Almost all electronic devices today rely on a battery as a power source. The DC-DC converter plays a significant role in maintaining battery operating time. A DC-DC buck-boost converter is the ideal choice for the most efficient and reliable battery range.
The buck-boost converter provides the regulated voltage in the range of lithium (lithium-ion) batteries (a common battery choice for everyday devices such as smartphones). These converters are suitable when the output voltage is higher or lower than the input voltage.
For this project, we will use a 595-TPS63051RMWR buck-boost integrated circuit (IC). This IC provides 3.3V/1A output for a full range of lithium-ion batteries.
Basic design
Principle of Operation
Switch mode IC works on the principle of a switching mechanism or switched mode power supply (SMPS). The switching mode IC consists of an oscillator and a switching circuit. The oscillator works to generate the desired frequency and the switching circuit includes a transistor or diode as a switching element.
When the circuit is turned on, the IC turns on and off.
- During the ON period, the energy storage element charges, providing regulated voltage at the output.
- During the OFF period, the energy storage and output filtering element maintain the output voltage.
Necessary parts
Circuit design
The following components are required to design the Buck-Boost converter circuit along with a Buck-Boost IC.
1. Energy storage element – the inductor
Every switching regulator needs an energy storage element to operate, and an inductor is a good choice. An inductor stores energy in the form of a magnetic field. However, before choosing the inductor, it is important to calculate the nominal current to avoid unregulated output or circuit saturation.
It is necessary to calculate the inductor peak current during circuit operation using the equation below (Eq. 1).
Ipeak = (Iout / η(1-D)) + (Vin D/2fL)……Eq. 1
Since we are using a fixed 3.3V IC, there is no need to worry about the inductor value. Typically, for fixed switching mode ICs, the required inductor value is already in its datasheet. But it is important to note that we can change the value of the inductor and calculate its rated current using the above equation.
2. Filter element capacitor
Capacitors filter and eliminate voltage spikes in the power supply. In a switched converter, the capacitor performs two functions:
- Provides filtering
- Serves as a power source to manage output transient response
As the technical sheet of this switching mode IC has a predefined value for the IN and OUT capacitors, we can use them directly in our circuit.
3. Feedback Network Resistor Divider
The feedback network helps regulate the output voltage. This network is a resistor divider. Some fixed-switch mode ICs incorporate this feedback network internally.
This completes the basic circuit requirement of the switching mode IC. Now, let's discuss the extra features of IC TPS63051RMWR.
IC Features:
- Adjustable input current limit
- PFM/PWM mode
- Soft start
- Good power
- Overvoltage protection
- Thermal shutdown
- Output short circuit protection
- Undervoltage lockout
Pin Description:
1. EN (Enable) – HIGH (voltage level to set high 1.2V-5.5) IC enabled, LOW (maximum voltage to set LOW 0.3V) IC disabled
ILIM1 and ILM0 – Defines the input current limit
2. PFM/PWM and efficiency
- PFM mode selection – Low/0
- PWM mode selection – High/1
Efficiency in both modes:
- PWM Mode – Good efficiency for 350mA and above
- PFM mode – Good efficiency for lighter loads
To switch between these two modes, simply select PFM/PWM mode.
3. SS (soft start) – Adjustable soft start. The circuit sets the default soft start time when this pin floats. The default period in Buck mode – 280us and in Boost mode – 600us.
4. PG (good power) – This is an output pin used to detect system power regulation. The PG pin is only set to HIGH when the output is present and requires a pull-up resistor that connects to a voltage of less than 5.5V.
Converter output current limit
- For an input voltage, Vin = 2.5-3.3V Buck Mode
- For an input voltage, Vin = 3.3-5.5V Boost mode
NOTE: In “Boost mode”, the maximum output current is 500mA and in “Buck mode” it is 1A.
The value of the output current depends on the input current. If we limit the input current (see table above), the output will also be limited.
The equation for Buck and Boost mode output current is:
- Increase output current, I out = η.I IN (1-D)
- Buck output current, I out = η.I IN /D
Key points
We design the circuit according to the application. However, initially, our circuit did not work as expected. It provided zero voltage at the output terminals.
Possible causes:
- A welding problem
- Defective component(s)
- A faulty circuit design
- A PCB problem, such as short or continuity
After evaluating these potential causes, we discovered a problem in the circuit design.
Review and rectification
The circuit design has three signal lines: PFM/PWM, ILIM1 and ILIM0. The signal lines supply a certain voltage to the device to turn it on or off or to select a mode set on the device. These lines carry a low current (uA), generally known as base or quiescent current.
In our initial design, we connected the signal pins directly to VCC, which allows a high current through these pins. For optimal operation, we must add a pull-up resistor to limit the current, which solves the problem.
Practical observation
- Current limit – IIN_max. VILM1 and VILM0 pins HIGH (on Vin)
- Iout impulse mode (max) – 500mA
- Current limit- I IN_max . VILM1 and VILM0 HIGH pins (in V in )
- EU Buck Mode Off (Max) –1A
IC performance
Boost mode efficiency – 85% for load <200mA
– 70% for load >200mA
Buck mode efficiency – 96% for load <200mA
– 75% for load >200mA
Boost mode
Buck Mode
Thermal management
Each component dissipates heat when the current drawn from it increases. Sometimes the heat generated is so high that the component fails, so we need the heatsink to dissipate the extra heat.
Application
- Portable devices
- Regulated DC Power
- For use with a single lithium-ion battery or two 1.5V coil cells
Precautions
A capacitor must be connected between the IN pin and ground to regulate the DC input voltage. The circuit capacitor must have a higher rated voltage than the input supply voltage. Otherwise, the capacitor will leak current from its plates (and potentially burst) due to excess voltage.
Some other important points:
- It is essential to ensure The entire capacitor is discharged before working on a DC power supply.
- The rated current of the inductor must be 1.15 times greater than the output current. DO NOT use an input voltage higher than the operating input voltage range.
- Avoid shorting the output terminals – this will reverse the current flow in the IC, causing it to fail or fail.
- Avoid shorting the input terminals – this will generate a large current in the circuit and circuit components will fail.
- Do not pull directly down or up on the signal pins (PWM/PFM and ILIM1/ILM0 pins) as this will allow high current to flow through the signal pin. Always connect a 10k resistor to the signal pins to limit quiescent current.
PCB Design Guidelines
- Keep power strokes thick and short
- Place the input and output capacitor as close as possible to the input and output pins of the IC
- Minimize inductor path length
- Keep voltage and switching nodes away from each other
Printed circuit board layout