Designing a Step-Up DC-to-DC Boost Converter

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.

Step 1: Introduction

The Pocket Step-Up Converter 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.

The Pocket Step-Up 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 Step-Up 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).

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).

Step 2: 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

Quiescent Current - 19 µA (typical)

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

Load Disconnect During Shutdown

Overtemperature Protection

Step 3: 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]

Step 4: 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 Step-Up Converter. [Board Design files available on GitHub].

a.) BreadBoard Compatibility - It was decided that the Pocket Step-Up 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)

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.

Step 5: Calculations

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

In TPS61070, the diode is replaced with a low Rds(on) PMOS switch integrated into the converter. Thus, Diode calculations may be neglected.

The following parameters are required to calculate the power stage:
i.) Minimum Input Voltage, Vin(min) - 0.9 V

ii.) Desired Output Voltage, Vout = 5 V

iii.) Maximum Output Current, Iout(max) = 100 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 - [{Vin(min) * η}/{Vout}]

where, D = Duty Cycle
Vin(min) = minimum input voltage (this will lead to maximum switch current)

Vout = desired output voltage

η = Efficiency of the converter. For TPS61070, η = 90%.

Thus,

D = 1 - [{0.9 V * 0.9}/{5 V}

D = 0.838

All formulae with detailed explanations are available in the pictures above as the editor does not allow the use of Math Equations.

Step 6: Testing

Following tests have been conducted:

(i.) Applying input voltage = 1.491 V from a Single Alkaline Cell (Duracell®).

(ii.) Choosing a Load of 200 Ω, 1/4 Watt Resistor (2x100 Ω Resistors in Series).

(iii.) 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.

(iv.) Measurements across individual 100 Ω resistances give output currents of 16.35 mA and 16.44 mA.

(v.) Thus, Output Power = 3.280 V * 16.4 mA = 53.792 mW

Ripple Voltage (Vpp) calculations:

3.3 V at no load = 50 mV

3.3 V at 100 Ω Load = 68 mV

5 V at no load = 150 mV

5 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

Step 7: 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.