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Texas Instruments Input Current Limit Solution for USB-Supplied Power System Application notes
Application Report
SLVA912 – August 2017
Input Current Limit Solution for USB-Supplied Power
System
Helen Chen
ABSTRACT
The maximum allowed output current from a USB rail is limited in a USB-powered system. A unit load is
defined as 100 mA in USB 2.0. A device may draw a maximum of five unit loads (500 mA) from a port in
USB 2.0. Most boost converters cannot limit input current to such a low value during start-up, especially in
a high output capacitance application. This application note details an input current limit solution for a
USB-powered system. With this solution, the designer can limit the input current well below the value set
during start-up and normal operation conditions.
1
2
3
Contents
Introduction ................................................................................................................... 2
Design Process .............................................................................................................. 2
Conclusion .................................................................................................................... 5
List of Figures
1
Schematic – Input Current Limit Solution ................................................................................. 3
2
Start-Up Waveforms of VIN, IIN, VADJ........................................................................................ 4
3
Start-Up Waveforms of VIN, IIN, VFB......................................................................................... 5
List of Tables
1
Performance Specification Summary
.....................................................................................
2
Trademarks
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1
Introduction
1
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Introduction
In a USB-powered application, the maximum supplied current must be limited below the maximum output
current capability of the USB port; if not, the USB voltage drops, perhaps even to zero, due to overcurrent
protection. This application note details an input current limit solution for a USB-powered system and
contains a circuit design, test result, schematic, and bill of materials (BOM).
2
Design Process
2.1
Specification
Table 1 shows the specification of this application note. The maximum input current is limited below
200 mA during start-up and normal operation.
Table 1. Performance Specification Summary
2.2
SPECIFICATION
TEST CONDITIONS
MIN
TYP
MAX
VIN
—
4.75
5.00
5.25
UNIT
V
IIN
During start-up and normal operation
—
150
200
mA
VOUT
VIN = 5 V, 60-mF capacitive load
8.7
9.0
9.3
V
Schematic
Figure 1 shows the schematic of the input current limit solution. The U1 TLV61046 is a 28-V output
voltage boost converter with power diode and isolation switch. The U2 INA138 is a high-side
measurement current shunt monitor. U3 is an operational amplifier.
Resistor R5 and diode D1 are connected from the input to the output side. These components are used to
charge up the output capacitance because the output voltage must be equal to the input voltage before
enabling the TLV61046 integrated circuit (IC). If the output voltage is not equal to the input voltage, the
input current rises to an uncontrolled high value while the VOUT charges up to the VIN stage, especially in
the big output capacitance application. Enable the TLV61046 IC after the VOUT finishes charging up to VIN
through R5 and D1.
The input current immediately rises up to the input current limit point after the boost converter has been
enabled. The output voltage of the INA138 VO_INA138 jumps high and D4 conducts. VADJ is equal to VO_ INA138.
The FB pin voltage VFB immediately rises up to VREF. So, the input current is immediately regulated and is
well limited below the setting value. When the output voltage charges up to the target point, the input
current goes low and D4 stops conducting.
2
Input Current Limit Solution for USB-Supplied Power System
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Design Process
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D1
L1
Rs
Vin
2
1
Vin-
0.5
6.2µH
J1
C1
10µF
U1
2
1
Vin
5
EN
1 SW
VIN
SW
4
EN
VOUT
5
FB
3
GND
2
VOUT
2
1
V+
J5
J3
PGND
U2
FB
R1
107k
C2
1µF
C3
10µF
2
1
GND
J4
TLV61046DBVR
PGND
2
3
4
VIN-
VIN+
1 Vin
2
3
C9
0.1µF
1
INA138NA/3K
PGND
OUT
J2
6
VCC
R4
PGND
VCC
C6
10µF
PGND
5
V_INA138
4.70
D4
3
R3
1
U3
R9
82.5k
PGND
C8
1000pF
PGND
Radj
10.5k
1N4148WT
2
4.70
Vadj
4
V+
V-
R2
10.5k
PGND
PGND
Copyright © 2017, Texas Instruments Incorporated
Figure 1. Schematic – Input Current Limit Solution
2.3
Component Selection
The maximum voltage drop across the precharge resistor R5 is 5 V, which happens at the VIN = 5 V,
VOUT = 0 V condition. So a 499-Ω, 0603 resistor is chosen in this application.
The maximum differential input voltage for the INA138 is 0.5 V and the recommended value is 0.1 V;
therefore, choose RS = 0.5 Ω in this application. When IIN = 0.2 A, the sensed voltage is just 0.1 V.
The highest input current occurs when the TLV61046 is enabled. As Equation 1 shows, the initial output
voltage is equal to:
VO _ INI = V IN = 5 V
(1)
To effectively limit the input current during start-up, the superposition voltage at the FB pin must be equal
to the feedback voltage VREF at this time (see Equation 2).
æ R P1 ö
æ
ö
R P2
VO _ INI ´ ç
÷ + VADJ ´ ç
÷ = VREF
ç R P1 + R 1 ÷
ç R P2 + R ADJ ÷
è
ø
è
ø
(2)
where,
• R ADJ = 10.5 kW initially,
•
•
R P1 =
R ADJ ´ R2
R ´ R2
, R P2 = 1
,
R ADJ + R 2
R1 + R2
V REF = 0.807 V.
Using the calculation from Equation 2 results in the following Equation 3:
æ R P1 ö
VREF - VO _ INI ´ ç
÷
ç R P1 + R1 ÷
è
ø
V ADJ =
R P2
R P2 + R ADJ
(3)
VADJ = 1.203 V; therefore, the output voltage of the INA138 device is also equal to 1.203 V.
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Design Process
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Calculate Equation 4 using the specifications from the INA138 data sheet:
VO _ INA138 = IS ´ RS ´ (200 mA/V ) ´ R9
(4)
Calculate R9 using the following formula in Equation 5:
VO _ INA138
R9 =
IS ´ RS ´ (200 mA/V )
(5)
where,
• RS = 0.5 Ω.
The designer must leave some margin during the theoretical calculation to limit the input current below
200 mA. When leaving a 25% margin, let IIN = 150 mA, then R9 = 82.5 kΩ.
2.4
Test Result
Figure 2 and Figure 3 show the start-up waveforms of the VIN, IIN, VADJ, and VFB pins with a 60-mF
electrolytic capacitor at the output side.
The input current increases immediately when the IC is enabled. The voltage VADJ jumps up from 0.45 V to
1.2 V, which makes the VFB immediately jump up to VREF. So the TLV61046 device can regulate the input
current immediately at the time in which it is enabled. Thus the input current is well limited below 200 mA
during start-up at the big output capacitance condition.
Figure 2. Start-Up Waveforms of VIN, IIN, VADJ
4
Input Current Limit Solution for USB-Supplied Power System
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Conclusion
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Figure 3. Start-Up Waveforms of VIN, IIN, VFB
3
Conclusion
This application note describes an input current limit solution for the boost converter TLV61046 in a USBpowered application. This solution is realized by a high-side measurement current shunt monitor INA138
and a single low-voltage rail-to-rail output operational amplifier LMV321. The input current can be well
limited below the setting point after start-up. When the output voltage rises up to the target value, the input
current goes low and the current limit circuit stops working and ceases to affect the normal operation.
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