Texas Instruments | Using the TPS54x02 to Create an Inverting Power Supply | Application notes | Texas Instruments Using the TPS54x02 to Create an Inverting Power Supply Application notes

Texas Instruments Using the TPS54x02 to Create an Inverting Power Supply Application notes
Application Report
SLVA933 – June 2018
Create an Inverting Power Supply Using a TPS54202 Buck
Converter With Internal Compensation
Bruce Lu, Milo Zhu
ABSTRACT
The TPS54202 device is a 4.5-V to 28-V input voltage range, 2-A synchronous buck converter. By
integrating MOSFETs, internal loop compensation, internal 5-ms soft start, and employing the SOT-23
package, the TPS54202 device achieves high power density and offers a small footprint on the PCB. This
device is well-suited for applications such as a 12-V, 24-V distributed power-bus supply, white goods,
audio, STB, DTV, and printers. Moreover, with its internal compensation, the TPS54202 device can be
configured in an inverting buck-boost topology, where the output voltage is inverted or negative with
respect to ground, even without any more compensation components. This application report describes
the TPS54202 device in an inverting buck-boost topology, for use in low-current negative rails for an
operational amplifier, optical module biasing, or line drivers and other low-power applications. This
application report also discusses how to choose an output LC filter in the buck-boost topology to achieve
applicable transient and steady performance.
1
2
3
4
5
6
Contents
Configuring the Buck Converter for Inverting Buck-Boost Topology Application .................................... 3
Choosing the Right Buck Converter for Inverting Power Application ................................................. 4
2.1
Output Voltage Range ............................................................................................. 4
2.2
Input Voltage Range................................................................................................ 4
2.3
Output Current Range .............................................................................................. 4
Selecting Applicable External Components for Inverting Power Application......................................... 6
3.1
Resistor Divider ..................................................................................................... 6
3.2
Inductor and Output Capacitor Selection ........................................................................ 7
3.3
Input Capacitors ................................................................................................... 15
3.4
Bypass Capacitor.................................................................................................. 16
3.5
Enabling and Adjusting UVLO ................................................................................... 16
Experimental Results ...................................................................................................... 17
Summary ................................................................................................................... 23
References .................................................................................................................. 24
List of Figures
1
Buck Converter Application................................................................................................. 3
2
Buck-Boost Converter Application ......................................................................................... 3
3
Inverting Buck Boost Configuration
4
Output Current Range Versus Inductor L
5
6
7
8
9
10
11
12
....................................................................................... 4
................................................................................ 5
12 V To –12 V Reference Design ......................................................................................... 6
Bode Plot of Internal Compensator........................................................................................ 9
Models of Buck-Boost Converter ......................................................................................... 10
Bode Plot of the Buck-Boost Converter ................................................................................. 11
Bode Plot of Power Stage Versus COUT ................................................................................. 12
Bode Plot of Open Loop Versus COUT ................................................................................... 13
Bode Plot of Power Stage Versus L ..................................................................................... 14
Bode Plot of Open Loop Versus L ....................................................................................... 14
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
1
www.ti.com
13
14
15
16
17
18
19
...................................................................................
Bode Plot Measurement Result ..........................................................................................
Load Transient, L = 27 µH ................................................................................................
Load Transient, C = 22 µF × 2 ...........................................................................................
Output Voltage Ripple, Iload = 0 A .........................................................................................
Output Voltage Ripple, Iload = 0.4 A ......................................................................................
Output Voltage Ripple, Iload = 0.8 A.......................................................................................
Enabling and Adjusting UVLO Circuit
16
18
19
20
21
22
22
List of Tables
1
Design Parameters .......................................................................................................... 6
2
Different L and COUT Comparison ......................................................................................... 17
Trademarks
All trademarks are the property of their respective owners.
2
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Configuring the Buck Converter for Inverting Buck-Boost Topology Application
www.ti.com
1
Configuring the Buck Converter for Inverting Buck-Boost Topology Application
The inverting buck-boost topology is similar to the buck topology. In the buck configuration, shown in
Figure 1, the positive connection (VOUT) is connected to the inductor, and the return connection is
connected to the integrated circuit (IC) ground (GND). However, in the inverting buck-boost configuration,
shown in Figure 2, the IC GND is used as the negative output voltage pin. What was the positive output in
the buck configuration is used as the GND. This inverting topology allows the output voltage to be inverted
and always lower than the GND.
3
6
VIN
VIN
BOOT
Cin
Cboot
Lo
2
1
VOUT
GND TPS54x02 SW
R1
5
4
EN
EN
FB
Co
R2
Figure 1. Buck Converter Application
Cin
3
VIN
6
VIN
BOOT
Cbypass
Cboot
1
2
GND TPS54x02
Lo
SW
R1
5
EN
4
EN
FB
Co
R2
VOUT
Figure 2. Buck-Boost Converter Application
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
3
Choosing the Right Buck Converter for Inverting Power Application
www.ti.com
The circuit operation in the inverting buck-boost topology is different from the buck topology. Figure 3 (a)
shows that the output voltage terminals are reversed, though the components are wired the same as a
buck converter. During the on time of the control MOSFET, shown in Figure 3 (b), the inductor is charged
with current while the output capacitor supplies the load current. The inductor does not provide current to
the load during that time. During the off time of the control MOSFET and the on time of the synchronous
MOSFET, shown in Figure 3 (c), the inductor provides current to the load and the output capacitor. These
changes affect many parameters as described in the upcoming sections.
VIN
+
Cin
VIN
PGND
T1
+
VIN
PGND
Cin
T1
+
Cin
PGND
T1
ton
Cbypass
SW
Lo
Cbypass
PGND
Co
T2
Load
SW
Lo
Cbypass
PGND
Co
T2
Load
SW
Lo
PGND
Co
T2
IC GND
L
toff
+
VOUT
IC GND
(a)
VOUT
(b)
IC GND
VOUT
(c)
Figure 3. Inverting Buck Boost Configuration
2
Choosing the Right Buck Converter for Inverting Power Application
When choosing the TPS54202 device for inverting power application, you must confirm whether this
device can withstand the I/O voltage and output current of the inverting power application.
2.1
Output Voltage Range
The output voltage range is the same as when configured as a buck converter, but negative. So, the
output voltage for the inverting buck boost topology should be set between –0.6 V and –26 V. The output
voltage is set the same way as in the buck configuration, with two resistors connected to the FB pin. Due
to the increased noise of the inverting buck boost topology, and for a more robust design, use smaller
value resistors than what are used for the buck configuration.
2.2
Input Voltage Range
The input voltage that can be applied to an inverting buck boost converter IC is less than the input voltage
that can be applied to the same buck converter IC. This is because the ground pin of the IC is connected
to the (negative) output voltage. Therefore, the input voltage across the device is VIN to VOUT, not VIN to
ground. Thus, the input voltage range of the TPS54202 device is 4.5 V to 28 V – VOUT, where VOUT is a
positive value.
2.3
Output Current Range
In the buck configuration, the average inductor current equals the average output current because the
inductor always supplies current to the load during both the on and off times of the control MOSFET.
However, in the inverting buck boost configuration, the load is supplied with current only from the output
capacitor and is completely disconnected from the inductor during the on time of the control MOSFET.
During the off time, the inductor connects to both the output capacitor and the load (see Figure 3).
4
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Choosing the Right Buck Converter for Inverting Power Application
www.ti.com
So, the peak current of the MOSFET and inductor can easily be calculated, as follows in Equation 1,
Equation 2, Equation 3, and Equation 4.
I peak
û, L
2
I Lavg
(1)
Where:
IOUT
1- D
VIN u D VOUT u (1 D)
û, L
fS u L
fS u L
VOUT
D
VIN VOUT
I Lavg
(2)
(3)
(4)
When VIN is increased and VOUT is kept constant, the duty cycle, D, and ILavg decrease, while ΔIL increases.
You can see that the sum of ILavg and ΔIL decreases. So, when VIN is at the minimum, you can get the
maximum Ipeak. You must choose an applicable inductor, L, to keep the maximum Ipeak lower than the
minimum current limit, Icl(min) of the device. Therefore, you get Equation 5, as follows:
I OUTmax
1 D max I LIM_HS
VINmin D max 1 D max
2fS L
(5)
You can get the IOUTmax versus Lmin graph of the TPS54202 device, shown in Figure 4. For the TPS54202
device, Ilim_HS = 2.5 A and fs = 500 kHz. From Figure 4, you can see that by increasing the inductor and
VINmin, or decreasing the output voltage level, this device can hold more output current in the buck-boost
application.
10 4
VINmin = 8V VOUT = -12V
VINmin = 12V VOUT = -12V
VINmin = 8V VOUT = -8V
VINmin = 12V VOUT = -8V
L (µH)
10 3
10 2
10 1
10 0
0
0.5
IOUTmax (A)
1
1.5
Figure 4. Output Current Range Versus Inductor L
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
5
Selecting Applicable External Components for Inverting Power Application
3
www.ti.com
Selecting Applicable External Components for Inverting Power Application
When the appropriate buck converter is chosen, shown in Figure 5, you must choose the correct external
components such as a resistor divider, inductor, input capacitor, output capacitor, and bypass capacitor,
for high, steady, and transient performance.
U1
VIN
3
8V ~ 16V
C1
10PF
6
VIN
C2
10PF
C3
0.1PF
C4
10PF
BOOT
1
C5
0.1PF
1
C6
0.1PF
1
2
GND TPS54202
SW
EN
FB
5
EN
Not Installed
L1 27PH
R1
49.9Q
4 VSENSE
C7
PF
C8
PF
C9
PF
R2
50lQ
VSENSE
R3
2.61lQ
-12V,0.8A
VOUT
Copyright © 2017, Texas Instruments Incorporated
Figure 5. 12 V To –12 V Reference Design
For this design example, use the input parameters listed in Table 1.
Table 1. Design Parameters
3.1
Design Parameter
Example Value
Input voltage range
12-V nominal 8 V to 16 V
Output voltage range
–12 V
Transient response, 50% load step
∆VO = 2.5%
Output ripple voltage
1%
Output current rating
Maximum 0.8 A
Resistor Divider
The output voltage of the TPS54202 device is externally adjustable using a resistor divider network. In this
example, this divider network is comprised of R2 and R3. Use Equation 6 to calculate the relationship of
the output voltage to the resistor divider.
R3
R 2 u Vref
VOUT Vref
(6)
As previously discussed, due to the increased noise of the inverting buck boost topology, and for a more
robust design, use smaller value resistors than what are used for the buck configuration. For this design,
Vref = 0.6 V, set R2 = 50 kΩ and R3 = 2.61 kΩ. The 49.9-Ω resistor, R1, is provided as a convenient location
to break the control loop for stability testing.
6
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Selecting Applicable External Components for Inverting Power Application
www.ti.com
3.2
Inductor and Output Capacitor Selection
The inductor and output capacitor must be selected based on the needs of the application and the stability
criteria of the device. The selection criterion for the inductor and output capacitor are different from the
buck converter.
3.2.1
Inductor Selection
3.2.1.1
Output Current
When selecting the inductor value for the inverting buck boost topology, you must select a large enough
inductor to keep ILmax lower than the minimum current limit value (2.5 A) of the device for a reliable design.
From Figure 4 and Equation 5 for this example, you can see that an at least 9.6-µH inductor is needed for
a 0.8-A output application.
3.2.1.2
Inductor Current Ripple
Considering the current ripple in the inductor, when the inductor value is too small, then the current ripple
will be so large that it causes more power loss in the inductor and capacitors, and also reduces the
lifetime of the components. Too large of an inductor value causes a larger size and it is not good for the
power density. Usually, you can choose an applicable inductor value that lets r = 0.4, to get Equation 7.
û, Lmax
I
VINmax u D min
d 0.4 u OUTmax
1 D min
L min u f S
(7)
For this example, VINmax = 16 V, VOUT = –12 V, IOUTmax = 0.8 A, and fs = 500 kHz, so Lmin = 24.5 µH.
3.2.2
Output Capacitor Selection
3.2.2.1
Large Load Transient
The desired response to a large change in the load current is the first criterion. The output capacitor must
supply the load with current when the converter cannot. Usually the converter requires two or more
switching periods for the control loop to notice the change in load current and output voltage, and to adjust
the duty cycle to react to the change. The output capacitor must be sized to supply the extra current to the
load until the control loop responds to the load change, during which the capacitor voltage droops at the
same time. Use Equation 8 to calculate the minimum required output capacitance.
C OUT t
û, OUT u 3TS
û9droop
(8)
Where ΔIOUT is the change of output current, Ts is the switching period of the converter, and ΔVdroop is the
allowable change in the output voltage.
For this example, ΔIOUT = 50% × IOUT = 0.4 A, Ts = 1/fs, and ΔVdroop = 2.5% × VOUT = 0.3 V, so you need at
least 8 µF for the large load transient condition.
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
7
Selecting Applicable External Components for Inverting Power Application
3.2.2.2
www.ti.com
Output Ripple Voltage
The output capacitor must supply the current when the high-side switch is off. Use the minimum input
voltage to calculate the output capacitance needed. This is when the duty cycle and the peak-to-peak
current in the output capacitor are the maximum. Using the 1% voltage ripple specification and Equation 9,
COUTmin is 8 μF. Use Equation 10 to calculate the maximum ESR an output capacitor can have to meet the
output-voltage ripple specification. Equation 10 indicates the ESR should be less than 53.2 mΩ. In this
case, the ESR of the ceramic capacitor is much smaller than 53.2 mΩ.
I OUTmax u D max
f S u Vripple
Vripple
d
I OUTmax VINmin u D max
1 D max
2 u fS u L
C OUTmin t
R ESR
(9)
(10)
An output capacitor that can support the inductor ripple current must be specified. Some capacitor data
sheets specify the RMS value of the maximum ripple current. Use Equation 11 to calculate the RMS ripple
current that the output capacitor must support. For this application, Equation 11 yields 0.98 A for the
output capacitor.
I Coutrms
3.2.3
I OUTmax u
D max
1 D max
(11)
Selecting L and COUT for Stability
Because the TPS54202 device includes internal loop compensation, the compensation cannot be
changed externally. So the stability and transient performance are only determined by the power stage,
which means that selecting the applicable inductor and output capacitor is very important. Also, using a
buck boost regulator to generate a negative output voltage does not close the feedback loop, because a
buck power supply does. So, a different design method is needed.
From the TPS54202 4.5-V to 28-V Input, 2-A Output, EMI Friendly Synchronous Step Down Converter
Data Sheet, you can get information about the internal loop compensation. The internal loop compensator
has 1 original pole, 1 negative pole, and 1 negative zero. The transfer function of the internal loop
compensator is shown in Equation 12, Equation 13, Equation 14, and Equation 15.
&z
s
GO u
s
1
&p
1
CS
(12)
Where:
GO
&Z
&p
8
gmea u R C u R 3
R2 R3
1
R CCC
1
R C C C2
(13)
(14)
(15)
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Selecting Applicable External Components for Inverting Power Application
www.ti.com
With known parameters, the bode plot of the internal compensation can be obtained, as shown in
Figure 6. From the bode plot of the internal compensator, you can see that there is a pole at 2.42 kHz and
a zero at 2640 kHz, which gives a –9 dB gain between them.
Magnitude (dB)
20
Frequency (kHz): 2.42
Magnitude (dB): -5.93
0
Frequency (kHz): 2.64e+03
Magnitude (dB): -11.9
-9dB
-20
-40
Phase (deg)
-60
0
-45
-90
10 -1
Frequency (kHz): 2.42
Phase (deg): -45
10 0
10 1
10 2
Frequency (kHz)
Frequency (kHz): 2.64e+03
Phase (deg): -45
10 3
10 4
10 5
Figure 6. Bode Plot of Internal Compensator
Now let us see the inverting power supply transfer function. In the usual case, you can change the
external compensator to set the cross frequency below ¼ of RHPZ, and also assume that the slope
compensation pole is high frequency, so that they can neglect the influence of it. However, because of the
internal compensation and relatively larger inductor, compared with the Buck application, you cannot just
neglect the influence of the RHPZ and the slope compensation pole. Figure 7 shows three kinds of models
of the Buck-Boost topology. The third model is the most accurate one, and it reveals that the buck-boost
topology has three poles. There is one RloadCout pole, which is the dominant pole, one slope compensation
pole and one high frequency pole, one RHPZ zero, and one ESR zero. The first model is the most
simplified one, and it neglects the high frequency pole and the slope compensation pole, which lets the
phase differ from the real one more. The second simplified model only neglects the high frequency pole
and can converge with the real one at up to 200 kHz. This report applies the second model to discuss the
selection of L and C. For a detailed information about the three models, see Section 6 [1] [2] [3].
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
9
Selecting Applicable External Components for Inverting Power Application
www.ti.com
Magnitude (dB)
50
Model #1
0
Model #2
-50
Model #3
Phase (deg)
-100
0
-45
Model #1
-90
-135
Model #2
-180
-225
Model #3
-270
-315 -2
10
10 -1
100
101
102
103
104
105
Frequency (kHz)
Figure 7. Models of Buck-Boost Converter
Equation 16 shows the transfer function of the second model. The ESR zero, ωz1, is the same as in a buck
regulator, see Equation 17, and is a function of the output capacitor and its ESR. The other zero is a right
half-plane zero, ωzRHP. The frequency response of ωzRHP results in an increasing gain and a decreasing
phase. The ωzRHP frequency is a function of the duty cycle, output current, and inductor, see Equation 18.
The dominant pole, ωP1, is a function of the load current, output capacitor, and duty cycle, see
Equation 19. The slope compensation pole, ωpL, is a function of the input and output voltage, slope
voltage, and inductor, see Equation 20. GPS0 is the DC gain which is only determined by the input and
output voltage and the load current, see Equation 21. The gmps variable is the transconductance of the
power stage, which is 6.8 A/V for the TPS54202 device.
§
s · §
s ·
¨¨1
¸¸ u ¨¨1
¸
& z1 ¹ © & zRHP ¸¹
©
G PS0 u
·§
·
§
¨1 s ¸¨1 s ¸
¨ & ¸¨ & ¸
p1 ¹©
pL ¹
©
G PS s
(16)
Where:
& z1
1
ESR u C 0
(17)
VOUT
I OUT
Du L
1 D
2
1 D u
& zRHP
& P1
& pL
10
(18)
VOUT
u C OUT
I OUT
VIN VOUT
VSLOPE u gmps u L
(19)
(20)
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Selecting Applicable External Components for Inverting Power Application
www.ti.com
VOUT
I OUT
u gmps
2 u VOUT
VIN u
G PS0
VIN
(21)
80
60
Magnitude (dB)
40
20
0
Compensator
-20
Power stage
-40
-60
Open loop
-80
-100
0
Phase (deg)
-45
-90
-135
-180
-225
-270
-315
10-2
10-1
10 0
101
102
Frequency (kHz)
103
104
105
Figure 8. Bode Plot of the Buck-Boost Converter
You can see that the inductor L determines the RHPZ, ωzRHP, and the slope compensation pole, ωpL. The
larger L is, the closer ωzRHP is to ωpL. The capacitor COUT with the load determines the dominant pole, ωP1.
Now you can get the general bode plot of the loop gain as Figure 8 shows. At low frequency, the dominant
pole of the power stage determines the beginning frequency which the gain starts to slew down at, so
COUT mainly determines the cross frequency. You can use Equation 22 to estimate the relationship
between COUT and the cross frequency, from which you know that when COUT is doubled, the cross
frequency approximately decreases by half.
&cross
G PS0 u & P1
1/G C0
(22)
By substituting parameters, you can see that by using a 22-µF COUT, the cross frequency estimated is 8.75
kHz, which is close to 8.98 kHz (the real cross frequency).
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
11
Selecting Applicable External Components for Inverting Power Application
www.ti.com
In Figure 9 and Figure 10, as for in the phase margin, when COUT is increased from a small value, ωP1 and
ωz1 decrease, so that the phase starts to decrease at a lower frequency. But when COUT is too small (10
µF), then the cross frequency is so high that it is close to the slope compensation pole and RHP zero, and
the phase is too small. Then, when COUT is increased, the cross frequency decreases so that the phase
increases first. When you go on increasing COUT, the cross frequency decreases more so that the phase
starts to decrease at a lower frequency, and the phase at the cross frequency then decreases a little.
Here, considering the phase margin, TI recommends a 15 µF to 80 µF capacitor to make the cross
frequency at a applicable value between the dominant pole and the slope compensation pole/RHPZ.
Magnitude (dB)
60
40
20
C
0
-20
-40
-60
0
Phase (deg)
-45
C
-90
-135
-180
-225
-270
10-2
10 -1
10 0
101
10 2
103
Frequency (kHz)
10 4
105
10 6
Figure 9. Bode Plot of Power Stage Versus COUT
12
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Selecting Applicable External Components for Inverting Power Application
www.ti.com
80
60
Magnitude (dB)
40
20
C
0
-20
-40
-60
-80
-100
0
-45
C
Phase (deg)
-90
-135
-180
-225
-270
-315
10-2
10 -1
10 0
101
102
Frequency (kHz)
103
104
105
Figure 10. Bode Plot of Open Loop Versus COUT
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
13
Selecting Applicable External Components for Inverting Power Application
www.ti.com
The phase of the power stage decreases by 180 degrees in total when the frequency sweeps from the
slope compensation pole to the RHPZ. In Figure 11 and Figure 12, you can see that when the inductor L
is increased, the RHPZ, ωzRHP, and the slope compensation pole, ωpL, are decreased, which makes the
phase start to decrease at a lower frequency and they are also closer at the same time. It causes the
phase margin of the open loop to be lower and the gain margin is also lower too. To get enough phase
margin and gain margin, a lower inductor L must be guaranteed.
50
Magnitude (dB)
40
30
20
10
L
0
-10
-20
-30
-40
-50
0
Phase (deg)
-45
L
-90
-135
-180
-225
-270 -2
10
10-1
100
10 1
10 2
Frequency (kHz)
103
10 4
105
104
105
Figure 11. Bode Plot of Power Stage Versus L
80
Magnitude (dB)
60
40
20
L
0
-20
-40
-60
-80
-100
0
Phase (deg)
-45
-90
L
-135
-180
-225
-270
-315 -2
10
10-1
10 0
101
102
Frequency (kHz)
103
Figure 12. Bode Plot of Open Loop Versus L
14
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Selecting Applicable External Components for Inverting Power Application
www.ti.com
So, combined with the boundaries previously derived, and considering the derating of the ceramic
capacitor, you can choose two 22-µF ceramic capacitors for enough bandwidth, and a 27-µH inductor
(DCR = 40 mΩ) for enough phase margin and gain margin.
From Equation 23 and Equation 24, when a 27-µH inductor is used, the peak current of the inductor is 2.1
A and the maximum rms current is 2.02 A.
I peak
I Lrms
VIN(min) u D max
I OUT
1 D max
§ I OUT ·
¨
¸
©1 D ¹
2 u fs u L
2
(23)
1 § VIN u D ·
¸
u¨
12 ¨© f s u L ¸¹
2
(24)
At last, the criterion can be summarized when choosing L and C for Buck-Boost application of the
TPS54202 device:
• Inductor L selection criterion:
– Output current ability: must be large enough to keep the peak current lower than the minimum
HSlim of the IC.
– Inductor ripple current: must be large enough to keep the ripple lower than 0.4 times of the inductor
average current.
– Phase margin and gain margin: must be small enough to get enough phase margin and gain
margin.
• Output Capacitor COUT selection criterion:
– Large load transient: must be large enough to hold the output voltage before the control system
works.
– Output ripple voltage: must be large enough to keep the output ripple voltage lower than the
specific demand.
– Applicable cross frequency: must be selected to achieve an applicable frequency.
3.3
Input Capacitors
The input capacitors between VIN and ground are used to limit the voltage ripple of the input supply.
Equation 25 to Equation 28 are used to estimate the capacitance, maximum ESR, and current rating for
the input capacitor, CIN. Using Equation 26, the estimated average input current is 1.2 A. Using
Equation 25 and Equation 27, the minimum required input capacitance is 12 μF, and the maximum ESR is
66.7 mΩ. Using Equation 28, the input capacitor needs at least a 0.98-A current rating. Two, 10-μF, 35-V
X7R in parallel are used for the input capacitor, because of the low ESR and size.
I OUT u D max
û9IN u f sw
I OUT u D max
I INavg
1 D max
û9IN
ESR cin d
I INavg
C IN
I INrms | I OUT
(25)
(26)
(27)
D
1 D
SLVA933 – June 2018
Submit Documentation Feedback
(28)
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
15
Selecting Applicable External Components for Inverting Power Application
3.4
www.ti.com
Bypass Capacitor
The TPS54202 device needs a tightly coupled, ceramic bypass capacitor, connected to the VIN and GND
pin of the device. Because the device GND is the power supply output voltage, the voltage rating of the
capacitor must be greater than the differences in the maximum input and output voltage of the power
supply. The voltage of the VIN to GND pin is at least the VOUT voltage, and the input capacitor and output
capacitor in series can supply the VIN and GND pin of the TPS54202 device, so there is no need to add
another 10-µF capacitor from the VIN pin to GND in this case. Another 0.1-µF capacitor can be added as a
bypass capacitor to clear high-frequency noise.
3.5
Enabling and Adjusting UVLO
The TPS54202 device is enabled when the voltage at the EN pin trips its threshold, and the input voltage
is above the UVLO threshold. It stops operation when the voltage on the EN pin falls below its threshold,
or the input voltage falls below the UVLO threshold. However, when configured as a Buck-Boost
application, the GND pin of the TPS54202 device is tied to the negative output voltage and not the zero
voltage (system ground), which can cause difficulties enabling or disabling the device. So, level-shifting
circuitry is needed to solve the problem, as shown in Figure 13.
VIN
R7
R8
R6
R4
Q2
R9
EN
Q1
R10
R5
VOUT
GND
Figure 13. Enabling and Adjusting UVLO Circuit
R9 and R10 are used to divide the input voltage into a small one, to ensure the EN pin can take the normal
action while not exceeding the maximum pin rating of 7 V.
§
¨¨ VIN
©
VOUT u
§
¨¨ VIN
©
VOUT u
R 10 ·
¸
R 9 R 10 ¸¹ min
VSTART u
R 10 ·
¸
R 9 R 10 ¸¹ max
VINmax
R 10
t VEN_RISING( max)
R 9 R 10
VOUT u
R 10
d 7V
R 9 R 10
1.28V
(29)
(30)
Because of the internal pull-up current source of the TPS54202 device, Equation 29 and Equation 30 are
a little smaller than the real EN pin voltage. So, keeping R9 and R10 small helps the accuracy of the setup
for VSTART. Here for example, you can set VSTART = 7.5 V for the 8-V minimum input voltage to get
Equation 31. So for VSTART = 7.5 V, you can choose R10 = 13.2 kΩ and R9 = 62.2 kΩ.
R 10
64
1
d
d
375 R 9 R 10 4
16
(31)
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Experimental Results
www.ti.com
R4 and R5 form a voltage divider to set the VSTOP voltage. When Q1 turns on, R6 and R7 form a voltage
divider to turn Q2 on. Then, the voltage of the EN pin equals the value Equation 30 gets. When Q1 turns
off, Q2 turns off, and the voltage of the EN pin equals the VOUT voltage. Then, the IC turns off at once.
From the MMBT2222A data sheet, the minimum VBE saturation voltage is 0.6 V. Given this value and the
stop voltage, Equation 32 and Equation 33 are derived as follows:
R5
0.6V
R4 R5
R7
VSTOP u
t 0.6V
R6 R7
VSTOP u
(32)
(33)
Here for example, set VSTOP = 7 V, to get Equation 34 and Equation 35. Here, R5 = 12 kΩ, R4 = 128 kΩ,
R7=12kΩ, and R6 = 72kΩ was chosen.
R5
R4 R5
3
35
R7
R6
4
R7
t
(34)
3
35
(35)
Experimental Results
The design shown in Figure 5 was used to generate –12-V output from 12-V input. Figure 14 to Figure 19
show some typical measured waveforms of this design.
Table 2 lists the experimental results regarding the comparison of different L and COUT. Figure 14 shows
the bode plot when L = 27 µH and COUT = 22 µF × 2.
Table 2. Different L and COUT Comparison
L (µH)
27
COUT (µF)
Cross Frequency (kHz)
Phase Margin (°)
Gain Margin (dB)
10
22.01
16.61
2.15
22
12.14
37.09
6.81
22 × 2
7.03
42.21
11.78
22 × 4
4.36
38.57
17.48
7.31
52.1
16.12
7.03
42.21
11.78
7.7
38.1
9.89
7.83
22.51
5.6
15
27
33
47
SLVA933 – June 2018
Submit Documentation Feedback
22 × 2
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
17
Experimental Results
www.ti.com
Figure 14. Bode Plot Measurement Result
18
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Experimental Results
www.ti.com
Figure 15 and Figure 16 show comparisons of the load transient waveforms from 0.4 A to 0.8 A. Table 2
shows that there are some deviations from the theoretical calculation, due to the DC bias rating and ESRfrequency curve of the ceramic capacitors. From Table 2, you can see that when COUT is increased, the
cross frequency decreases and the phase margin first increases and then decreases. Also, the gain
margin increases too. When you increase L, the cross frequency almost does not change, and the phase
margin and gain margin decrease significantly.Figure 15 shows that too small a COUT creates a high cross
frequency, but also a lack of phase margin. Figure 16 shows that too large an L creates a poor phase
margin.
C = 10µF
C = 22µF
C = 22x2µF
C = 22x4µF
Figure 15. Load Transient, L = 27 µH
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
19
Experimental Results
www.ti.com
L = 15µH
L = 27µH
L = 33µH
L = 47µH
Figure 16. Load Transient, C = 22 µF × 2
20
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Experimental Results
www.ti.com
Figure 17 to Figure 19 show the steady state of the Buck-Boost converter, which shows that the output
ripples are small when the load is 0 A, 0.4 A, and 0.8 A. Also, while the load is light, the converter works
at Eco-Mode which increases the efficiency significantly.
Figure 17. Output Voltage Ripple, Iload = 0 A
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
21
Experimental Results
www.ti.com
Figure 18. Output Voltage Ripple, Iload = 0.4 A
Figure 19. Output Voltage Ripple, Iload = 0.8 A
22
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
Summary
www.ti.com
5
Summary
The TPS54202 buck converter can be configured as an inverting buck boost converter to generate a
negative output voltage. This application report explains, due to the internal compensation, how to select
an applicable LC value and other external components. Measured data from the example design is
provided. This application report also applies to the TPS54302 and TPS54202H devices.
SLVA933 – June 2018
Submit Documentation Feedback
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
23
References
6
References
1.
2.
3.
4.
24
www.ti.com
Li, Jian, Current-Mode Control: Modeling and Its Digital Application. Diss. Virginia Tech, 2009
Texas Instruments, Understanding Buck-Boost Power Stages in Switch Mode Power Supplies
Texas Instruments, Using the TPS54335A to Create an Inverting Power Supply, Milo Zhu
Texas Instruments, TPS54202 4.5-V to 28-V Input, 2-A Output, EMI Friendly Synchronous Step Down
Converter Data Sheet
Create an Inverting Power Supply Using a TPS54202 Buck Converter With
Internal Compensation
Copyright © 2018, Texas Instruments Incorporated
SLVA933 – June 2018
Submit Documentation Feedback
IMPORTANT NOTICE FOR TI DESIGN INFORMATION AND RESOURCES
Texas Instruments Incorporated (‘TI”) technical, application or other design advice, services or information, including, but not limited to,
reference designs and materials relating to evaluation modules, (collectively, “TI Resources”) are intended to assist designers who are
developing applications that incorporate TI products; by downloading, accessing or using any particular TI Resource in any way, you
(individually or, if you are acting on behalf of a company, your company) agree to use it solely for this purpose and subject to the terms of
this Notice.
TI’s provision of TI Resources does not expand or otherwise alter TI’s applicable published warranties or warranty disclaimers for TI
products, and no additional obligations or liabilities arise from TI providing such TI Resources. TI reserves the right to make corrections,
enhancements, improvements and other changes to its TI Resources.
You understand and agree that you remain responsible for using your independent analysis, evaluation and judgment in designing your
applications and that you have full and exclusive responsibility to assure the safety of your applications and compliance of your applications
(and of all TI products used in or for your applications) with all applicable regulations, laws and other applicable requirements. You
represent that, with respect to your applications, you have all the necessary expertise to create and implement safeguards that (1)
anticipate dangerous consequences of failures, (2) monitor failures and their consequences, and (3) lessen the likelihood of failures that
might cause harm and take appropriate actions. You agree that prior to using or distributing any applications that include TI products, you
will thoroughly test such applications and the functionality of such TI products as used in such applications. TI has not conducted any
testing other than that specifically described in the published documentation for a particular TI Resource.
You are authorized to use, copy and modify any individual TI Resource only in connection with the development of applications that include
the TI product(s) identified in such TI Resource. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE TO
ANY OTHER TI INTELLECTUAL PROPERTY RIGHT, AND NO LICENSE TO ANY TECHNOLOGY OR INTELLECTUAL PROPERTY
RIGHT OF TI OR ANY THIRD PARTY IS GRANTED HEREIN, including but not limited to any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
regarding or referencing third-party products or services does not constitute a license to use such products or services, or a warranty or
endorsement thereof. Use of TI Resources may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
TI RESOURCES ARE PROVIDED “AS IS” AND WITH ALL FAULTS. TI DISCLAIMS ALL OTHER WARRANTIES OR
REPRESENTATIONS, EXPRESS OR IMPLIED, REGARDING TI RESOURCES OR USE THEREOF, INCLUDING BUT NOT LIMITED TO
ACCURACY OR COMPLETENESS, TITLE, ANY EPIDEMIC FAILURE WARRANTY AND ANY IMPLIED WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF ANY THIRD PARTY INTELLECTUAL
PROPERTY RIGHTS.
TI SHALL NOT BE LIABLE FOR AND SHALL NOT DEFEND OR INDEMNIFY YOU AGAINST ANY CLAIM, INCLUDING BUT NOT
LIMITED TO ANY INFRINGEMENT CLAIM THAT RELATES TO OR IS BASED ON ANY COMBINATION OF PRODUCTS EVEN IF
DESCRIBED IN TI RESOURCES OR OTHERWISE. IN NO EVENT SHALL TI BE LIABLE FOR ANY ACTUAL, DIRECT, SPECIAL,
COLLATERAL, INDIRECT, PUNITIVE, INCIDENTAL, CONSEQUENTIAL OR EXEMPLARY DAMAGES IN CONNECTION WITH OR
ARISING OUT OF TI RESOURCES OR USE THEREOF, AND REGARDLESS OF WHETHER TI HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
You agree to fully indemnify TI and its representatives against any damages, costs, losses, and/or liabilities arising out of your noncompliance with the terms and provisions of this Notice.
This Notice applies to TI Resources. Additional terms apply to the use and purchase of certain types of materials, TI products and services.
These include; without limitation, TI’s standard terms for semiconductor products http://www.ti.com/sc/docs/stdterms.htm), evaluation
modules, and samples (http://www.ti.com/sc/docs/sampterms.htm).
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2018, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising