A Step-Up converter is capable of boosting a low input voltage, say 1.5 V to a much higher voltage like, 5 V. Since, Power must be conserved, while boosting the voltage, output current is lowered. We take a look at the steps followed by all the necessary calculations to design a Step Up DC-to-DC Boost Converter.

Introduction:

The Pocket Boost Converter by Explore Labs is a DC-to-DC Boost Converter which generates a supply voltage of 5 V or 3.3 V from a single-cell alkaline battery. Output currents can go as high as 75 mA (maintaining 3.3 V output) while using 1xAA battery and discharge it all the way down to 0.9 V. [1]

Texas Instruments' TPS61070 - 90% Efficient PFM/PWM Boost Converter:

The Pocket Boost Converter utilises Texas Instruments' TPS61070. It has a typical start-up voltage specified as 1.1 V (with a worst case start-up voltage of 1.2 V) [since the open-load battery voltage of an almost empty alkaline cell is around 1.1 V]. However, once started, it can operate < 0.9 V. Output voltage is programmed by an external resistive feedback divider. In case of Pocket Boost Converter, two different networks of External Resistors are employed so as to choose between 5 V and 3.3 V.

At low load currents the TPS61070 enters the power-save mode to maintain a high efficiency over a wide load current range. Also, to prevent malfunctioning of the converter, an Undervoltage Lockout function shuts down the device if the supply voltage is lower than 0.8 V (which is not possible for a single AA cell). [2]

The TPS61070 works well in most single-cell boost applications. Though in some cases the discharging battery terminal voltage could be lower than the start-up voltage of the device, the battery voltage readily recovers above 1.1 V once the load is disconnected. Therefore, the TPS61070 does not have difficulty starting up again even when the battery energy capacity is minimal at these terminal voltages.

Regardless of battery chemistry - alkaline, nickel cadmium (NiCd), or nickel metal hydride (NiMH), the discharging of a battery in an actual application exhibits constant power discharge rather than a constant current discharge. This shortens battery lifetime at lower voltages when nearing its end of discharge, further reducing the battery voltage (during constant power discharge, the discharge current increases as terminal voltage decreases to supply the same power to the load). However, once the load is disconnected, or the device is shut down, the battery voltage recovers to a higher voltage (typically above 1.1 V). [3]

Technical Specifications:

• Minimum Startup Voltage - 1.2 V (Once started, it may operate at 0.9 V)
• Precharge Current required for starting up - 30 mA @ 1.2 V to 135 mA @ 5V - [1] (See Page 13 Figure 22 of datasheet)
• Quiescent Current - 19 µA (typical) - [1] (See Page 8 Figure 9 of datasheet)
• Supports Input voltages from 0.9 V to 4.5 V - (like, one-cell, two-cell, or three-cell alkaline, NiCd or NiMH, or one-cell Li-ion or Li-polymer battery)
• Output Voltage: 3.3 V or 5 V (+/- 1 %) - as selected from the voltage selection switch
• Output from 1xAA Alkaline cell - 3.3 V @ 75 mA (typical) or, Output from 1x Li Ion cell or 3.3V Power source - 5 V @ 200 mA (typical)
• Output Voltage Ripple: 10 mV Vpp (Theoretical) For test values, see Testing section
• Switching Current: 600 mA (typical)
• Switching Frequency: 1.2 MHz (typical)

Features:

• 90 % Efficiency
• Power-Save Mode for Improved Efficiency at Low Output Power
• Overtemperature Protection

Designing the Circuit:

After studying the design guidelines and examples from the TPS61070 Datasheet, following schematic design was created. A JST connector to accommodate different types of inputs, a voltage selection switch (to switch between 5 V and 3.3 V) and 2x2 male headers to interface with a BreadBoard were added. Enable Pin of the TPS61070 IC was connected to the Input Voltage for always ON configuration. [Schematic available on GitHub]

Board Design:

Once schematic design is complete, it's time for designing the board. It takes patience as well as creative thinking in laying out the board. Be ready for multiple iterations before deciding the best design. Here is an example of the design process followed in designing the Pocket Boost Converter. [Board Design files available on GitHub]Pocket Boost Converter

a.) BreadBoard Compatibility - It was decided that the Pocket Boost Converter should mate with a standard BreadBoard's Power Rails. Careful measurements need to be taken so that the headers fit snug into a BreadBoard. Luckily, the dimensions were readily available from our BreadBoard Power Supply - Fully Assembled. The distance between these power rails is 41.34 mm from bottom rail's origin or 1650 mils [in EAGLE lingo]

b.) Reverse Mount LED - Since the whole board would be used facing down with  only the Cell Holder visible on top; there is no way to make sure that the circuit is turned ON and providing power or not. Thus, it was decided to use a Reverse mount LED so that it glows through a hole on the TOP side of the board. The reverse mount LED could shine through the two mounting holes of the single-cell battery holder. Also, there is a solder jumper (SJ1) to enable/disable this LED. Thus, current consumption can further be reduced.

c.) JST Connector - This has become a standard for connecting LiPo batteries to boards. Thus, it was decided to use this connector. We came to know that this JST connector could be used for 2xAA battery holder from SparkFun as well. Both these inputs can provide sufficient output current for your high-current project [driving a motor is still far away]. The design team added two variants of the JST connector - a through hole version and an SMD version. It makes us less dependent on a single component, plus, the through holes can accommodate wires soldered directly to the board from a compatible input.

There is an onboard JST connector for providing additional sources of Input like a 2xAA Battery Holder, a single-cell Li-Polymer Battery (typical 3.7 V) or Solar Cells (with Voc ~ 3V).

d.) Layout Considerations - A double layered configuration was chosen. The input capacitor, output capacitor, and the inductor were placed as close as possible to the IC. Extra wide and short traces were used for the main current path and for the ground tracks to control stability and EMI problems as seen in the below figure. These big copper pads also enhance the thermal performance by improving the power dissipation capability of the Printed Circuit Board.

Calculations:

The following is a basic configuration of a boost converter where the switch is integrated in the IC used (here, TPS61070). [4]

In TPS61070, the diode is replaced with a low $$R_{DS_{(ON)}}$$ PMOS switch integrated into the converter. Thus, Diode calculations may be neglected.

The following parameters are required to calculate the power stage:

1. Minimum Input Voltage, $$V_{IN(min)}=0.9\hspace{2pt}V$$
2. Desired Output Voltage, $$V_{OUT}=5\hspace{2pt}V$$
3. Maximum Output Current, $$I_{OUT(max)}=100\hspace{2pt}mA$$, (desired)

a.) Calculating the Duty Cycle

We start with calculating the Duty Cycle, $$D$$ for a minimum input voltage of $$0.9 V$$.

$$D=1-\left [ \frac{V_{IN(min)}*\eta}{V_{OUT}} \right ]$$

where, $$D=$$ Duty Cycle

$$V_{IN(min)}=$$ minimum input voltage (this will lead to the maximum switch current)

$$V_{OUT}=$$ desired output voltage

$$\eta=$$ Efficiency of the converter. For TPS61070, $$\eta=90\hspace{2pt}%$$

Thus,

$$D=1-\left [ \frac{0.9\hspace{2pt}V*0.9}{5\hspace{2pt}V} \right ]$$

$$D=0.838$$

b.) Choosing the Inductor

This is one of the most crucial component in designing a DC/DC Converter (whether it be buck or boost). The higher the inductor value, the higher is the possible maximum output current because of the reduced ripple current. We can use the following formula:

$$L=\frac{V_{IN}*\left ( V_{OUT}-V_{IN} \right )}{\Delta I_{L}*f_{s}*V_{OUT}}$$

where, $$L=$$ Inductance in Henry

$$V_{IN}=$$ typical input voltage, here, $$0.9 V$$

$$f_{s}=$$ minimum switching frequency of the converter, here, 960 kHz (from datasheet)

$$\Delta I_{L}=$$ estimated inductor ripple current as discussed below:

A good estimation for the inductor ripple current is 20 % to 40 % of the output current. A smaller ripple reduces the magnetic hysteresis losses in the inductor, as well as output voltage ripple and EMI. But in the same way, regulation time rises at load changes. In addition, a larger inductor increases the total system costs. Inductor Ripple Current can be found out as below:

$$\Delta I_{L}=\left ( 0.2\hspace{2pt}to\hspace{2pt}0.4 \right )*I_{OUT(max)}*\frac{V_{OUT}}{V_{IN}}$$

where, $$I_{OUT(max)}$$= maximum output current desired in the application, here, we would like the output current to be approximately 100mA (This is just an approximate. Actual value will be calculated and verified at a later stage).

Taking an estimate for the inductor ripple current as 30 % of the desired output current, we get,

$$\Delta I_{L}=0.3*100\hspace{2pt}mA*\frac{5\hspace{2pt}V}{0.9\hspace{2pt}V}$$

$$\Delta I_{L}=167\hspace{2pt}mA$$

Substituting in the formula for Inductance, L, we get,

$$L=\frac{0.9\hspace{2pt}V*\left ( 5\hspace{2pt}V-0.9\hspace{2pt}V \right )}{167\hspace{2pt}mA*960\hspace{2pt}kHz*5\hspace{2pt}V}$$

$$L=4.603\hspace{2pt}\mu H$$ (choose a value above the calculated one) [5] (See Page 3 Section 3.2 Equation 4)

For our application of boosting a single-cell battery's input of 0.9 V - 1.5 V to 3.3 V or 5 V, an inductance value of 4.7 µH was chosen. The exact part being SDE6603-4R7M with an rms current rating and saturation current rating of 1.5A and a maximum DC Resistance of $$90 mΩ$$. [6]

c.) Calculating the Maximum Switch Current

Using the recently calculated value of Inductance, L, we can verify the Inductor Ripple Current, $$\Delta I_{L}$$, using the following formula:

$$\Delta I_{L}=\frac{V_{IN(min)}*D}{f_{s}*L}$$

where, $$f_{s}=$$ minimum switching frequency of the converter, 960 kHz

$$L=$$ as calculated above, $$L=4.7\hspace{2pt}\mu H$$

Thus,

$$\Delta I_{L}=\frac{0.9\hspace{2pt}V*0.838}{960\hspace{2pt}kHz*4.7\hspace{2pt}\mu H}$$

$$\Delta I_{L}=167\hspace{2pt}mA$$, same as calculated above.

It is a good idea to verify if our selected IC (TPS61070) can deliver the desired output current of 100 mA or not. We can do that as follows:

$$I_{MAXOUT}=\left [ I_{LIM(min)}-\frac{\Delta I_L}{2} \right ]*\left ( 1-D \right )$$

where, $$I_{LIM(min)}=$$ minimum value of the current limit of the integrated switch, from datasheet, 500 mA.

Thus, $$I_{MAXOUT}=\left [ 500\hspace{2pt}mA-\frac{167\hspace{2pt}mA}{2} \right ]*\left ( 1-0.838 \right )$$

$$I_{MAXOUT}=67\hspace{2pt}mA$$

Uh Oh! Our desired Output Current was 100 mA but the calculations show a peak value to be 67 mA. This is true as it can be seen on Page 6 of the datasheet (Graph between Maximum Output Current vs Input Voltage]. The graph starts at around 60 mA to 70 mA. [1] (See Page 6 Figure 1 of datasheet).

Next, we move on to find out the Maximum Switch Current, $$I_{SW(max)}$$, as follows,

$$I_{SW(max)}=\frac{\Delta I_{L}}{2}+\frac{I_{OUT(max)}}{1-D}$$

Keeping our desired output current to be 100 mA, we get,

$$I_{SW(max)}=\frac{167\hspace{2pt}mA}{2}+\frac{100\hspace{2pt}mA}{1-0.838}$$

$$I_{SW(max)}=700\hspace{2pt}mA$$

This is our IC's switching current limit. It can also be verified from the datasheet.

d.) Setting the Output Voltage through Resistive Feedback Divider

To set the Output Voltage to a fixed value, Resistance values of the Feedback Divider of a DC/DC Converter are chosen as follows:

The current through the resistive divider must be at least 100 times the size of the feedback bias current:

$$I_{R{\frac{1}{2}}}\geq 100*I_{FB}$$

where, $$I_{R{\frac{1}{2}}}=$$ Current through the Resistive Divider to GND

and, $$I_{FB}=0.01\hspace{2pt}\mu A$$, feedback bias current (from data sheet)

Neglecting the current into the FB pin, the resistors are calculated as follows:

$$R_{2}=\frac{V_{FB}}{{I_{R{\frac{1}{2}}}}}$$

$$R_{1}=R_{2}\ast\left [ \frac{V_{o}}{V_{FB}}-1 \right ]$$

$$R_{1}=180\hspace{2 pt}k\Omega \ast\left [ \frac{V_{o}}{500\hspace{2 pt}mV}-1 \right ]$$, R2 = 180 kΩ chosen according to datasheet [1] (See Page 16 of datasheet).

$$R_{1}=180\hspace{2 pt}k\Omega \ast\left [2 \ast V_{o}-1 \right ]$$

$$R_{1}=1620\hspace{2 pt}k\Omega,\hspace{2 pt}for\hspace{2 pt}V_{o} = 5\hspace{2 pt}V or R_{1}=1008\hspace{2 pt}k\Omega,\hspace{2 pt}for\hspace{2 pt}V_{o} = 3.3\hspace{2 pt}V$$

We can verify our Resistor selection by applying Kirchoff's Current Law in the below figure, [7]

$$V_{FB}=R_{2}*\frac{V_{out}-I_{FB}*R_{1}}{R_{1}+R_{2}}$$

Thus,

$$V_{FB}=180\hspace{2pt}k\Omega *\left [ \frac{5\hspace{2pt}V-(0.01\hspace{2pt}\mu A*180\hspace{2pt}k\Omega)}{(180\hspace{2pt}k\Omega+1620\hspace{2pt}k\Omega)} \right ]$$

$$V_{FB}=0.49999982\hspace{2pt}V$$, close to the expected value of 500 mV.

Here, to change the output between 5V and 3.3V, a voltage selection switch would change the feedback resistor divider network. Thus, as seen in the below schematic portion, resistors in the feedback divider were connected to an SPDT switch as follows:

e.) Input Capacitor Selection

Two 4.7 µF Ceramic Capacitors in parallel combination were chosen so that the combined ESR (Equivalent Series Resistance) would be lowered (Size - 0603, Dielectric - X5R , Voltage Rating - 10 V). [1] (See Page 15 of datasheet).

f.) Output Capacitor Selection

To calculate the minimum output capacitance needed, we use the following formula:

$$C_{OUT(min)}=\frac{I_{OUT(max)}*D}{f_{s}*\Delta V_{OUT}}$$

where, $$C_{OUT(min)}=$$ minimum output capacitance needed

$$I_{OUT(max)}=$$ maximum output current of the desired application

$$D=$$ Duty Cycle of the converter

$$f_{s}=$$ minimum switching frequency of the converter, here, 960 kHz (from datasheet)

$$\Delta V_{OUT}=$$ desired output voltage ripple. Here, 10 mV.

Thus,

$$C_{OUT(min)}=\frac{100\hspace{2pt}mA*0.838}{960\hspace{2pt}kHz*10\hspace{2pt}mV}$$

$$C_{OUT(min)}=8.729\hspace{2pt}\mu F$$

With a chosen ripple of 10 mV, a minimum capacitance of 8.8 µF was required. Best solution would be to place two 4.7 µF Ceramic Capacitors in parallel (similar to Input Capacitors).

If we consider the ESR (Equivalent Series Resistance) of our Output Capacitors, we get an additional ripple as follows:

$$\Delta V_{OUT(ESR)}=ESR*I_{SW(max)}$$

where, $$\Delta V_{OUT(ESR)}=$$ Additional output voltage ripple due to ESR

ESR = 15 mΩ (We know that the typical ESR of ceramic capacitors is in the range of 10 mΩ to 100 mΩ. We choose a value of 30 mΩ for our application. Since, we have arranged the two output capacitors in parallel configuration, the combined ESR becomes 15 mΩ. [8].

$$\Delta V_{OUT(ESR)}=15\hspace{2pt}m\Omega*700\hspace{2pt}mV$$

$$\Delta V_{OUT(ESR)}=10.5\hspace{2pt}mV$$

If we consider this ripple voltage caused by the ESR of capacitors, our minimum value of the output capacitance would get halved. But, considering the ripples caused by load transients, we keep the Output Capacitor well above 4.7 µF.

Testing - Following tests have been conducted:

1. Applying input voltage = 1.491 V from a Single Alkaline Cell (Duracell®).
2. Choosing a Load of 200 Ω, 1/4 Watt Resistor (2x100 Ω Resistors in Series).
3. Selecting 3.3 V through the voltage selection switch and taking measurements across a load of 200 Ω, we get output current as 16.40 mA.
4. Measurements across individual 100 Ω resistances give output currents of 16.35 mA and 16.44 mA.
5. Thus, Output Power = 3.280 V * 16.4 mA = 53.792 mW

Ripple Voltage (Vpp) calculated for 3.3 V at no load = 50 mV

Ripple Voltage calculated for 3.3 V at 100 Ω Load = 68 mV

Ripple Voltage calculated for 5 V at no load = 150 mV

Ripple Voltage calculated for 3.3 V at 100 Ω Load = 168 mV

Also, applying a load resistance of 100 Ω, we get

Output current at 3.3 V = 30.10 mA,

And, Output current at 5 V = 46.40 mA

For 3.3 V Output:

Ripple calculated for single-cell alkaline battery input - 138 mV

Output Current - 75.6 mA without sacrificing output voltage (remember constant power discharge).

Applications - The Pocket Step-Up Converter can be used as a Portable power supply for powering up most of the projects involving Microcontrollers, Sensors, etc. It can not, however, supply the necessary current to drive high torque DC motors or Servo motors.

Q. 1) Can i connect multiple devices in Series/Parallel combination? A. 1) Yes, treat them as normal batteries.

Q. 2) What if need more current than the advertised 75 mA? A. 2) You need a good power supply that can provide high current when required by your load. But, it should be less than the Maximum Switch Current (as we calculated above) of the TPS61070 IC.

Q. 3) What is the minimum current required to turn on the circuit? A. 3) It is the Pre-Charge current required to charge the input capacitor.

Q. 4) I want to charge my cell phone (USB Charger). Can i do it? A. 4) Yes, you can. If you are comfortable with basic soldering skills and not worry about charging your phone for hours with a non-commercial charger. The Pocket Step-Up Converter can provide approximately 40 mA using a single cell battery (until it goes all the way down to 0.9 V, efficiency will drop to 70 %) - figure 4 datasheet. If you need more current, input power may be increased by using a 3.3V power rail or a Lithium Polymer battery (3.7 V). This may be able to provide 230 mA (Vin = 3.3 V) to 265 mA (Vin = 3.7 V) (calculated from the above mentioned formulae). It is far from the USB Standard Downstream Port's High Power mode of 500 mA but well above the 100 mA Low Power mode. [9]

Q. 5) What if i want 500 mA @ 5V? A. 5) You will have to provide a suitable input power source that can relate to this graph in the datasheet [1]. It is not suggested to increase the input beyond 4.5 V. Also, it defers the use of a Step -Up Boost Converter.

Q. 6) Can i use a Solar Cell as Input to the Pocket Step-Up Converter? A. 6) We are yet to find a panel with suitable rating that will work with the Pocket Step-Up Converter. You will be the first to know about our findings.

Q. 7) Can i run my robot off a single-cell AA battery? A. 7) No, This circuit cannot provide the necessary current to drive motors. It can however be used for powering up the sensors of the robot or even a low power microcontroller.

Here are some unsuccessful tests with a Solar cell of 4 V / 100 mA:

References: