Texas Instruments | TPS65311-Q1/TPS65310A-Q1 GPFET Soft Start Using a Capacitor | Application notes | Texas Instruments TPS65311-Q1/TPS65310A-Q1 GPFET Soft Start Using a Capacitor Application notes

Texas Instruments TPS65311-Q1/TPS65310A-Q1 GPFET Soft Start Using a Capacitor Application notes
Application Report
SLVA792 – November 2016
TPS65311-Q1/TPS65310A-Q1 GPFET Soft Start Using a
Capacitor Between GPFET Pin and PMOS-Protection
Switch Source Pin
Krishnamurthy Hegde .......................................................................................................... MSA-ASP
ABSTRACT
TPS65311-Q1 and TPS65310A-Q1 devices are High-Voltage Power-Management Integrated Circuits
(ICs) for use in Automotive Applications such as Advanced Driver Assistance Systems (ADAS). These
devices provide the gate drive signal for the external PMOS-protection switch, which connects input
voltage to the BUCK1 external-switching MOSFET. With the help of a small capacitor connected from the
source of the external PMOS-protection switch to the gate drive pin (GPFET pin), it is possible to adjust
the turnon time of this external PMOS-protection switch to reduce the inrush current when PMOSprotection switch turns on.
This application report provides guidelines for choosing the optimum soft-start capacitor between the
GPFET pin and external PMOS-protection switch source pin to reduce the inrush current during turn on.
This method was verified on the TPS65311-Q1 device. However, this part of the circuit is identical
between TPS65310A-Q1 and TPS65311-Q1 devices and the results are applicable to the TPS65310A-Q1
device also.
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
The TPS65311-Q1/TPS65310A-Q1 devices have an external PMOS gate drive output (GPFET), which
turns on and off the main input supply (VINPROT) and BUCK1 input supply with the help of an external
PMOS switch based on certain device operating conditions. The TPS65311-Q1 datasheet provides further
details related to GPFET pin operating conditions.
Internal to the device, a PMOS transistor connected from VIN pin to GPFET pin pulls the GPFET pin high
(to Vin voltage level) to keep the external PMOS switch in off condition. During the turn on process of the
external PMOS switch, the internal PMOS switch connected from Vin to GPFET is turned off and GPFET
pin is pulled low with the help of 20-µA (typical) pulldown current source. Detailed specifications of the
GPFET pin is provided in Table 1.
As shown in Figure 1, with the help of an external soft-start capacitor (Css_gpfet) connected between
source pin of external PMOS-protection switch and GPFET pin, it is possible to adjust the turn on
characteristics of the external PMOS-protection switch connected to the GPFET pin.
Table 1. GPFET Pin Specifications From TPS65311-Q1 Datasheet (SLVSCA6 Rev B)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
EXTERNAL PROTECTION
VCLAMP
Gate to source clamp voltage VIN – GPFET, 100 µA
14
20
V
IGPFET
Gate turn on current
VIN = 14 V, GPFET = 2 V
15
25
µA
RDSONGFET
Gate driver strength
VIN = 14 V, turn off
25
Ω
SLVA792 – November 2016
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TPS65311-Q1/TPS65310A-Q1 GPFET Soft Start Using a Capacitor Between
GPFET Pin and PMOS-Protection Switch Source Pin
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1
Test Setup and Procedure
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VSSENSE
1 k:
1
VIN
To VBAT
2
Cin_1
Css_gpfet
PCH
GPFET
20 µA
VINPROT
PMOS Protection
Switch
3
L_in
(Optional)
4
50 ± 60 V
Cin_2
GND
Cin_3
(Optional)
To BUCK1 Controller
(Switching MOSFET)
56
Figure 1. GPFET PMOS Switch Drive Control Circuitry With Css_gpfet
2
Test Setup and Procedure
A
•
•
•
•
•
•
•
TPS65311EVM was used to check the effect of Css_gpfet with the following test conditions:
Vbat (Input supply) = 12 V (Bench-top power supply with 4-A current rating)
BUCK1 Output Voltage, VBUCK1 = 3.3 V
External PMOS switch: SI7461DP
Lout_BUCK1 = 10 µH
Cout_BUCK1 = 100 µF (total capacitance)
Input LC filter inductor value (L_in) = 10 µH
Input capacitor values are as follows:
– Cin_1 = 22 µF
– Cin_2= 47 µF
– Cin_3= 47 µF
Functionality is tested at room temperature with no external load conditions.
3
Test Procedure and Measurement Results
To understand the effect of the Css_gpfet capacitor on overall system behavior, different measurements
were done as below.
• TPS65311-Q1 start-up behavior with Vbat ramped from 0 V to 12 V is captured without an external
Css_gpfet capacitor as a reference as shown in Figure 2.
• The measurement is repeated with the following different Css_gpfet values: 1 nF, 2.2 nF,4.7 nF, 10 nF,
15 nF, 100 nF, and 470 nF.
• For each case, different timing measurements are taken and listed in Table 2.
• Input inrush current during start up is measured and its rise time and peak current are tabulated in
Table 3. Timing diagram for this measurement is shown in Figure 3.
• Shutdown behavior is also measured to see the effect of Css_gpfet on device shutdown.
2
TPS65311-Q1/TPS65310A-Q1 GPFET Soft Start Using a Capacitor Between
GPFET Pin and PMOS-Protection Switch Source Pin
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Test Procedure and Measurement Results
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3.1
Device Start Up Behavior Measurement Results
C1 (Yellow): VBUCK1 output voltage (3.3 V)
C2(Red): GPFET pin Voltage
C3(Blue): VINPROT voltage
C4(Green): Vbat (input supply) voltage
T1: Time from VBat start (0V) to VINPROT start (0 V)
T2: Time from VBat start (0V) to VBUCK1 start (0 V)
T3: Time from VBat=4.2 V (VPOR) to VBUCK1 start (0 V)
T4: Time from VBat=4.2 V(VPOR) to VINPROT start (0 V)
T5: Time from VINPROT start (0V) to VBUCK1 start (0 V)
T6: Time from VBUCK1 start (0V) to VBUCK1 end (3.3 V)
T7: Time from VINPROT start (0V) to VBUCK1 end (3.3 V)
T8: Time from VINPROT start (0V) to VINPROT = 10 V
Figure 2. Start-up Timing Diagram
Table 2. Timing Measurement Results With Different Soft-Start Capacitors Across GPFET Pin and
External PMOS Switch Source Pin
Css_gpfet
T1 (ms)
T2 (ms)
T3(ms)
T4 (ms)
T5 (µS)
T6 (ms)
T7 (ms)
T8 (ms)
No Css_gpfet
2.001
2.404
1.741
1.339
404.32
1.214
1.603
1.665
1nF
2.105
2.513
1.867
1.460
431.32
1.219
1.630
1.691
2.2nF
2.234
2.649
1.995
1.578
447.56
1.215
1.645
1.645
4.7nF
2.473
2.964
2.300
1.827
483.43
1.212
1.694
1.527
10nF
2.994
3.544
2.892
2.342
537.95
1.201
1.743
1.293
15nF
3.467
4.204
3.555
2.819
737.33
1.214
1.945
1.373
100nF
11.577
13.673
12.974
10.646
2276.06
1.216
3.363
3.115
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3
Test Procedure and Measurement Results
3.2
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Input Inrush Current Measurement Results
C1 (Yellow): VBUCK1 output voltage
(3.3 V)
C2(Red): Vbat voltage
C3(Blue): VINPROT voltage
C4(Green): Input (Vbat) inrush current
Trise_Iin: Inrush current rise time
measured from ~100 mA current to its
peak value
Figure 3. Start-up Timing Diagram With Input Inrush Current
Table 3. Input Inrush Current Measurement Results
With Different Css_gpfet Capacitors
3.3
Css_gpfet
Trise_Iin (µs)
Peak Inrush Current (A)
No Css_gpfet
310
1.9
1 nF
337
1.8
2.2 nF
382
1.8
4.7 nF
408
1.8
10 nF
553
1.7
15 nF
614
1.6
100 nF
1949
0.8
Device Shutdown Behavior Results
For each case (No Css_gpfet, Css_gpfet = 1 nF, 2.2 nF, 4.7 nF, 10 nF, 15 nF, and 100 nF), device
shutdown behavior is checked to see whether Css_gpfet influences the shutdown behavior. Based on the
measurement results, it is clear that this capacitor does not have much influence on the shutdown
behavior. This is also expected per design due to the presence of a strong internal pullup transistor (with
max Rdson resistance = 25 ohm) between GPFET pin and VIN pin. Figure 4 and Figure 5 show the
shutdown behavior without Css_gpfet and with Css_gpfet = 100 nF, respectively.
4
TPS65311-Q1/TPS65310A-Q1 GPFET Soft Start Using a Capacitor Between
GPFET Pin and PMOS-Protection Switch Source Pin
Copyright © 2016, Texas Instruments Incorporated
SLVA792 – November 2016
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Test Procedure and Measurement Results
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Figure 4. Shutdown VINPROT High to Low - No Css_gpfet
3.4
Figure 5. Shutdown VINPROT High to Low - With
Css_gpfet = 100 nF
Device Start Up Behavior With Large Css_gpfet Capacitor
Since the external PMOS-protection switch is turned off with the help of a weak pulldown current source
(20 µA, typical, internal to the GPFET pin), a very large Css_gpfet may prevent the PMOS-protection
switch from turning on. To check this behavior, Css_gpfet value was increased to 470 nF. With this
capacitor value, the external PMOS-protection switch does not turn on correctly as shown in Figure 7.
Figure 6. Start-up behavior With Css_gpfet = 100 nF
4
Figure 7. Start-up behavior With Css_gpfet = 470 nF
Observations and Conclusions
Measurement results show that with the help of Css_gpfet capacitor, it is possible to control the turnon
characteristics of external P-MOS protection switch to reduce the inrush current during its turn on. An
optimum Css_gpfet capacitor value depends mainly on external application conditions, hence the exact
value of this capacitor cannot be provided. Select this capacitor value based on measurements done on
the actual application. Use this application note only as a reference document for optimizing the Css_gpfet
capacitor.
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5
Revision History
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Based on the measurements done with the application condition mentioned in the Section 3 section, the
following observations and conclusions are made:
• With a Css_gpfet capacitor (less than 100 nF) between GPFET pin and source of external PMOS
transistor, it is possible to adjust the timing between start of BUCK1 output or start of VINPROT
voltage with respect to Vbat.
• With the help of Css_gpfet capacitor, input inrush current magnitude and its slew rate can be reduced.
• Rise time of BUCK1 output is not affected by Css_gpfet.
• Rise time of VINPROT, with reference to the time at which VBUCK1 reaches 3.3 V, is slowed down
with increasing Css_gpfet.
• Higher value capacitors, like 470 nF for Css_gpfet, will not allow GPFET to turn on since the 20 µA
pulldown current source is too weak to pull the GPFET pin sufficiently low.
• Shutdown behavior does not change much with different Css_gpfet values.
• Since the effect of Css_gpfet mainly depends on actual components (Example: Cout, Lout, LC filter,
PMOS-protection switch, and so forth) used on the board, it is recommended to finalize this capacitor
value based on actual measurements done on the customer board. Keep this capacitor as small as
possible.
• Also, results may vary depending on the input power supply used. The recommendation is to use a
real car battery for more accurate measurements.
• All timing specifications related to device power-up sequencing in the datasheet are specified without
considering the Css_gpfet capacitor and some of the power-up timing parameters may change with the
addition of Css_gpfet capacitor. For example, tSTART timing (TPS65311-Q1 datasheet, Figure 24.
Power-up Sequencing) may be changed with addition of Css_gpfet capacitor.
Revision History
6
DATE
REVISION
NOTES
November 2016
*
Initial Release
Revision History
SLVA792 – November 2016
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