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Texas Instruments XWR1xxx Power Management Optimizations - Low Cost LC Filter Solution Application notes
Application Report
SWRA577 – October 2017
XWR1xxx Power Management Optimizations – Low Cost
LC Filter Solution
Abdulraheem Killedar
ABSTRACT
This application report discusses the Power supply ripple and noise specifications, reference solutions and
introduces its trade off, an LC filter-based low cost solution.
1
2
3
4
5
Contents
Introduction ................................................................................................................... 2
Power Supply Ripple and Noise Specifications ......................................................................... 2
Reference Solution .......................................................................................................... 3
Low Cost LC Filter Solution ................................................................................................ 5
Summary ................................................................................................................... 18
List of Figures
................................................................................
1
Power Management Scheme With LDOs
2
Reference Schematic ....................................................................................................... 4
3
Power Management Scheme With LC filter
4
Capture With LDO Scheme, RX Gain-48dB
5
6
7
8
9
10
11
4
............................................................................. 5
............................................................................. 9
Capture With LDO Scheme, RX Gain-30dB ............................................................................ 10
Capture With LC Filter Scheme, RX Gain-48dB ....................................................................... 11
Capture With LC Filter Scheme, RX Gain-30dB ....................................................................... 12
LDO Scheme –Spur Level ................................................................................................ 14
LDO Scheme - Noise Floor With Tx –OFF ............................................................................. 15
LC Filter Scheme – Spur Level ........................................................................................... 16
LC Filter Scheme - Noise Floor With Tx –OFF ......................................................................... 17
List of Tables
1
XWR1xx Supply Ripple (µVrms) ........................................................................................... 2
2
XWR1xx Noise Spectral Density (µVrms/RtHz) ......................................................................... 2
3
XWR1xxx Supply Ripple (µVrms)
4
5
6
7
8
9
10
......................................................................................... 3
XWR1xxx Noise Spectral Density (µVrms/RtHz) ........................................................................ 3
Recommended Ferrite Bead Components ............................................................................... 5
Sensor Configuration ....................................................................................................... 5
Spur/Noise Level Measurements With Tx to Rx Waveguide Loopback .............................................. 6
Voltage Transient Behavior ................................................................................................ 7
Sensor Configuration ........................................................................................................ 8
SNR Comparison Between LDO and LC Filter Scheme for RX Gain = 30 and 48 ................................ 13
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1
Introduction
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Trademarks
All trademarks are the property of their respective owners.
1
Introduction
XWR1xx is a family of Texas Instrument’s mmWave Radar sensor devices with RFCMOS technology.
These devices are intended for the automotive and industrial applications. The devices are powered by
four voltage rails, namely 3.3 V, 1.2 V, 1.8 V and 1.0 V.
2
Power Supply Ripple and Noise Specifications
Out of the four Power supply rails mentioned above, the 1.0 V and the 1.8 V supplies are sensitive to the
supply ripple and noise because these supplies feed the critical blocks in the device like PLL, baseband
ADC, synthesizer, and so forth. For more details on the power supply rail and the logic that it feeds in the
device, see the device-specific data sheet. Table 1 and Table 2 provide the ripple and noise spec for 1.0 V
Table 1. XWR1xx Supply Ripple (µVrms)
Sl. Number
Frequency (KHz)
1
17.1875
2
34.375
3
68.75
4
137.5
5
275
6
550
7
1100
8
2200
9
4400
10
6600
XWR1xx Supply Ripple (µVrms)
35.48133892
Table 2. XWR1xx Noise Spectral Density (µVrms/RtHz)
2
Sl. Number
Frequency (KHz)
XWR1xx Noise Spectral Density
(µVrms/RtHz)
1
17.1875
0.01
2
34.375
0.01
3
68.75
0.01
4
137.5
0.015
5
275
0.02
6
550
0.02
7
1100
0.009
8
2200
0.002
9
4400
0.05
10
6600
0.002
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Reference Solution
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Table 3 and Table 4 provide the ripple and noise spec for 1.8 V.
Table 3. XWR1xxx Supply Ripple (µVrms)
Sl Number
Frequency (KHz)
1
17.1875
2
34.375
3
68.75
4
137.5
5
275
6
550
7
1100
8
2200
9
4400
10
6600
XWR1xxx Supply Ripple (µVrms)
35.48133892
Table 4. XWR1xxx Noise Spectral Density (µVrms/RtHz)
3
Sl Number
Frequency (KHz)
XWR1xxx Noise Spectral Density
(µVrms/RtHz)
1
17.1875
0.08
2
34.375
0.07
3
68.75
0.07
4
137.5
0.07
5
275
0.07
6
550
0.07
7
1100
0.07
8
2200
0.07
9
4400
0.07
10
6600
0.07
Reference Solution
TI recommends the reference solution with the PMIC and the LDOs as shown in Figure 1, which is also
incorporated in the XWR1xxx EVMs. The reference solution meets these specifications. The XWR1xx
device would expect 1.0 V to be fed on the H5, G5, J5, D2 and C2 pins. The PMIC needs to be
reconfigured for the Buck outputs shown in Figure 1 using the Inter-Integrated Circuit (I2C) interface from
the host. Initially during the power up, the buck outputs would be 3.3 V, 1.2 V, 1.3 V and 2.1 V. Using I2C
interface, the buck outputs needs to be reconfigured as shown in Figure 1 before the nRESET of XWR1xx
is released.
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Reference Solution
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Figure 1. Power Management Scheme With LDOs
The above reference solution consists of the LP87524BRNFRQ1 PMIC with the switching frequency
localized at 4 MHz, so that the fundamental and the harmonics could easily be filtered out during RADAR
data post processing. The reference schematic is shown in Figure 2.
Figure 2. Reference Schematic
4
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4
Low Cost LC Filter Solution
For the cost sensitive use cases, a low cost LC filter (Ferrite bead + Device Decoupling capacitor) based
solution could be used, which would effectively replace the LDOs on the 1.8 V and 1.0 V rails. This
approach would have performance and the system level trade-off.
Figure 3 shows the Power management scheme.
Figure 3. Power Management Scheme With LC filter
NOTE: The capacitors shown above in the LC filter are the device decoupling capacitors.
The recommended Ferrite bead components are shown in Table 5.
Table 5. Recommended Ferrite Bead Components
LC Filter
Inductor
Value (µH)
DC
Resistance
(mΩ)
Impedance
@ 100 MHz
(Ω)
Ferrite Bead
0.1909
25
120
4.1
4.1.1
Pole
Frequencies
w.r.t. 1V Rail
Decaps/1.8V
Rail Decaps
Size (inches)
(KHz)
0603
113.9
DC Current
(A)
Theoretical
Rejection
Offered by
the Filter
(dB)
3
64
Manufacture
r Part
Number
BLM18KG12
1TH1
LDO vs LC Filter Scheme Comparison
Waveguide Loopback Tests
provides the summary of comparison between LDO based and LC filter based implementation. Two
consecutive chirps have been analyzed with 10 µSec and 100 µSec of idle time and the below chirp
configuration.
Table 6 provides the sensor configuration for the below tests.
Table 6. Sensor Configuration
Parameter
Value
Start frequency (GHz)
77
Frequency slope (MHz)
35.003
Tx start time (µsec)
0
ADC start time (µsec)
5
ADC samples
1024
Sample rate (Kbps)
10000
Ramp end time (µsec)
110
Rx gain
33
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4.1.2
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Spur/Noise Floor Level
The measurements shown in Table 7 are done with TX to RX loopback with Tx Back-0ff=0 and Complex
Rx gain=33dB.
Table 7. Spur/Noise Level Measurements With Tx to Rx Waveguide Loopback
PM Scheme
Idle Channel Noise Floor
(Rx-alone) at the ADC
Output (dBFs/Hz)
Amplitude Level of PMIC
Ripple Frequency Spur
(4.05M-4.17M) Seen at
the ADC Output (dBFs)
(Tx+Rx) SNR
dBc/Hz
Additive (Rx1 - alone)
Amplitude Level of PMIC
Ripple Frequency Spur
(4.05M-4.17M) Seen at
the ADC Output (dBFs)
Loop back (Tx2-Rx1)
LDO Scheme
-138.80
Not seen
-118.74
Not seen (less than noise
floor)
LC Filter Scheme
-138.27
Not seen
-120.6
-104
6
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4.1.3
Voltage Transient Behavior
Table 8. Voltage Transient Behavior
Voltage Setting Parameters on Chirp 1
PM Scheme
LDO
LC Filter
XWR1xx
Supply Rail
Undershoot Voltage
(mV)
Voltage Setting Parameters on Chirp 2
Undershoot Period
(µs)
Undershoot Voltage
(mV)
Undershoot Period
(µs)
Steady State DC
Value (mV) at the
Input of XWR1642
Rail
Calculated IR Drop (mV)
(difference between
steady state DC and
actual input voltage)
Idle time (µs)
10
100
10
100
10
100
10
100
10
10/100
RF1_1V0
55.6
79.0
4.4
6.7
67.0
214.5
5.0
10.0
982.0
17
RF2_1V0
33.9
34.6
7.2
11.1
42.0
208.3
5.6
12.2
982.0
17
CLK_1V8
20.0
20.0
1.6
2.5
24.0
20.0
2.4
2.6
1760.0
35
RF1_1V0
59.0
51.2
6.2
6.1
59.0
51.2
4.4
5.0
949.5
49.5
RF2_1V0
44.1
47.0
5.9
5.5
44.1
41.1
4.3
5.7
944.6
54.4
CLK_1V8
30.5
17.9
3.1
4.0
30.5
17.9
3.5
3.5
1737.0
58
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4.2
4.2.1
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System Tests
Sensor Configuration
Table 9. Sensor Configuration
Parameter
Value
Start frequency (GHz)
Frequency slope (MHz)
68.65
Tx start time (uSec)
0
ADC start time (uSec)
5.99
Idle time (uSec)
12
ADC samples
256
Sample rate (Kbps)
5000
Ramp end time
58.22
Rx gain
4.2.2
Comments
77
Inter Chirp power saving enabled for idle time+Tx start time >10usec
48
System Test With Static Object
The test involves detecting an object at approximately 1m from the sensor. Both captures with LDO and
LC filter schemes are shown below. No major degradation in the signal power is detected as shown
below.
8
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Figure 4. Capture With LDO Scheme, RX Gain-48dB
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Figure 5. Capture With LDO Scheme, RX Gain-30dB
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Figure 6. Capture With LC Filter Scheme, RX Gain-48dB
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Figure 7. Capture With LC Filter Scheme, RX Gain-30dB
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Table 10. SNR Comparison Between LDO and LC Filter Scheme for RX Gain = 30 and 48
PM Scheme
LDO
LC Filter
4.2.3
RX Gain(dB)
SNR(dB)-RBW 152.6Hz
30
81.93
48
83.9
30
79.6
48
81.9
Spur/Noise Floor Level
For Spur level measurements, a continuous tone at 77 GHz is transmitted and the received signal is
analyzed with 524288 samples for the presence of the PMIC switching frequency spur at ~4 MHz. The Rx
gain is set at 48dB with an observation that PMIC spur level tracks the Rx gain.
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Figure 8. LDO Scheme –Spur Level
NOTE: No PMIC switching Frequency (approximately 4 MHz) Spur seen.
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Figure 9. LDO Scheme - Noise Floor With Tx –OFF
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Figure 10. LC Filter Scheme – Spur Level
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NOTE: PMIC spur is observed at -84.53 dBFS at RX gain of 48dB.
Figure 11. LC Filter Scheme - Noise Floor With Tx –OFF
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Summary
5
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Summary
This application report proposes two power management schemes for XWR1xxx devices. Section 4 puts
forth an optimal scheme with respect to bill of material and power dissipation, using LC filters. Section 3
explains a scheme using LDOs for achieving better power supply ripple/noise rejection. The LC Filter
solution scores better on the Cost and overall System Power saving front in comparison to the LDO
scheme."
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