Writing negative integer as 0xnn hex into peripheral

I am dealing with a hardware peripheral which exposes a writable characteristics for Measured Power. As per the specs from the hardware vendor Measured Power should be sent in the format "0xnn". I am not sure how do we convert a negative integer (-59) into 2 bytes hex representation (0xnn).



By far I have tried below


int16_t power = CFSwapInt16HostToBig(iPower);

NSData *powerData = [NSData dataWithBytes:&power length:sizeof(power)];

But this writes 0xFF into peripheral which is 255 in decimal. Any idea?