Designing a boost converter for a photo cammera

[ealex]

Hello

I've been following Dave's blogs and the forum for some time now and got some great ideas / design tips but now I have an interesting problem on my hands and I don't know how to proceed.

I have a Canon camera that uses a 2CR5 battery - basically 2 Li primary cells in series, hard to find and expesive. The "normal" voltage is around 6.8-7V.
There is also a grip that will take 4xAA batteries - 6V if using normal cells and ~ 5V if using NiMh rechargeable.
The camera complains when using normal cells and will not start / barely work when using rechargeable ones.

I could alter the grip to accept available Li rechargeable ( some 700-800mAh prismatic cells ) but then i'll have to carry those all over me with no way to easily recharge them on the field or use readily available AA's .

With all 4 cells inserted i have an unused area of ~ 25x30x5 mm, larger if i follow the case profile. If you have a BP-50 grip i'm referring to the area in witch the 2CR5 logo is present.
I could fit in there a small boost converter that will add the extra 2-3V required when i'm using NiMh or ordinary cells.

The problem consist in choosing the right controller and configuration that will fit in that area and be able to supply ~ 7V at 2A peak.
In normal usage the camera draws ~ 500mA when driving the focus mechanism and AF assist lamps, film wind and less than 10mA when idle.

The first controller that came to mind is MAX668 (got some samples) but it's in a large package and quite expensive on farnell.
I've used some 6 terminal boost controllers for low power, but this is way over my head.

Do you know if there are come cheaper / simpler boost controller available that will run from 4V and could fit in that area ?
A already built module would be even better - it's a one-off and i don't like risking the camera .

The attached image shows the area i'm referring to.

Thanks

[peter.mitchell]

get 4x 14500 lithium batteries, go series-parallel, get a wall charger for them.

[ealex]

I thought about that, but i could find only 400mAh cells in that format. I'll have to carry at least a spare set.
I've been looking at LM3478: http://www.ti.com/lit/ds/symlink/lm3478.pdf.
It's quite cheap and the documentation looks ok.
I'll add later the computed values.

[ealex]

These are the design parameters
Vin(min) = 4V
Vout = 7V
Iout = 2A
Fsw = 500kHz
Vdiode = 0.4V ( estimated )
Vq = 0.1V ( estimated voltage drop across the switching transistor )

Using the formula form the data sheet:

L > 0.9uH -> chosen value L = 4.7uH
delta IL for 4.7uH = 0.704A
average IL = 3.795A
peak IL = 4.5A ( 5A inductor chosen )
peak switch current: IswMax = 4.198A
diode peak current: Idpk = 4.5A
current sense resistor: Rsens = 0.025 ohms
output capacitor RMS curren Icout = 1.87A ( maybe 10-20uF ceramic ? )

These are the values under worst case conditions.

Next step will be to source all the parts.
For the actual layout I am thinking on building it dead-bug style, but using a thin copper sheets instead of the PCB to save space.