Texas Instruments | Extending the Soft Start Time Without a Soft Start Pin (Rev. B) | Application notes | Texas Instruments Extending the Soft Start Time Without a Soft Start Pin (Rev. B) Application notes

Texas Instruments Extending the Soft Start Time Without a Soft Start Pin (Rev. B) Application notes
Application Report
SLVA307B – August 2008 – Revised June 2017
Extending the Soft Start Time Without a Soft Start Pin
Chris Glaser .......................................................................................... Low Power DC-DC Applications
ABSTRACT
In battery-powered equipment, extending the soft start time can be crucial to a glitch-free start-up.
Especially toward the end of a battery's life, the voltage drop and increasing impedance of the battery from
excessive inrush current into the power supply can be a problem. This application report demonstrates a
simple circuit that extends the soft start time and reduces the inrush current on the TPS6107x family of
boost converters.
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5
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Contents
TPS6107x Soft Start Operation ............................................................................................
Extending the Soft Start Time ..............................................................................................
Setting the Soft Start Time .................................................................................................
Transient Response .........................................................................................................
Inrush Current Reduction ...................................................................................................
Load Regulation..............................................................................................................
Conclusion ....................................................................................................................
References ...................................................................................................................
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2
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5
6
7
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List of Figures
1
TPS61070EVM With Added Soft Start Circuit ........................................................................... 2
2
TPS61070EVM Soft Starting Into 50-Ω Load – 743-μs Soft Start Time .............................................. 3
3
Figure 1 Circuit Soft Starting Into 50-Ω Load with C6 = 1000 pF and R3 = 162 kΩ – 971-μs Soft Start Time
4
Figure 1 Circuit Soft Starting Into 50-Ω Load with C6 = 1800 pF and R3 = 162 kΩ – 1.379-ms Soft Start
Time ........................................................................................................................... 4
5
TPS61070EVM Transient Response Without Soft Start Circuit ....................................................... 5
6
Figure 1 Circuit Transient Response With C6 = 1800 pF and R3 = 162 kΩ ......................................... 5
7
TPS61070EVM Start-up Powered From Two AA Batteries Without Soft Start Circuit
.
3
9
.............................
Figure 1 Circuit Start-up Powered From Two AA Batteries With C6 = 1800 pF and R3 = 162 kΩ ...............
Load Regulation Effect of the Soft Start Circuit ..........................................................................
1
Soft Start Times and Inrush Currents of the Circuit With Different Values of C6 and R3 .......................... 4
8
6
6
7
List of Tables
Trademarks
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SLVA307B – August 2008 – Revised June 2017
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1
TPS6107x Soft Start Operation
1
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TPS6107x Soft Start Operation
The TPS6107x has three cycles or phases of soft start:
• Cycle 1 (Precharge) charges the output capacitors to the input voltage. The internal synchronous FET
operates in the linear region and delivers a DC, fixed current to the output capacitor during this phase,
as shown in Figure 22 in the data sheet (SLVS510). The TPS6107x enters cycle 1 when enabled and
Vin is above the undervoltage lockout. It exits cycle 1 after charging the output voltage up to a value
that is almost equal to the input voltage.
• Cycle 2 (Fixed Duty Cycle Control) operates after cycle 1 ends. During cycle 2, the TPS6107x switches
with a fixed 70% duty cycle. The TPS6107x enters cycle 2 when the cycle 1 is complete and the output
voltage is less than 1.8 V. If the output voltage is greater than 1.8 V at the end of cycle 1, then cycle 2
is skipped. The TPS6107x only leaves cycle 2 when the output voltage is greater than 1.8 V.
• Cycle 3 (Reduced Current Limit) starts normal boost converter operation. During the third cycle of soft
start, the error amplifier takes over and switches at the proper duty cycle, based on input and output
voltage. During this cycle, the current limit is reduced to 50% of the nominal current limit to minimize
inrush current. The TPS6107x enters cycle 3 directly from cycle 1 when the input voltage is greater
than 1.8 V. It enters cycle 3 directly from cycle 2 when the input voltage is less than 1.8 V. The
TPS6107x leaves cycle 3 and begins normal operation with the standard current limit when the output
capacitor charges to the nominal output voltage.
2
Extending the Soft Start Time
The technique used in this application report to extend the soft start time adds an RC circuit connected
through a diode to the FB pin. The diode provides isolation of the capacitor, so that it affects neither the
steady state operation nor the dynamic response of the converter. Figure 1 shows the TPS61070EVM
with the added soft start circuit (C6, R3, and D1). By feeding a current onto R2 through C6 and D1, the
TPS61070's error amplifier reacts as if the output voltage is higher than it actually is and reduces the duty
cycle accordingly. Thus, the output voltage increases slower. As the output voltage comes into regulation,
C6 charges up and becomes more and more of an open circuit, while R3 pulls the remaining charge to
ground. Thus, soft start is ended and the error amplifier regulates the output voltage to the proper level.
VIN
C1
10 mF
L1
4.7 mH
U1
TPS61070DDC
6
1
VBAT
SW
2 GND VOUT 5
3
FB 4
EN
VOUT
R1
909 kW
C2
4.7 mF
C3
4.7 mF
R2
100 kW
C6
D1
R3
Added SS Circuit
Figure 1. TPS61070EVM With Added Soft Start Circuit
Figure 2 shows the TPS61070EVM without any external soft start circuit starting into a 50-Ω load with 3.6V input voltage. Figure 3 and Figure 4 show the same EVM and load conditions but the soft start circuit
has been added. The diode used in these tests was the small signal, silicon diode 1N4148.
2
Extending the Soft Start Time Without a Soft Start Pin
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Extending the Soft Start Time
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5 V/div
1 V/div
200 mS/div
Figure 2. TPS61070EVM Soft Starting Into 50-Ω Load – 743-μs Soft Start Time
5 V/div
1 V/div
200 mS/div
Figure 3. Figure 1 Circuit Soft Starting Into 50-Ω Load with C6 = 1000 pF and
R3 = 162 kΩ – 971-μs Soft Start Time
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3
Setting the Soft Start Time
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5 V/div
1 V/div
200 mS/div
Figure 4. Figure 1 Circuit Soft Starting Into 50-Ω Load with C6 = 1800 pF and
R3 = 162 kΩ – 1.379-ms Soft Start Time
3
Setting the Soft Start Time
The soft start time is roughly proportional to the product of R3 and C6. As this product goes up, the soft
start time increases and the inrush current is reduced. If the product is too low, hardly any soft start time is
added. Note that the allowable inrush current during the first cycle of soft start [Figure 22 in the TPS61070
data sheet (SLVS510)] cannot be reduced.
Table 1 shows measured soft start times with various values of R3 and C6. The amount of output
capacitance and expected load during start-up are also critical factors in determining the soft start time for
a particular application. Table 1 should be used as a guide to selecting R3 and C6. Laboratory verification
is necessary to ensure a particular soft start time for a given system with a given output capacitance and
load in the midst of component variation over tolerance and temperature.
Table 1. Soft Start Times and Inrush Currents of the Figure 1 Circuit With
Different Values of C6 and R3
(1)
(2)
4
R3 (Ω)
C6 (nF)
Soft Start Time (1) (ms)
Peak Inrush Current (2)
(mA)
10 k
1.8
0.8
200
10 k
15
1.1
120
10 k
47
3.2
100
10 k
100
6.4
100
10 k
150
9.5
100
162 k
1
1.0
120
162 k
1.8
1.4
100
162 k
4.7
3.8
100
162 k
10
7.8
100
162 k
15
11.2
100
Measured on the Figure 1 Circuit with 3.6 V input and 50-Ω load
Measured on the Figure 1 Circuit with two depleted AA batteries (~2.1 V) and 500-Ω load
Extending the Soft Start Time Without a Soft Start Pin
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Transient Response
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4
Transient Response
One way to see if the control loop is affected is to look at the response of the circuit to a load step and see
if this response is changed significantly by the addition of the soft start circuitry.
Figure 5 and Figure 6 show the resulting output voltage deviation of the Figure 1 circuit when subjected to
a 20-mA to 180-mA load step. (Note: during light load (20 mA), the TPS61070 goes into PFM mode, so
the output ripple is larger).
100 mV/div-AC
100 mA/div-DC
100 mS/div
Figure 5. TPS61070EVM Transient Response Without Soft Start Circuit
100 mV/div-AC
100 mA/div-DC
100 mS/div
Figure 6. Figure 1 Circuit Transient Response With C6 = 1800 pF and R3 = 162 kΩ
Because Figure 5 and Figure 6 are nearly identical, the added soft start circuit has not affected the control
loop significantly.
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5
Inrush Current Reduction
5
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Inrush Current Reduction
Figure 7 and Figure 8 demonstrate how the additional soft start circuitry reduces the sagging battery
voltage caused by the large inrush current at turn on. Two AA batteries connected in series with a
combined voltage of 2.1 V power the TPS61070EVM, which has a 500-Ω load. Without the soft start
circuitry, the TPS61070 has an inrush current of 200 mA, which results in a battery voltage droop of 260
mV. With the soft start circuit, the inrush current is only 100 mA, which results in a 140-mV droop in
battery voltage.
5 V/div
5 V/div
1 V/div
100 mA/div
400 mS/div
Figure 7. TPS61070EVM Start-up Powered From Two AA Batteries Without Soft Start Circuit
5 V/div
5 V/div
1 V/div
100 mA/div
400 mS/div
Figure 8. Figure 1 Circuit Start-up Powered From Two AA Batteries With C6 = 1800 pF and R3 = 162 kΩ
6
Extending the Soft Start Time Without a Soft Start Pin
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Load Regulation
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6
Load Regulation
The added soft start circuit has the possibility of introducing some additional output voltage regulation
(load regulation) depending on the operating conditions of the circuit and the specific device and
component values used. Figure 9 shows that the added soft start circuit with C6 = 15 nF and R3 = 162 kΩ
does not worsen the load regulation more than the TPS61070EVM without the soft start circuit. Therefore,
the soft start circuit does not create any additional load regulation on the TPS61070. If this circuit is used
with other devices, the load regulation should be checked.
5.10
Output Voltage (V)
5.05
3.6 VIN with
Series1
SS
Circuit
2 VIN with
Series2
SS
Circuit
3.6 VIN without
Series4
SS
Circuit
2 VIN without
Series5
SS
Circuit
5.00
4.95
4.90
4.85
4.80
1
10
100
Load Current (mA)
1000
C001
Figure 9. Load Regulation Effect of the Soft Start Circuit
7
Conclusion
This application report has demonstrated a simple circuit to extend the soft start time of the TPS6107x
family of boost converters, while also reducing the inrush current drawn from a battery. The addition of a
resistor, capacitor, and diode creates a user-programmable soft start time that does not affect the control
loop and thus does not affect the circuit’s response to a load step or its load regulation. This basic circuit
is applicable in principal to any TPS6xxxx device. Checking the load regulation and control loop stability
should be done to assess its applicability to specific devices besides the TPS6107x family.
8
References
•
•
TPS61070 Datasheet (SLVS510)
Design considerations for a resistive feedback divider in a DC/DC converter (SLYT469)
Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from A Revision (February 2013) to B Revision ............................................................................................. Page
•
Changed document title.
.................................................................................................................
SLVA307B – August 2008 – Revised June 2017
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Revision History
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