Does your iPhone lose power just when you need it most? (ie. In the subway when you need to look up an address). Well, this has happened to me quite often. Most recently on a trip to Vienna I needed to keep Google maps on all the time and needless to say, the battery drained quite quickly.

Because I didn’t want to shell out $50 for one, I decided to make my own…

Considerations in Design

I started looking at MintyBoost (http://www NULL.instructables NULL.com/id/MintyBoost!---Small-battery-powered-USB-charger/) which uses a boost regulator to convert power from 2xAA’s (3V) to USB required power (5V) at 500mA.

A linear regulator, 7805 that converts 9V to 5V, is not ideal in this situation as battery power that is not used is lost due to heat. For instance, a linear regulator that converts a 9V battery supply input into 5V output at 500mA will lose 4V *500mA = 2W. 9V batteries also have low amp-hours (number of hours that it can provide the current) and high internal resistance (~2ohms). So at 250mA, 0.25A*2ohm = 0.5V will be lost due to internal resistance.

MintyBoost uses 2 AA because they have more power 3000mAh at 3V compared to 500mAh for 9V batteries.

I wanted to reuse a Li-Ion battery from my former cell phone as the power supply. 1 battery has 1020mAh and typically 3.7V, 3.8Wh. But it is a lot smaller and portable. On thing to consider with Li-Ion that is very different than AA batteries is that Li-Ion batteries die at 3V. It is imperative to have circuitry to disconnect at 3V.

Converting around 3V to the required 5V (for USB) requires a boost regulator (or DC/DC converter).

Boost Regulator Specs

In the spirit of designing the circuit myself, I look for a boost chip with the following criteria:
– 8-DIP package
– 500mA @5V output (the original MintyBoost design suggests 100mA output @ 5V but with larger smart phone batteries, the charging will take forever), more info at http://www.ladyada.net/make/mintyboost/changelog.html (http://www NULL.ladyada NULL.net/make/mintyboost/changelog NULL.html)
– 3V minimum input voltage to accommodate Li-Ion batteries

I decide to go with MAX because they are great for sending free samples. The other ones such as Linear Technology, does not send to Canada.

Most chips today are surface mount and too tiny to solder. MAX does not have a lot of selections: MAX608, MAX1771. MAX608 is a good choice as the output voltage is factory set to 5V meaning fewer components but some of the components cannot be easily bought from my local electronics store (50mohm resistor).

Design for 2xAA Power Supply

iPhone Battery Booster using 2xAA power supply connected at JP1 and JP2

I used all the components that MAX1771 datasheet suggested: 22uH inductor (L1), 1N5817 schottky diode (D1), 0.1uF input bypass capacitor (C2), 0.1uF REF bypass capacitor (C3). R4 was chosen as 100mohm from the graph in Figure 4a of the datasheet. C1 and C4 were chosen as they are similar to the 9V Output example in Figure 2c of the datasheet. R1, R2, R3, and C5 were calculated as per datasheet to adjust output to 5V and mimics behaviour in Figure 5. A BS170 N channel MOSFET power transistor was chosen for its low total gate charge (Qg), on-resistance (rDS(ON)) and reverse transfer capacitance (CRSS).

Typically a USB connection only requires Power (5V) and Ground connections. The two other pins are for data (D+ and D-) for transferring information. However iPhones require 2V at each of D+ and D- for 500mA charging rate. Using 2.8V and 2 V results in a 1A charging rate. Please see http://www.ladyada.net/make/mintyboost/icharge.html for more details.

The above design is specific for 2xAA battery supply because it does not provide any method to put the IC in Shutdown mode if the battery supply falls below a specific voltage. The IC will be in Shutdown mode if pin 4 is HIGH. Currently pin 4 is Grounded.

Finally

This post details the considerations required to build a Battery Booster for the iPhone using 2xAA batteries and provides a schematic for a possible design. In my next post, I will specify how to replace the 2xAA batteries with a Li-Ion supply.

Stay tuned!