Application Report
`SLUA928A–December 2018–Revised March 2019
`
`Low Dropout Operation in a Buck Converter
`
`Ryan Hu
`
`ABSTRACT
`Certain applications require the DC/DC to maintain output voltage regulation when the input voltage is
`only slightly higher than the target output voltage. They still require it to be regulated although the input
`voltage is lower than the target output voltage in some extreme cases. For a buck converter, these low
`dropout operation conditions require high duty cycle operation that may approach 100%. A similar
`description can be found like the following sentence in some buck converters datasheet: “To improve drop
`out, the device is designed to operate at 100% duty cycle as long as the BOOT-to-PH pin voltage is
`greater than 2.1 V (typical)". However, when operating near 100% duty cycle with light
`loads,
`the
`Bootstrap capacitor can be discharged below the BOOT UVLO threshold, causing high ripple voltage on
`the output side. This application report discusses the cause and various operating modes associated with
`the low dropout operation. The TPS54231 is an example to introduce this operation behavior and
`providing work-around to maintain the Bootstrap capacitor voltage.
`
`Contents
`Introduction ................................................................................................................... 2
`TPS54231 Low Dropout Operation........................................................................................ 2
`Solutions for Non-synchronous Part....................................................................................... 5
`Design Example.............................................................................................................. 6
`Low Dropout Operation Improved in Synchronous Part .............................................................. 15
`Conclusion .................................................................................................................. 16
`References.................................................................................................................. 16
`
`List of Figures
`TPS54231 Test Circuit...................................................................................................... 3
`TPS54231 Waveforms at VIN = 4.25 V, IOUT = 2 A ....................................................................... 3
`TPS54231 Waveforms at VIN = 3.87 V, IOUT = 2 A ....................................................................... 4
`TPS54231 Waveforms at VIN = 5 V, IOUT = 0.1 A ........................................................................ 4
`TPS54231 Waveforms at VIN = 5 V, IOUT= 10 mA........................................................................ 5
`TPS54231 Waveforms at VIN=5 V, IOUT=0 A.............................................................................. 5
`TPS54231 Circuit with Solution (1) + (A)................................................................................. 6
`TPS54231 Circuit with Solution (2) + (A)................................................................................. 6
`Current in Auxiliary Diode and Inductor................................................................................... 7
`Current in Auxiliary Diode and Inductor (Zoom In) ...................................................................... 7
`Reverse Current under Different Reverse Voltage...................................................................... 8
`Bootstrap Capacitor Voltage Waveform with VIN = 4 V, IOUT = 3 mA................................................. 10
`Bootstrap Capacitor Voltage Waveform with VIN = 4 V, IOUT = 10 mA ............................................... 10
`Bootstrap Capacitor Voltage Waveform with VIN = 4 V, IOUT = 1 A................................................... 11
`Bootstrap Capacitor Voltage Waveform with VIN = 4 V, Variation IOUT............................................... 11
`Startup Waveform with VIN = 4 V, IOUT = 2 A ............................................................................ 12
`Startup Waveform with VIN = 4 V, IOUT = 3 mA.......................................................................... 12
`Load Transient of Solution (1)............................................................................................ 13
`Load Transient of Solution (2)............................................................................................ 13
`
`1
`2
`3
`4
`5
`6
`7
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`12
`13
`14
`15
`16
`17
`18
`19
`
`SLUA928A–December 2018–Revised March 2019
`Submit Documentation Feedback
`
`Copyright © 2018–2019, Texas Instruments Incorporated
`
`Low Dropout Operation in a Buck Converter
`
`1
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`Introduction
`20
`21
`22
`23
`24
`25
`
`www.ti.com
`Load Transient of Original Converter.................................................................................... 13
`Efficiency of Solution (1) with 3.3 V Output under Different Input Voltage ......................................... 14
`Efficiency Comparison with 5 V Input and 3.3 V Output .............................................................. 14
`Efficiency Comparison with 8 V Input and 5 V Output ................................................................ 15
`TPS54202 Waveforms, VIN = 4.9 V, VOUT = 5 V (Target), IOUT = 2 A ................................................. 15
`TPS56339 Waveforms, VIN = 4.9 V, VOUT = 5 V (Target), IOUT = 3 A ................................................. 16
`
`List of Tables
`The Entry and Recovery Voltages with Different Schottky Diodes.................................................... 8
`1
`The Entry and Recovery Voltages with VOUT = 3.3 V.................................................................... 8
`2
`The Entry and Recovery Voltages with VOUT = 5 V ...................................................................... 9
`3
`Trademarks
`Eco-mode is a trademark of others.
`
`1
`
`2
`
`Introduction
`An N-channel MOSFET is widely used in a buck converter as the high-side (HS) switch. To drive this HS
`MOSFET properly, a bootstrap circuit is designed to generate a floating power supply between gate node
`and source node of the MOSFET and an external small ceramic capacitor between the BOOT and PH
`pins is required. This bootstrap capacitor is refreshed when the HS MOSFET is off and the catch diode or
`low-side (LS) MOSFET conducts. An undervoltage lock-out (UVLO) circuit is also required for the gate
`drive supply to keep the converter from attempting to switch when the gate drive may be too low.
`Certain applications require the DC/DC to maintain output voltage regulation when the input voltage is
`only slightly higher than the target output voltage. They still require it to be regulated although the input
`voltage is lower than the target output voltage in some extreme cases. For a buck converter, these low
`dropout operation conditions require high duty cycle operation that may approach 100%. However, when
`operating near 100% duty cycle with light loads, the Bootstrap capacitor can be discharged below the
`BOOT UVLO threshold, causing high ripple voltage on the output side.
`This application report discusses the cause and various operating modes associated with the low dropout
`operation. This report uses the TPS54231 as an example to introduce this operation behavior and
`providing work-around to maintain the Bootstrap capacitor voltage. The end of this report introduces some
`synchronous parts with the low dropout operation has been improved.
`
`TPS54231 Low Dropout Operation
`The input voltage range for the TPS54231 is 3.5 V to 28 V, and the rated output current is 2 A. The
`TPS54231 is non-synchronous and its low-side switching element is an external catch diode. The
`TPS54231 can operate in both continuous conduction mode (CCM) and discontinuous conduction mode
`(DCM) depending on the output current.
`Figure 1 shows the TPS54231 circuit used for testing. Some modifications are needed to investigate low
`dropout.
`1. Remove R1 and R2 to float EN pin. Internal input voltage UVLO is used to allow operation at low input
`voltages without the device shutting off.
`2. Change R6 to 53.6 kΩ when testing the 5 V output.
`Use the TPS54231EVM-372 for evaluation which is an official EVM board for the TPS54231 device with
`3.3 V output. Similar modifications are needed if using the official EVM board.
`
`2
`
`Low Dropout Operation in a Buck Converter
`
`Copyright © 2018–2019, Texas Instruments Incorporated
`
`SLUA928A–December 2018–Revised March 2019
`Submit Documentation Feedback
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`
`TPS54231 Low Dropout Operation
`
`Figure 1. TPS54231 Test Circuit
`
`2.1
`
`Low Dropout Operation in CCM
`In the CCM operation, when the HS MOSFET is turned off, the inductor current continues to flow in the
`catch diode, and the diode clamps the PH node voltage at one diode drop below ground. The BOOT to
`PH voltage is (VIN + 0.7) V, allowing the Bootstrap capacitor to be fully charged. Therefore, low dropout
`operation can be obtained in CCM. Figure 2 illustrates the TPS54231 output waveforms at an input
`voltage of 4.25 V and an output current of 2 A. As the input voltage is 6 V, the Bootstrap capacitor voltage
`can remain higher than 2.1 V. The TPS54231 works in normal CCM with a nominal switching frequency of
`580 kHz.
`To improve dropout, the TPS54231 device is designed to operate at 100% duty cycle as long as the
`BOOT-to-PH pin voltage is greater than 2.1 V. For a buck converter, the duty cycle D = VOUT / VIN. Note
`that the effective duty cycle during dropout of the regulator is mainly influenced by the voltage drops
`across the power MOSFET, inductor resistance, catch diode, and printed circuit board resistance. With the
`decreasing of VIN, the duty cycle is increased to maintain the output voltage. When the duty cycle
`approaches 100%, the on-time of the HS MOSFET can be extended with the effective switching frequency
`decreased. Figure 3 illustrates the TPS54231 output waveforms at an input voltage of 3.87 V and an
`output current of 2 A. The switching frequency has been reduced to approximately 34 kHz from the
`nominal of 580 kHz.
`
`Figure 2. TPS54231 Waveforms at VIN = 4.25 V, IOUT = 2 A
`
`SLUA928A–December 2018–Revised March 2019
`Submit Documentation Feedback
`
`Copyright © 2018–2019, Texas Instruments Incorporated
`
`Low Dropout Operation in a Buck Converter
`
`3
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
`
`VOUT = 3.3V, 2A Max
`
`J3
`
`1 2
`
`TP8
`VOUT
`
`C9
`22uF
`
`C10
`22uF
`
`C11
`22uF
`
`DNP
`Rdummy
`
`R6
`49.9
`
`R7
`10.2k
`
`R8
`3.24k
`
`D2
`
`DNP
`
`SS14FL
`
`TP5
`BST
`
`D3
`
`DNP
`
`SS14FL
`
`2
`
`3
`
`4
`
`6
`
`U1
`
`VIN
`
`BOOT
`
`EN
`
`PH
`
`SS
`
`VSENSE
`
`COMP
`
`GND
`
`TPS54231D
`
`1
`
`8
`
`5
`
`7
`
`TP6
`SW
`
`C7
`
`0.1uF
`
`L1
`
`3.3uH
`
`TP7
`LOOP
`
`D1
`B240-13-F
`
`TP4
`EN
`
`C4
`15nF
`
`C5
`27pF
`
`C6
`1.5nF
`
`R3
`47.0k
`
`GND
`
` Copyright © 2018, Texas Instruments Incorporated
`
`TP9
`
`TP1
`VIN
`
`VIN= 3.5V to 8V
`
`J1
`
`12
`
`R1
`133k
`
`R2
`56.2k
`
`TP3
`
`123
`
`J2
`
`C1
`10V
`220uF
`
`C2
`10uF
`
`C3
`0.1uF
`
`Vin_start=4.08V
`Vin_stop=3.68V
`
`TP2
`
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`TPS54231 Low Dropout Operation
`
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`
`Figure 3. TPS54231 Waveforms at VIN = 3.87 V, IOUT = 2 A
`
`2.2
`
`Low Dropout Operation in DCM
`Decreasing the output current makes the device work in DCM. Figure 4 and Figure 5 show the waveforms
`of the PH node with a different output current. There are three states during each duty cycle. During ON
`state, HS MOSFET is on, the catch diode D1 is off, and the PH node voltage equals to VIN. During the
`OFF state, HS MOSFET is off, D1 is on, and the PH node voltage is clamped one diode drop below
`ground. During IDLE state, both HS MOSFET and D1 is off, the PH voltage waveform oscillates because
`the output inductor, the junction capacitance of the catch diode and HS MOSFET constitute a resonant LC
`network. The Bootstrap capacitor can be charged during OFF state and IDLE state. Thus, low dropout
`operation can be obtained in DCM.
`
`Figure 4. TPS54231 Waveforms at VIN = 5 V, IOUT = 0.1 A
`
`4
`
`Low Dropout Operation in a Buck Converter
`
`Copyright © 2018–2019, Texas Instruments Incorporated
`
`SLUA928A–December 2018–Revised March 2019
`Submit Documentation Feedback
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
`
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`
`TPS54231 Low Dropout Operation
`
`Figure 5. TPS54231 Waveforms at VIN = 5 V, IOUT= 10 mA
`
`2.3
`
`Low Dropout Operation in No-Load
`In the no-load condition, there is no current flowing in the output inductor, so the OFF state is gone. The
`PH node voltage is very close to VOUT, and the BOOT to PH voltage is (VIN-VOUT). When running in low
`dropout operation, the (VIN-VOUT) can be significantly less than the BOOT UVLO voltage. If the (VIN-
`VOUT) is less than 2.1 V (the BOOT UVLO threshold), the device stops switching and the output voltage
`decays. As the output voltage decaying, the Bootstrap capacitor is charged until the output voltage decays
`to (VIN-2.1) and the HS MOSFET turns on again. Figure 6 shows a high ripple voltage was presented on
`the output.
`
`Figure 6. TPS54231 Waveforms at VIN=5 V, IOUT=0 A
`
`When decreasing the input voltage from nominal 8 V with no load, the output begins to exhibit this ripple
`at a certain input voltage. This voltage is defined as the entry voltage. If the input voltage is increased
`again, the converter returns back to expected operation. This voltage level is defined as the recovery
`voltage. There is hysteresis between the entry and recovery voltages.
`
`3
`
`Solutions for Non-synchronous Part
`Based on the above description, two basic methods can be used to improve the low dropout operation.
`Solution (A) is adding a dummy load at the output to keep the OFF state and extend the duration time.
`Solution (B) is adding an external voltage at BOOT pin to raise the BOOT to PH voltage directly.
`
`SLUA928A–December 2018–Revised March 2019
`Submit Documentation Feedback
`
`Copyright © 2018–2019, Texas Instruments Incorporated
`
`Low Dropout Operation in a Buck Converter
`
`5
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
`
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`Design Example
`www.ti.com
`Providing Continuous Gate Drive Using a Charge Pump Application Report: Used a charge pump to boost
`the BOOT to PH voltage on the basic of solution (B), however, it requires more additional counts with
`complex design, increasing BOM cost and solution size.
`Methods to Improve Low Dropout Operation with the TPS54240 and TPS54260 Application Report:
`Provided four additional solutions on the basic of solution (B).
`(1) Diode tied from input to BOOT
`(2) Diode tied from output to BOOT
`(3) Charge pump tied from output to BOOT
`(4) Diode and resistor at PH
`
`4
`
`Design Example
`Figure 7 and Figure 8 show the circuit of solution (1) + (A) and solution (2) + (A) with 3.3 V output.
`
`Figure 7. TPS54231 Circuit with Solution (1) + (A)
`
`Figure 8. TPS54231 Circuit with Solution (2) + (A)
`
`4.1 Auxiliary Diode Selection
`The reverse voltage on the auxiliary diode is limited by the Bootstrap capacitor voltage. The reverse
`voltage equals the Bootstrap capacitor voltage when HS MOSFET is ON, and the absolute maximum
`rating of BOOT to PH is 8 V.
`The charging current in the auxiliary diode is very small in normal running, but considering the extreme
`condition, a large peak charging current can be observed when powering on with a low output voltage, as
`shown in Figure 9 and Figure 10. A diode with 0.5-A average rectified forward current and 2-A peak
`repetitive forward current are required.
`
`6
`
`Low Dropout Operation in a Buck Converter
`
`Copyright © 2018–2019, Texas Instruments Incorporated
`
`SLUA928A–December 2018–Revised March 2019
`Submit Documentation Feedback
`
`VOUT = 3.3V, 2A Max
`
`J3
`
`1 2
`
`TP8
`VOUT
`
`C9
`22uF
`
`C10
`22uF
`
`C11
`22uF
`
`Rdummy
`
`R6
`49.9
`
`R7
`10.2k
`
`R8
`3.24k
`
`D2
`
`DNP
`
`SS14FL
`
`TP5
`BST
`
`D3
`
`SS14FL
`
`2
`
`3
`
`4
`
`6
`
`U1
`
`VIN
`
`BOOT
`
`EN
`
`PH
`
`SS
`
`VSENSE
`
`COMP
`
`GND
`
`TPS54231D
`
`1
`
`8
`
`5
`
`7
`
`TP6
`SW
`
`C7
`
`0.1uF
`
`L1
`
`3.3uH
`
`TP7
`LOOP
`
`D1
`B240-13-F
`
`TP4
`EN
`
`C4
`15nF
`
`C5
`27pF
`
`C6
`1.5nF
`
`R3
`47.0k
`
`GND
`
` Copyright © 2018, Texas Instruments Incorporated
`
`TP9
`
`TP1
`VIN
`
`VIN= 3.5V to 8V
`
`J1
`
`12
`
`R1
`133k
`
`R2
`56.2k
`
`TP3
`
`123
`
`J2
`
`C1
`10V
`220uF
`
`C2
`10uF
`
`C3
`0.1uF
`
`Vin_start=4.08V
`Vin_stop=3.68V
`
`TP2
`
`VOUT = 3.3V, 2A Max
`
`J3
`
`1 2
`
`TP8
`VOUT
`
`C9
`22uF
`
`C10
`22uF
`
`C11
`22uF
`
`Rdummy
`
`R6
`49.9
`
`R7
`10.2k
`
`R8
`3.24k
`
`D2
`
`SS14FL
`
`TP5
`BST
`
`D3
`
`DNP
`
`SS14FL
`
`2
`
`3
`
`4
`
`6
`
`U1
`
`VIN
`
`BOOT
`
`EN
`
`PH
`
`SS
`
`VSENSE
`
`COMP
`
`GND
`
`TPS54231D
`
`1
`
`8
`
`5
`
`7
`
`TP6
`SW
`
`C7
`
`0.1uF
`
`L1
`
`3.3uH
`
`TP7
`LOOP
`
`D1
`B240-13-F
`
`TP4
`EN
`
`C4
`15nF
`
`C5
`27pF
`
`C6
`1.5nF
`
`R3
`47.0k
`
`GND
`
` Copyright © 2018, Texas Instruments Incorporated
`
`TP9
`
`TP1
`VIN
`
`VIN= 3.5V to 8V
`
`J1
`
`12
`
`R1
`133k
`
`R2
`56.2k
`
`TP3
`
`123
`
`J2
`
`C1
`10V
`220uF
`
`C2
`10uF
`
`C3
`0.1uF
`
`Vin_start=4.08V
`Vin_stop=3.68V
`
`TP2
`
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`
`Design Example
`
`Figure 9. Current in Auxiliary Diode and Inductor
`
`Figure 10. Current in Auxiliary Diode and Inductor (Zoom In)
`
`There is a path for the leakage current from the Bootstrap capacitor to the VIN (VOUT) side through the
`auxiliary diode. The reverse current has an influence to the recovery voltage, especially under no-load.
`Three different schottky diodes are tried during the test. Table 1 shows the difference of entry and
`recovery voltage under light load. Figure 11 shows the reverse current under different reverse voltage
`based on bench test. The SS14 has the lowest reverse current of 1SS404 and B0520LW, thus the SS14
`has a lower entry and recovery voltage.
`
`SLUA928A–December 2018–Revised March 2019
`Submit Documentation Feedback
`
`Copyright © 2018–2019, Texas Instruments Incorporated
`
`Low Dropout Operation in a Buck Converter
`
`7
`
`VIN
`
`VOUT
`
`ID2
`
`IL
`
`VIN
`
`VOUT
`
`ID2
`
`IL
`
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`Design Example
`
`www.ti.com
`
`Figure 11. Reverse Current under Different Reverse Voltage
`
`Table 1. The Entry and Recovery Voltages with Different Schottky Diodes
`DIODE TIED FROM INPUT
`DIODE TIED FROM OUTPUT
`RECOVERY
`RECOVERY
`VOLTAGE
`VOLTAGE
`(V)
`(V)
`4.23
`4.33
`5.56
`6.19
`5.6
`6.19
`4.23
`4.33
`4.35
`4.55
`4.55
`6.19
`3.73
`3.77
`3.972
`4.42
`4.05
`4.51
`
`ENTRY VOLTAGE
`(V)
`
`4.32
`4.80
`4.59
`4.32
`4.42
`4.59
`3.76
`3.70
`3.83
`
`LOADING
`CURRENT
`(mA)
`
`ENTRY VOLTAGE
`(V)
`
`0
`0
`0
`1
`1
`1
`5
`5
`5
`
`4.22
`4.24
`4.43
`4.22
`4.12
`4.34
`3.7
`3.67
`3.77
`
`DIODE P/N
`
`SS14
`1SS404
`B0520LW
`SS14
`1SS404
`B0520LW
`SS14
`1SS404
`B0520LW
`
`4.2 Entry Voltage and Recovery Voltage
`Table 2 and Table 3 show the entry and recovery voltage of solution (1), solution (2), and the original
`converter under a different dummy load current with 3.3 V and 5 V output. The solution (1) has lowest
`entry and recovery voltage and the smallest hysteresis voltage when compared to solution (2) and the
`original converter.
`If you use a 1SS404 or B0520LW to do the same above test, their recovery voltage is higher than SS14
`especial under light load. The root cause is the revere current of the auxiliary diode.
`
`Table 2. The Entry and Recovery Voltages with VOUT = 3.3 V
`ORIGINAL CURRENT
`DIODE TIED FROM INPUT
`DIODE TIED FROM OUTPUT
`ENTRY
`RECOVERY
`ENTRY
`RECOVERY
`ENTRY
`RECOVERY
`VOLTAGE
`VOLTAGE
`VOLTAGE
`VOLTAGE
`VOLTAGE
`VOLTAGE
`(V)
`(V)
`(V)
`(V)
`(V)
`(V)
`5.10
`5.11
`4.22
`4.23
`4.32
`4.33
`5.10
`5.11
`4.22
`4.23
`4.32
`4.33
`
`LOADING
`CURRENT
`(mA)
`
`0
`1
`
`8
`
`Low Dropout Operation in a Buck Converter
`
`SLUA928A–December 2018–Revised March 2019
`Submit Documentation Feedback
`
`Copyright © 2018–2019, Texas Instruments Incorporated
`
`SS14
`B0520LW
`1SS404
`
`1
`
`2
`
`3
`
`4
`5
`6
`7
`Reverse Voltage (V)
`
`8
`
`9
`
`10
`
`Figu
`
`16
`
`14
`
`12
`
`10
`
`02468
`
`Reverse Current (PA)
`
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`
`LOADING
`CURRENT
`(mA)
`
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`12
`13
`14
`15
`
`LOADING
`CURRENT
`(mA)
`
`0
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`12
`13
`14
`15
`
`Design Example
`
`Table 2. The Entry and Recovery Voltages with VOUT = 3.3 V (continued)
`ORIGINAL CURRENT
`DIODE TIED FROM INPUT
`DIODE TIED FROM OUTPUT
`ENTRY
`RECOVERY
`ENTRY
`RECOVERY
`ENTRY
`RECOVERY
`VOLTAGE
`VOLTAGE
`VOLTAGE
`VOLTAGE
`VOLTAGE
`VOLTAGE
`(V)
`(V)
`(V)
`(V)
`(V)
`(V)
`5.10
`5.11
`4.11
`4.12
`4.22
`4.23
`5.10
`5.11
`3.98
`3.99
`4.14
`4.15
`5.10
`5.11
`3.82
`3.87
`3.9
`3.91
`4.76
`4.77
`3.7
`3.73
`3.76
`3.77
`4.64
`4.65
`3.68
`3.69
`3.72
`3.73
`4.56
`4.57
`3.58
`3.6
`3.61
`3.62
`4.58
`4.59
`3.56
`3.59
`3.6
`3.61
`4.59
`4.60
`3.55
`3.56
`3.57
`3.58
`4.59
`4.60
`3.51
`3.52
`3.53
`3.54
`4.50
`4.51
`3.5
`3.51
`3.52
`3.53
`4.43
`4.44
`3.49
`3.5
`3.51
`3.52
`4.31
`4.32
`3.46
`3.47
`3.48
`3.5
`4.23
`4.24
`3.46
`3.47
`3.48
`3.5
`4.25
`4.26
`3.44
`3.45
`3.46
`3.47
`
`Table 3. The Entry and Recovery Voltages with VOUT = 5 V
`ORIGINAL CONVERTER
`DIODE TIED FROM INPUT
`DIODE TIED FROM OUTPUT
`ENTRY
`RECOVERY
`ENTRY
`RECOVERY
`ENTRY
`RECOVERY
`VOLTAGE
`VOLTAGE
`VOLTAGE
`VOLTAGE
`VOLTAGE
`VOLTAGE
`(V)
`(V)
`(V)
`(V)
`(V)
`(V)
`6.47
`7.82
`5.84
`5.85
`5.98
`5.99
`6.47
`7.82
`5.83
`5.84
`5.97
`5.98
`6.47
`7.79
`5.74
`5.76
`5.85
`5.86
`6.34
`7.72
`5.65
`5.67
`5.78
`5.79
`6.25
`7.7
`5.51
`5.6
`5.58
`5.64
`6.17
`7.6
`5.38
`5.39
`5.42
`5.51
`5.85
`7.76
`5.36
`5.37
`5.4
`5.5
`5.77
`7.75
`5.26
`5.27
`5.28
`5.38
`5.57
`7.68
`5.25
`5.26
`5.27
`5.43
`5.57
`7.73
`5.24
`5.25
`5.26
`5.47
`5.58
`7.43
`5.19
`5.2
`5.2
`5.43
`5.57
`7.73
`5.19
`5.2
`5.2
`5.27
`5.51
`7.68
`5.18
`5.19
`5.19
`5.41
`5.45
`7.69
`5.15
`5.16
`5.15
`5.24
`5.42
`7.57
`5.15
`5.16
`5.14
`5.34
`5.42
`7.62
`5.14
`5.15
`5.14
`5.43
`
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`Design Example
`4.3
`Input Capacitance
`Large input capacitance can be required to ensure a low ripple voltage at the input side, which is helpful
`for the low dropout operation.
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`4.4 Bootstrap Capacitor Voltage
`Figure 12 through Figure 15 show the bootstrap capacitor voltage of solution (1) under different load
`conditions. From Figure 14, the minimum load current is 3 mA to keep the bootstrap capacitor voltage
`higher than 2.1 V.
`
`Figure 12. Bootstrap Capacitor Voltage Waveform with VIN = 4 V, IOUT = 3 mA
`
`Figure 13. Bootstrap Capacitor Voltage Waveform with VIN = 4 V, IOUT = 10 mA
`
`10
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`Low Dropout Operation in a Buck Converter
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`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
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`Design Example
`
`Figure 14. Bootstrap Capacitor Voltage Waveform with VIN = 4 V, IOUT = 1 A
`
`Figure 15. Bootstrap Capacitor Voltage Waveform with VIN = 4 V, Variation IOUT
`
`4.5 Startup
`Figure 16 and Figure 17 show the startup waveforms of solution (1) under different conditions.
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`11
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`2A
`
`1A
`
`0.1A
`
`5mA
`
`3mA
`
`0A
`
`2mA
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
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`Design Example
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`Figure 16. Startup Waveform with VIN = 4 V, IOUT = 2 A
`
`Figure 17. Startup Waveform with VIN = 4 V, IOUT = 3 mA
`
`4.6
`
`Load Transient Response
`With a verified the load transient response, there is no significant difference in solution (1) and (2)
`compared to the original converter. Figure 18 through Figure 20 show the typical load transient response
`comparison results. The input voltage is 4 V, the output current step is from 25% to 75% of 2 A.
`
`12
`
`Low Dropout Operation in a Buck Converter
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`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
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`Design Example
`
`Figure 18. Load Transient of Solution (1)
`
`Figure 19. Load Transient of Solution (2)
`
`Figure 20. Load Transient of Original Converter
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`13
`
`VOUT
`
`IOUT
`
`VOUT
`
`IOUT
`
`VOUT
`VOUT
`
`IOUT
`IOUT
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`Design Example
`4.7 Efficiency
`Figure 21 through Figure 23 show the efficiency of solution (1) and (2) with 3.3 V / 5 V output under
`different VIN. The efficiency of solution (1) is lower than the efficiency of solution (2) and the original
`converter. Solution (2) has a closer efficiency performance to the original converter. The lower the input
`voltage, the higher the efficiency.
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`Figure 21. Efficiency of Solution (1) with 3.3 V Output under Different Input Voltage
`
`Figure 22. Efficiency Comparison with 5 V Input and 3.3 V Output
`
`14
`
`Low Dropout Operation in a Buck Converter
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`Diode tied from input
`Diode tied from output
`Original converter
`
`0.01
`
`0.1
`
`IOUT (A)
`
`1
`
`2
`
`Figu
`
`95%
`
`90%
`
`85%
`
`80%
`
`75%
`
`70%
`
`65%
`
`60%
`
`55%
`
`50%
`
`Efficiency
`
`45%
`0.001
`
`VIN=4 V
`VIN=5 V
`VIN=6 V
`VIN=7 V
`VIN=8 V
`
`1
`
`2
`
`Figu
`
`100%
`
`90%
`
`80%
`
`70%
`
`60%
`
`50%
`
`40%
`
`30%
`
`Efficiency
`
`20%
`0.001
`
`0.01
`
`0.1
`
`IOUT (A)
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`Low Dropout Operation Improved in Synchronous Part
`
`Figure 23. Efficiency Comparison with 8 V Input and 5 V Output
`
`5
`
`Low Dropout Operation Improved in Synchronous Part
`Some synchronous parts like TPS54202, TPS54302 and TPS56339, are all designed to improve the drop
`out operation.
`The TPS54202 and TPS54302 has a boot cap refresh function. When the BOOT UVLO is triggered, the
`LS MOSFET is forced to be OFF to charge the Bootstrap capacitor to maintain the Bootstrap capacitor
`voltage higher than 2.1 V. Figure 24 shows the typical operating waveform.
`In the TPS56339, a frequency foldback scheme is employed to extend the maximum duty cycle when
`TOFF_MIN is reached. The switching frequency decreases once a longer duty cycle is needed under low VIN
`conditions. With the duty increase, the on time increases, until up to the maximum ON-time, 5-μs. The
`wide range of frequency foldback allows the TPS56339 output voltage to stay in regulation with a much
`lower supply voltage VIN, leading to a lower effective dropout voltage. The boot cap voltage is always
`higher than 2.1 V. Figure 25 shows the typical operating waveform.
`
`Figure 24. TPS54202 Waveforms, VIN = 4.9 V, VOUT = 5 V (Target), IOUT = 2 A
`
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`Low Dropout Operation in a Buck Converter
`
`15
`
`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
`
`Diode tied from input
`Diode tied from output
`Original converter
`
`0.01
`
`0.1
`
`IOUT (A)
`
`1
`
`2
`
`Figu
`
`100%
`
`90%
`
`80%
`
`70%
`
`60%
`
`50%
`
`40%
`
`Efficiency
`
`30%
`0.001
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`Conclusion
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`Figure 25. TPS56339 Waveforms, VIN = 4.9 V, VOUT = 5 V (Target), IOUT = 3 A
`
`6
`
`7
`
`Conclusion
`This application report compares the two different solutions for improving the low dropout operation and
`provides guideline to the auxiliary diode selection.
`The reverse current of the auxiliary diode has an influence to the entry and recovery voltage. A small
`ripple voltage at the input side would be helpful for the low dropout operation.
`Solution (1): diode tied from input to BOOT, it has the lowest entry voltage and smallest hysteresis, lower
`efficiency, and a maximum input voltage is only 8 V.
`Solution (2): diode tied from output to BOOT has no the above input voltage limitation, has the 8 V
`limitation of the maximum output voltage, and has a closer efficiency performance to the original
`converter.
`The low dropout operation has been improved in synchronous part like the parts of TPS54202, TPS54302,
`TPS56339, and their family series part.
`
`References
`• Texas Instruments, TPS54231 2-A, 28-V Input, Step-Down DC-DC Converter with Eco-mode Data
`Sheet
`• Texas Instruments, TPS54202 4.5-V to 28-V Input, 2-A Output, EMI Friendly Synchronous Step Down
`Converter Data Sheet
`• Texas Instruments, TPS56339 4.5-V to 24-V Input, 3-A Output Synchronous Step-Down Converter
`Data Sheet
`• Texas Instruments, Providing Continuous Gate Drive Using a Charge Pump Application Report
`• Texas Instruments, Methods to Improve Low Dropout Operation with the TPS54240 and TPS54260
`Application Report
`
`16
`
`Low Dropout Operation in a Buck Converter
`
`Copyright © 2018–2019, Texas Instruments Incorporated
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`SLUA928A–December 2018–Revised March 2019
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`VIN
`
`VOUT
`
`BOOT-SW
`
`SW
`
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`Revision History
`
`Revision History
`NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
`
`Changes from Original (December 2018) to A Revision ................................................................................................ Page
`• Edited application report for clarity. ..................................................................................................... 1
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`Revision History
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