I'd place my bids on TPS61170 as compatible.
I didn't check the pinout against what is on the S5 board, but it's the only one I've found in 6 pins package with specifications that would match the part on board regarding Vin and Vout.
The device marking is supposed to be BZS, but maybe the one on the S5 board is simply a clone from another manufacturer.

http://www.ti.com/lit/ds/symlink/tps61170.pdf