Texas Instruments | Methods to Improve Low Dropout Operation with the TPS54240 and TPS54260 (Rev. A) | Application notes | Texas Instruments Methods to Improve Low Dropout Operation with the TPS54240 and TPS54260 (Rev. A) Application notes

Texas Instruments Methods to Improve Low Dropout Operation with the TPS54240 and TPS54260 (Rev. A) Application notes
Application Report
SLVA547A – December 2012 – Revised October 2013
Methods to Improve Low Dropout Operation With the
TPS54240 and TPS54260
Jerry Chen, Steve Schnier, Anthony Fagnani, Dave Daniels
..................................................... DCS SWIFT
ABSTRACT
TPS54240 and TPS54260 devices are part of a family of non-synchronous, step-down converters with an
integrated high-side FET and 100% duty cycle capability. However, when operating near 100% duty cycle
(when the input voltage is slightly higher than the output voltage) with light loads, the output regulation of
the converter may degrade. This application note investigates the cause of this operation and introduces
several suggestions to improve the low dropout operation. Comparisons of the pros and cons illustrate the
tradeoffs of each solution. These solutions can be used with TPS54040A, TPS54060A, TPS54140A, and
TPS54160A.
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3
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5
6
Contents
Introduction .................................................................................................................. 2
TPS54260 Low Dropout Operation ....................................................................................... 2
Basic Solutions .............................................................................................................. 6
Additional Solutions ......................................................................................................... 8
Conclusion .................................................................................................................. 13
References ................................................................................................................. 13
List of Figures
1
Schematic of Original Converter ..........................................................................................
2
2
Boot and PH Waveforms ...................................................................................................
3
3
PH Waveform in DCM, VIN = 8V IOUT = 100 mA .........................................................................
3
4
PH Waveform in DCM, VIN = 8 V IOUT = 200 mA ........................................................................
3
5
PH Waveform in DCM, VIN = 12 V IOUT = 100 mA .......................................................................
4
6
Low Dropout Brownout Waveform ........................................................................................
5
7
Dummy Load With 5-V Output
...........................................................................................
Dummy Load With 3.3-V Output ..........................................................................................
Required External Voltage on BOOT Pin ................................................................................
Solutions (1), (2), and (3) ..................................................................................................
Solution (4) ...................................................................................................................
Operation Waveform of Solution (1) ....................................................................................
Efficiency With 8-V Input .................................................................................................
Efficiency With 12-V Input ................................................................................................
Dynamic Load .............................................................................................................
Load Transient Response, Solution (1): Diode Tied from Input to BOOT .........................................
Load Transient Response, Solution (2) Diode Tied from Output to BOOT ........................................
Load Transient Response, Solution (3): Charge Pump Tied from Output to BOOT..............................
Load Transient Response, Original Converter ........................................................................
Load Transient Response, Solution (4): Diode and Resistor at PH ................................................
6
8
9
10
11
12
13
14
15
16
17
18
19
20
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6
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8
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1
Introduction
1
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Introduction
The TPS54240 and TPS54260 devices are a series of 42-V and 60-V, non-synchronous, step-down
regulators with an integrated high-side N-channel MOSFET. A low dropout condition is defined by when
the input voltage approaches the nominal output voltage level. These regulators use a bootstrap circuit to
charge a capacitor connected between the BOOT and PH pins to provide the gate-drive voltage for the
high-side FET. To improve the low dropout performance, this family of parts is designed to operate at
100% duty cycle, as long as the BOOT to PH pin voltage is greater than 2.1 V. When the voltage from
BOOT to PH drops below 2.1 V, to protect the high-side FET, a UVLO circuit turns off the high-side
MOSFET because there is insufficient gate-drive voltage. This event allows the low-side diode to conduct,
pulling the PH pin to ground and recharging the BOOT capacitor. Be attentive to high duty cycle
applications experiencing extended time periods with light loads or no load. In this condition, there may
not be enough inductor current to turn on the low-side diode pulling the PH pin to GND for a long enough
period to recharge the BOOT capacitor. As a result, the high-side MOSFET of the converter stops
switching due to the 2.1-V BOOT UVLO. This operation only appears in low dropout conditions and with
light loads.
This application note investigates the operation of low dropout in light load and presents two basic
solutions and four additional solutions to improve the operation. The TI TPS54260 and TPS54260EVM597 devices are used to evaluate the pros and cons of each solution. The conclusions can be applied to
TPS54040A, TPS54060A, TPS54140A, and TPS54160A.
2
TPS54260 Low Dropout Operation
The TPS54260EVM-597 is the evaluation module for the TPS54260 device, with 3.3-V output as
described in the TI User’s Guide. The low-side switching element is an external catch diode D1. Some
modifications are needed to investigate low dropout:
1. Remove R1 and R2 resistors to float EN pin. The internal pull-up enables the device at the internal
UVLO threshold of 2.5 V.
2. Change R6 to 53.6-kΩ when testing a 5-V output.
As a result, the board can operate at low-input voltages without the device shutting off due to the external
resistor divider at the EN pin, and the low dropout operation can be evaluated. This board is referred to as
the original converter. Because the TPS54260 device is non-synchronous, it operates in both continuous
conduction mode (CCM) and discontinuous conduction mode (DCM), depending on the output load
current. The TPS54260 circuit used for testing is shown in Figure 1. The BOOT and PH waveforms are
shown in Figure 2 with an 8-V input and a 2-A load.
(B)ExternalVoltage
C5 0.1μF
PH
BOOT
L1
10μH
U1
VIN
GND
2
2
1
C1
not installed
C2
2.2μF
3
C3
2.2μF
4
5
R2
not installed
PH
VIN
GND
EN
COMP
SS/TR
VSNS
RT/CLK
PWRGD
10
9
D1
B360B-13-F
8
C8
C9
47μF 47μF
R1
not installed
VOUT
GND
51
GND
GND
6
R4
R6
53.6k
PWRGD
20K
C4
1
2
R5
7
TPS54260DGQ
11
GND
BOOT
PwPd
1
(A)Dummy Load
C7
R7
not installed
10K
GND
0.01μF
R3
412K
C6
4700pF
GND
GND
GND
GND
Figure 1. Schematic of Original Converter
2
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VOUT
Figure 2. Boot and PH Waveforms
2.1
Discontinuous Conduction Mode (DCM) Operation
Reduction in output-load current makes the power stage operate in DCM. Figure 3 through Figure 5 show
the waveform of switch node (PH pin) in this condition. Observe there are three unique states during each
switching period in DCM operation. The first state is the ON state, when the MOSFET switch is on and
catch diode D1 is off. The OFF state is when MOSFET is off and D1 is on. The IDLE state is during both
MOSFET and D1 are off. Ignoring the voltage drop on MOSFET and the catch diode, the voltage on PH
node equals input voltage in ON state and equals zero in OFF state. The remainder of the switching cycle
is the IDLE state when both MOSFET and D1 are off. The current in the inductor is zero, and the voltage
of the PH node is expected to be equal to the output voltage. In reality, the PH voltage waveform
oscillates because the output inductor and the junction capacitance of the catch diode constitute a
resonant LC network.
Figure 3 through Figure 5 show the waveforms of the PH node of the circuit in Figure 1 with different input
voltages and loads. Figure 3 VIN = 8 V, IOUT = 100 mA; Figure 4 VIN = 8 V, IOUT = 200 mA; Figure 5 VIN = 12
V, IOUT = 100 mA. Compare Figure 3 with Figure 4 and Figure 5. It can be observed that the off state
duration is shorter, if the input voltage is lower, or the load is lighter. Figure 3 toff = 640 ns; Figure 4 toff =
940 ns; Figure 5 toff = 840 ns. In no load, the OFF state duration is at its minimum.
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TPS54260 Low Dropout Operation
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Figure 3. PH Waveform in DCM, VIN = 8V IOUT = 100 mA Figure 4. PH Waveform in DCM, VIN = 8 V IOUT = 200 mA
Figure 5. PH Waveform in DCM, VIN = 12 V IOUT = 100 mA
2.2
Low Dropout Operation in Light Load
In no load and minimum input voltage, the OFF state period is lowest, as the results show in Section 2.1.
There is not always enough time to charge the BOOT capacitor to keep its voltage charge above the 2.1-V
threshold in the OFF state. Also shown previously, when increasing the load, the off state duration is
longer. However, the time may still be insufficient, if the load is not heavy enough. In this condition, if the
resonant oscillation in the IDLE state is ignored, the voltage of PH node equals the output voltage. If the
input voltage is 2.1 V higher than the output, the BOOT capacitor can be charged to 2.1 V in IDLE state. If
the input voltage is lower than output plus 2.1 V, the BOOT capacitor cannot be fully charged and the
BOOT to PH voltage falls below 2.1 V, forcing the high-side switch off. This operation mode causes the
output voltage regulation to degrade. Figure 6 shows this operation when the input is 6 V with a light
output load. The VBOOT – VPH cannot remain higher than 2.1 V, and the output has an approximately 2.1-V
sawtooth ripple. When decreasing the input voltage from nominal 12 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. The entry and recovery
voltages of the original converter in Figure 1 are 7.7 V and 8 V, respectively.
4
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VOUT
Figure 6. Low Dropout Brownout Waveform
When the converter operates in CCM, the PH voltage is clamped to ground during the high-side switch off
time, allowing the BOOT capacitor to be fully charged. As a result, this regulation problem does not exist
in CCM. However, when operating near 100% duty cycle, there is a drop on the output voltage when the
BOOT UVLO is triggered. The high-side MOSFET turns off and the output capacitor must supply the load
during this time period (see Figure 7).
This operation is observed because in light load, the converter enters Eco-mode™ to reduce power-loss.
In this operation mode, the device stops switching during a time period with bursts of switching to maintain
output voltage. When the MOSFET switches, the BOOT capacitor is charged to a certain level higher than
2.1 V, then the converter sleeps with switching stopped. The capacitor is discharged slowly through the
characteristic impedance of the BOOT pin, and the BOOT voltage decays gradually. When the voltage is
lower than 2.1 V, the BOOT UVLO inhibits switching, until the VIN – VOUT exceeds 2.1 V. The BOOT
capacitor is then charged sufficiently again and begins another cycle of this operation. The heavier the
load is, the longer the regulation maintains; this is due to more charging during the off state with a higher
load, which results in slower decay of the BOOT voltage. At a sufficient dummy load, the converter enters
fixed-frequency current mode operation. At this moment, the brownout issue disappears and the Ecomode™ function is disabled.
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Basic Solutions
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Basic Solutions
According to the description presented above, there are two basic ways to improve the low dropout
operation issue. As Figure 1 shows, solution (A) applies a dummy load at the output. Solution (B) applies
an external voltage at the BOOT node through a diode. Solution (A) extends the off state duration, so the
BOOT capacitor can be charged to 2.1 V. Solution (B) raises the voltage on BOOT node directly to sustain
sufficient voltage on the BOOT capacitor.
3.1
Dummy Load Solution
When the input voltage approaches the output voltage, a heavier load is needed to sustain enough off
state duration. If a dummy load is applied, the user can see the converter maintain regulation for a longer
period before the output drops down. This occurrence is followed by an immediate recovery to regulation
with another cycle of this operation. The period of maintaining regulation may last for minutes, depending
on the size of the dummy load and the input to output ratio, followed by a quick recovery time.
Figure 7 shows the values of the dummy load with different input voltages when the output is 5 V. The
minimum load is selected to provide at least one minute of regulation followed by an immediate recovery
from brownout. It also includes the corresponding dummy resistor values.
For 3.3-V application, in addition to changing R6 back to 31.6 kΩ, TI recommends adding a 220-μF bulk
capacitor on the input to keep the input voltage stable. When the input voltage approaches the minimum
input voltage of the TPS54260 device (3.5 V), the noise and ripple at the input while switching may shut
down the regulator. Figure 8 shows the dummy load needed with the corresponding resistance to keep a
3.3-V output in regulation for at least one minute.
3.2
External Voltage at BOOT
When supplying a voltage to the BOOT node that is 2.1 V higher than the output, the output maintains
regulation. A lower voltage source helps charge the BOOT capacitor, as long as the source is stable with
sufficient load capability and fast transient response. The external voltage sources current in parallel to the
internal BOOT regulator. Figure 9 shows the minimum required voltage at the BOOT node with various
input voltage in no load when the output is 5 V and 3.3 V.
If there is another voltage on the final system board, it can be used to improve the low dropout regulation
of the TPS54260 family. As Figure 9 indicates, an external 6 V is sufficient to keep the 3.3-V converter in
regulation at any input voltage. The similar conclusion is 7.5 V for a 5-V converter.
12
12
10
10
8
8
6
6
4
4
2
2
0
0
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
10
9
9
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
0
0
3.6
3.8
Methods to Improve Low Dropout Operation With the TPS54240 and
TPS54260
4
4.2
4.4
4.6
4.8
5
Input Voltage (V)
C001
Figure 7. Dummy Load With 5-V Output
6
11
Dummy Resistor
10
7.0
Input Voltage (V)
12
Dummy Load
11
14
Load Current (mA)
Load Current (mA)
Dummy Resistor
5.2
12
16
Dummy Load
14
Load Resistance (k
16
LOAD CURRENT AND LOAD RESISTANCE
vs
INPUT VOLTAGE
Load Resistance (k
LOAD CURRENT AND LOAD RESISTANCE
vs
INPUT VOLTAGE
C002
Figure 8. Dummy Load With 3.3-V Output
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Basic Solutions
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External BOOT Voltage (V)
EXTERNAL VOLTAGE ON BST
vs
INPUT VOLTAGE
7.8
7.4
7.0
6.6
6.2
5.8
5.4
5.0
4.6
4.2
3.8
3.4
3.0
2.6
2.2
1.8
1.4
1.0
VOUT = 3.3 V
VOUT = 5 V
3.5
4.0
4.5
5.0
5.5
6.0
Input Voltage (V)
6.5
7.0
C003
Figure 9. Required External Voltage on BOOT Pin
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Additional Solutions
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Additional Solutions
The basic solutions introduce two ideas to improve the low dropout operation:
1. Increasing the voltage level of the BOOT pin or the charging current for the BOOT capacitor
2. Clamping the PH pin to ground when MOSFET is off
SLVA444, Providing Continuous Gate Drive Using a Charge Pump, introduces a circuit on the basis of
solution (1). This application note presents four additional soultions:
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
Figure 10 and Figure 11 show the schematics of these solutions with 5-V output.
(2)Diodetied from Outputto BOOT
(1)Diodetied from Input to BOOT
(3)ChargePumptied from Outputto BOOT
C5 0.1μF
PH
BOOT
L1
10μH
U1
1
GND
2
2
1
C1
not installed
C2
2.2μF
VIN
3
C3
2.2μF
GND
EN
4
COMP
SS/TR
5
R2
not installed
PH
PwPd
VIN
BOOT
RT/CLK
VSNS
PWRGD
10
9
D1
B360B-13-F
8
VOUT
GND
51
GND
6
GND
R6
53.6k
R4
11
1
2
R5
7
TPS54260DGQ
GND
C8
C9
47μF 47μF
PWRGD
20K
C4
R1
not installed
C7
R7
not installed
10K
GND
0.01μF
C6
R3
412K
4700pF
GND
GND
GND
Figure 10. Solutions (1), (2), and (3)
C5 0.1μF
(4)Diodeand
Resistorat PH
PH
BOOT
U1
VIN
GND
2
2
1
C1
not installed
C2
2.2μF
3
C3
2.2μF
4
5
R2
not installed
BOOT
PH
VIN
GND
EN
COMP
SS/TR
PwPd
1
RT/CLK
VSNS
PWRGD
9
11
D1
B360B-13-F
8
C8
C9
47μF 47μF
1
2
R5
VOUT
GND
51
GND
7
6
GND
R4
TPS54260DGQ
GND
L1
10μH
10
R6
53.6k
PWRGD
20K
C4
R1
not installed
C7
R7
not installed
10K
GND
0.01μF
R3
412K
C6
4700pF
GND
GND
GND
Figure 11. Solution (4)
The first three additional solutions work by supplying an additional voltage to the BOOT pin to aid in
charging the BOOT capacitor. The fourth solution works by pulling the PH voltage to ground with the
added resistor, allowing the BOOT capacitor to be charged during the IDLE state. The diode is required to
block current from flowing through the added resistor to ground. During the ON state, PH voltage equals
VIN and the resistor sink has a current equal to VIN / R. This solution was tested with a 39-kΩ resistor.
Pay attention to the power dissipation in the resistor at the maximum input voltage, which is equal to I2 ×
R.
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4.1
Entry Voltage and Recovery Voltage
The entry and recovery voltages are defined in Section 2.2. Table 1 and Table 2 show the entry and
recovery voltages of the four additional solutions in comparison with the original converter in Figure 1
when the outputs are 3.3 V and 5 V. The limitation of maximum input voltages shown in Table 1 and
Table 2 are discussed in Section 4.2.
Table 1. Entry and Recovery Voltages with VOUT = 3.3 V
Entry Voltage (V)
Recovery Voltage (V)
(1) Diode tied from input to BOOT
Solutions
4.1
4.2
Max VIN (V)
8
(2) Diode tied from output to BOOT
4.2
5.0
60
(3) Charge pump tied from output to BOOT
4.2
5.0
60
(4) Diode and resistor at PH
4.3
4.4
60
Original converter
4.9
5.0
60
Table 2. Entry and Recovery Voltages with VOUT = 5 V
Enter Voltage (V)
Recovery Voltage (V)
Max VIN (V)
(1) Diode tied from input to BOOT
Solutions
5.9
6.0
8
(2) Diode tied from output to BOOT
6.7
7.9
60
(3) Charge pump tied from output to BOOT
6.3
7.8
60
(4) Diode and resistor at PH
5.8
7.5
60
Original converter
7.7
8.0
60
Some initial observations from Table 1 and Table 2 are as follows. The hysteresis between entry and
recovery voltages is much smaller in 3.3-V application than 5 V. The entry voltages of these solutions in
3.3 V are also close to each other. In 5-V application, solutions (1) and (4) have the lowest entry voltages.
For the solution (3) charge pump, it helps to improve the low dropout operation in comparison with solution
(2) diode tied from output to BOOT, but requires two more components. Solution (1) possesses the
minimum hysteresis among these solutions.
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Additional Solutions
4.2
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Input and Output Voltage Range
Typically, the TPS54260 device regulates the BOOT voltage to 6.5 V for charging the BOOT capacitor as
Figure 2 shows. If the input voltage is lower than 6.5 V, the charging voltage approximately equals the
input voltage. Solutions (1), (2), and (3) introduce an external voltage on BOOT, so it could increase the
charging voltage. The absolute maximum rating of BOOT-PH is 8 V. In solution (1), the BOOT voltage
approximately equals the input voltage in the off state. As a result, if the input voltage is 8 V, the BOOT
capacitor can be charged to 8 V. If the input voltage is higher than 8 V, the BOOT capacitor voltage is
higher than 8 V and exceeds the absolute maximum rating. This limits the input voltage to 8-V max.
Figure 12 shows this operation status of solution (1) when VIN = 8 V, and the measured VBOOT to VPH is 8
V.
Figure 12. Operation Waveform of Solution (1)
In solutions (2) and (3), a diode connects output and BOOT. The BOOT voltage can then charge to the
output voltage. If the output voltage is higher than 8 V, VBOOT to VPH exceeds the absolute maximum rating.
Solution (4) does not supply a voltage to BOOT, so it has no limitation on input and output voltage.
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4.3
Efficiency
Figure 13 shows the efficiency of four additional solutions together with the original converter when VIN = 8
V and Figure 14 shows the efficiency without solution (1) when VIN = 12 V, because solution (1) cannot
operate with input higher than 8 V as described in Section 4.2.
EFFICIENCY
vs
LOAD CURRENT
100
100
90
90
80
80
70
70
Efficiency (%)
Efficiency (%)
EFFICIENCY
vs
LOAD CURRENT
60
50
60
50
40
Diode and Resistor at PH
30
Diode tied from VIN to BOOT
30
20
Diode tied from VOUT to BOOT
20
Charge Pump tied from VOUT to BOOT
10
0
0.001
Original Converter
0.01
0.1
1
Load Current (mA)
10
40
Diode and Resistor at PH
Diode tied from VOUT to BOOT
Charge Pump tied from VOUT to BOOT
10
0
0.001
0.01
0.1
1
Load Current (mA)
C006
Figure 13. Efficiency With 8-V Input
Original Converter
10
C007
Figure 14. Efficiency With 12-V Input
The main observation from these results is the lower efficiency of the diode and resistor at PH solution.
This lower efficiency results from the added diode in series with the output current. It presents an
additional power loss equal to the RMS output current multiplied by the forward voltage drop of the added
diode.
4.4
Transient Response
The test of the load transient response verified there is not a significant degradation in performance.
Figure 16 through Figure 20 show the load transient response of the additional solutions compared with
the original converter. The input voltage equals 8 V, and the load condition is shown in Figure 15. The
maximum output current of 2.5 A is defined as 100% load.
load
75%
25%
20 ms
100
mA/µs
20 ms
100
mA/µs
Figure 15. Dynamic Load
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Additional Solutions
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Figure 16. Load Transient Response, Solution (1):
Diode Tied from Input to BOOT
Figure 17. Load Transient Response, Solution (2)
Diode Tied from Output to BOOT
Figure 18. Load Transient Response, Solution (3):
Charge Pump Tied from Output to BOOT
Figure 19. Load Transient Response, Original
Converter
Figure 20. Load Transient Response, Solution (4): Diode and Resistor at PH
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All of the undershoots and overshoots are less than 100 mV, which is 4% of the output voltage, as shown
in Figure 16 to Figure 19. However, in Figure 20 a larger overshoot and undershoot with the diode and
resistor at PH solution is observable.
5
Conclusion
To improve the low dropout of the TPS54240 and TPS54260 family, this application note investigates the
low dropout operation in light load. Two basic solutions are introduced, which include (A) dummy load at
the output and (B) an external voltage at the BOOT node. Figure 7, Figure 8, and Figure 9 show the effect
of the basic solutions. Based on the basic solutions, the four additional solutions provided are:
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
Comparisons between the additional solutions in the previous sections describe the pros and cons.
For solution (1), diode tied from input to BOOT, although it has lowest entry voltage and smallest
hysteresis (Table 1 and Table 2), its maximum input voltage is only 8 V (Figure 12), which greatly limits its
practicability.
The efficiency of solutions (1) and (4) is lower than the efficiency of solutions (2) and (3) (Figure 13 and
Figure 14). Solutions (2) and (3) have the closest efficiency performance to the original converter.
If the output is lower than 8 V, solutions (2) and (3) can be used to improve the low dropout operation.
According to the comparisons from Section 4.1 to Section 4.4, the other performance of these two
solutions is closest to the original converter. TI recommends using solution (2) to improve the low dropout
operation in 3.3-V application and solution (3) for 5-V application. This recommendation is because the
entry and recovery voltages of solutions (2) and (3) are close to each other with a 3.3-V converter, as
Table 1 indicates. In a 5-V application, the entry voltage of solution (3) is lower than solution (2) as
Table 2 shows, but introduces two more external components.
These solutions and conclusions can also be used on TPS54040A, TPS54060A, TPS54140A, and
TPS54160A.
Additionally, the newer, higher-current family of non-synchronous regulators, integrate a FET for
recharging the BOOT capacitor to improve low dropout operation. This family includes the 60-V
TPS54360, TPS54560, TPS54361, TPS54561, and their 42-V equivalents.
6
References
1. TPS54260 3.5 V to 60 V Input, 2.5 A, Step Down Converter with Eco-mode™, Texas Instruments,
SLVSA86
2. TPS54260EVM-597 2.5 A, SWIFT™ Regulator Evaluation Module, Texas Instruments, SLVU372
3. Philip Meyer, John Tucker. (2011). Providing Continuous Gate Drive Using a Charge Pump, Texas
Instruments, SLVA444
SLVA547A – December 2012 – Revised October 2013
Submit Documentation Feedback
Methods to Improve Low Dropout Operation With the TPS54240 and
TPS54260
Copyright © 2012–2013, Texas Instruments Incorporated
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