be improved as it eliminates the
drop across the external sense resistor. This method will also help to save PCB space, as sense
resistor is usually bulky.
As DCR current sensing requires an inductor with a larger current rating and better tolerance along
with additional two components, it may offset the cost of an additional sense resistor. A cost
comparison between these two current sensing methods should be performed to determine the correct
level of cost and performance optimization.
Current sensing with external sense resistor, RS, is very stable across temperature.
With DCR Sensing Method, larger inductor-current ripple was observed at high temperature and might
affect the EMC performance.
Electrical and EMC performance of DCR Sensing Method must be studied by the customer in their
application to ensure the applicability to use this method in the specific application use case.
Revision History
8
DATE
REVISION
NOTES
September 2016
*
Initial Release
Revision History
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