Texas Instruments | How to Design a Simple Constant Current/Constant Voltage Buck Converter | Application notes | Texas Instruments How to Design a Simple Constant Current/Constant Voltage Buck Converter Application notes

Texas Instruments How to Design a Simple Constant Current/Constant Voltage Buck Converter Application notes
Application Report
SNVA829 – June 2018
How to Design a Simple Constant Current/Constant
Voltage Buck Converter
ABSTRACT
Technical Information about designing a constant current, constant voltage (CC/CV) power converter is
limited. The design implementation can be challenging from a complexity, efficiency, and cost perspective.
The LM5117 device with its current monitor (CM) pin greatly simplifies the design process, allows current
regulation without using a lossy current sense resistor, and saves cost in the process. This application
note details the design approach using the LM5117 CM pin and the LMV431 for current regulation and
voltage regulation, respectively.
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Contents
Introduction ...................................................................................................................
Application Examples .......................................................................................................
Traditional Methods of Implementing CC/CV ............................................................................
Drawbacks of the Traditional Method .....................................................................................
A Simple CC/CV Method Using the LM5117 Device....................................................................
CC Programming ............................................................................................................
CV Programming.............................................................................................................
Power Design of the LM5117 ..............................................................................................
Example Schematic .........................................................................................................
Results ........................................................................................................................
Summary ......................................................................................................................
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List of Figures
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CC/CV Implementation With a Buck Topology .......................................................................... 2
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CLM5117 CC/CV Basic Implementation .................................................................................. 4
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A 30 V-to-54 V Input, 27 V-at-6 A Output CC/CV Converter Using the LM5117.................................... 6
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Efficiency at 30 VIN ........................................................................................................... 7
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Load Regulation at 30 VIN With Increasing RLOAD (VOUT / IOUT)
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Steady-State Waveforms ...................................................................................................
Load-Transient Performance ...............................................................................................
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1
Introduction
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Introduction
A DC-to-DC converter is typically implemented as a constant voltage (CV) regulator. The control loop
adjusts the duty cycle in order to maintain a constant output voltage regardless of changes to the input
voltage and load current.
A constant current (CC) converter regulates current the same way: the control loop adjusts the duty cycle
to maintain a constant output current regardless of changes to the input voltage and output resistance. A
change in output resistance causes the output voltage to adjust as the load resistance varies; the higher
the output resistance, the greater the output voltage.
A CC/CV converter regulates both current and voltage depending on the output resistance level.
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Application Examples
Many applications limit the maximum output resistance and resulting output voltage so that components
connected to the output won’t be damaged, which is where constant voltage regulation engages. Some
examples of CC/CV converter uses are applications driving a light-emitting diode (LED) or charging
batteries or supercapacitors. The current is regulated for a range of output resistances; should the
resistance increase beyond a certain level, the voltage is regulated, or “clamped.”
Output-voltage accuracy may be crucial, particularly in battery applications and supercapacitor chargers.
Precise voltage regulation enables more energy storage because you can set the voltage regulation point
as close as possible to the maximum safe operating voltage rating of the storage device.
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Traditional Methods of Implementing CC/CV
Figure 1 outlines a typical discrete implementation of a CC/CV converter. The converter requires a sense
resistor (RSENSE), an amplifier and a voltage regulation circuit (Vz). The current flowing through RSENSE sets
the voltage across RFB, which is the feedback voltage of a controller. In this way, the current is regulated.
As ROUT increases, the voltage on the output rises to a point where the Zener diode conducts, and the
device transitions from a CC converter to a CV converter.
VIN
Buck Power Stage
+
CC Mode = IOUT =(RINuVFB)/(RFBuRSENSE)
CV Mode = VOUT ~VFB+VZ
RSENSE
±
RIN
ROUT
+
±
VZ
To FB
RFB
Figure 1. CC/CV Implementation With a Buck Topology
As previously mentioned, the current through RSENSE sets the feedback voltage, which regulates the output
current. Equation 1 expresses the relationship between the output current and VFB:
2
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Drawbacks of the Traditional Method
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IOUT
RIN u VFB
RFB u RSENSE
(1)
Assuming a resistive load, Equation 2 governs the voltage at the output:
VOUT
IOUT u ROUT
(2)
Equation 3 sets the voltage regulation level:
VCLAMP VREF VZ
(3)
As seen in Figure 1, a Zener diode regulates the voltage in CV mode. Using a Zener as a voltage clamp
yields relatively poor voltage-accuracy performance because of the variation in Zener voltages from device
to device. Two Zener diodes are sometimes used in series to prevent leakage current flowing from
cathode to anode, which if present causes errors in the current regulation loop.
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Drawbacks of the Traditional Method
The traditional method requires the use of a sense resistor in series with the output in order to sense
current. As a result, resistive losses will impact efficiency; Equation 4 shows the losses in the sense
resistor
RSENSEloss
IOUT 2 u RSENSE
(4)
Higher losses increase the operating temperature and reduce system efficiency because the resistor has
all of the output current flowing through it. Cost also increases because low milliohm current-sense
resistors are relatively expensive compared to a small signal resistor. The common-mode voltage range of
the amplifier needs to be rated to the maximum output voltage. A high output voltage might increase the
cost of the amplifier. To help save costs, you could use a floating bias supply to reduce the common-mode
voltage range requirement, but that will increase the component count. The solution presented in Figure 1
has many disadvantages, including added design complexity, board real estate required, cost and impact
on system efficiency.
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A Simple CC/CV Method Using the LM5117 Device
The LM5117 is an emulated peak current-mode synchronous buck controller suitable for high-current,
wide step-down conversions. The major benefit of using the LM5117 in a CC/CV application is that it has
a current monitor (CM) feature. The CM pin provides an accurate voltage that is proportional to the output
current of the buck power stage. The designer can use the CM pin as the current loop feedback, saving
the additional current-sense circuitry that the traditional method requires. The voltage present on the CM
pin is accurate to ±2%, provided that the converter is set to forced pulsed width modulation (FPWM) or is
in continuous conduction mode. Figure 2 shows a basic CC/CV regulator implementation using the
LM5117.
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CC Programming
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VIN
LM5117
VOUT
HO
VOUT
RTopVC
VOUT = (IOUTuROUT)
VCMAve
CM
LO
LMV431
Ref
RFBt
CS
BAT54
RS
FB
CSG
R1
RBotVC
RFBb
Figure 2. CLM5117 CC/CV Basic Implementation
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CC Programming
Equation 5 describes the relationship between the CM voltage and IOUT:
VCMAve
IPEAK + IVALLEY u RS u AS
(5)
Equation 6 simplifies Equation 5:
VCMAve 2 u IOUT u RS u AS
(6)
As can be seen, the CM pin enables the designer to omit the series power dissipative current-sense
resistor at the output. RS is the current-sense resistor of the power stage used to generate a ramp for the
current-mode pulse-width modulation (PWM) loop. AS is the current-sense amplifier gain of the LM5117,
which has a typical value of AS = 10.
For example, assume that IOUT = 10 A and RS = 10 mΩ. Using Equation 7:
VCMAve 2 u 10 A u 0.01: u 10
VCMAve
2V
(7)
Setting the resistor divider network from the CM pin to ground and connecting the divider node to the
feedback pin sets the current regulation point. With 2 V at the CM pin, selecting the proper resistor-divider
ratio sets the current regulation level. To set the resistor-divider values for 10-A current regulation, select
RFBb = 10kΩ and calculate RFBt using Equation 8:
RFBt
·
§ VCMAve
- 1¸ RFBb
¨
© VFB
¹
(8)
which yields an RFBt value of 15 kΩ.
Remember to account for the reduction in AS caused by placing series filter resistors from RS and ground
to CS and CSG, respectively. Refer to the LM5117 data sheet for more details on how resistors in series
with the current-sense pins affect the gain of the internal current-sense amplifier.
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CV Programming
CV programing is achieved by using an LMV431 as a voltage clamp. Assume a voltage clamp level of 12
V. The forward voltage drop Vfwd across BAT54 is 0.5 V, and the FB voltage of the LM5117 is 0.8 V. The
voltage clamp engages when the voltage across R1 is equal to the voltage calculated using Equation 9:
VR1
VFB + VFWD
(9)
4
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Power Design of the LM5117
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Therefore, VR1 = 1.3 V.
The voltage at the reference pin of the LMV431 must be a reference voltage above VR1. The LMV431 has
a reference voltage of 1.24 V, so the voltage at the reference pin of the LMV431 is equal to the voltage
calculated using Equation 10:
VREF
VR1 + VREF LMV431
(10)
Therefore, a VREF = 2.54 V is required for the LMV431 to conduct current from its cathode to anode.
Select RBotVC = 10 kΩ and calculate RTopVC using Equation 11:
·
§ VCLAMP
RTopVC
- 1¸ RBotVC
¨
© VREF
¹
RTopVC
8
37 k:
(11)
Power Design of the LM5117
The design approach for the power section of a CC/CV converter using the LM5117 is the same as it is for
a basic buck converter. TI suggests carrying out the design at the highest output power level (which is the
highest output resistance) using either WEBENCH® Designer or the LM5117 Quick Start Calculator. Refer
as well to the LM5117 data sheet for guidance on the design of the buck power stage.
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Example Schematic
Figure 3 shows a 30 V-to-54 V input, 27 V-at-6 A output CC/CV implementation using the LM5117.
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Example Schematic
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Figure 3. A 30 V-to-54 V Input, 27 V-at-6 A Output CC/CV Converter Using the LM5117
6
How to Design a Simple Constant Current/Constant Voltage Buck Converter
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Results
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10
Results
Figure 4 shows the efficiency results with increasing output resistance.
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0.98
0.96
Efficiency
0.94
0.92
0.9
0.88
0.86
0.84
0.82
Efficiency Plot
0.8
2.5
5
7.5
10
12.5
15 17.5
VOUT
20
22.5
25
27.5
D001
Figure 4. Efficiency at 30 VIN
Figure 5 shows load regulation and the voltage setpoint with increasing output resistance.
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IOUT
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Load Regulation
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0
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12
15
VOUT
18
21
24
27
30
D002
Figure 5. Load Regulation at 30 VIN With Increasing RLOAD (VOUT / IOUT)
Figure 6 shows the switch node (CH3), VOUT ripple (CH1) and output current (CH4) at 30 VIN, 25 VOUT at
6 A.
Figure 6. Steady-State Waveforms
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Summary
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Figure 7 shows the load-transient performance VOUT (CH1) and output current (CH4) when stepping a
constant resistive load from 60 Ω to 120 Ω.
Figure 7. Load-Transient Performance
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Summary
The LM5117 configured as a CC/CV converter provides accurate current regulation, while offering many
advantages over the traditional implementation. The design approach is relatively simple, and enables
significant reductions in size, cost, and power losses.
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How to Design a Simple Constant Current/Constant Voltage Buck Converter
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