Design Notes	
Lower Half Elex Board, Coastal Profiling Float

2018-05-01	ECM	create

Requirements
(from GM email 2018-01-30)
1. Manage 6 Inspired Energy 8S1P sticks, selectable 2 thru 6.  Voltage 20 – 33.6V.  
2. Battery stick ratings are 1.5A charge, 4A discharge.  Application discharge 8A total.
2. I2C c

Description

Design Notes

Oscillator/Sequencer

Use 4060 with RC osc (good for ~5% accuracy) and 14-stage divider; taps 12-14 should count at 2^-12 of osc freq.  Per conversation with Denis K, 1 minute is adequate step time to cycle between battery sticks, but near full charge, resumption of charge after a step sometimes takes 15-20 seconds, therefore he uses 2.5 minute (150 second) steps.  1/150 Hz * 2^12 = 27 Hz

74HC4060 datasheet (Nexperia) gives f = 1/(2.5 * Rt * Ct).  Reasonable values look like C = 0.22uF and R = 68.1k.  Use stock X7R capacitor, temp drift should be +/- 2.5% between 0-40C which seems OK  Main drawback is 10% tolerance.  C0G caps with 5% tol available, but 1210 case which means boardflex concerns.

Ripple counter driving decoder is classic case for runt pulses and spurious selects, but in this case the driven devices are all level (not edge) sensitive, and fairly slow (LTC4314 0.5-1 usec enable, ideal diodes probably several microseconds).  HC4060 specs 20 nsec per stage so 60 nsec for 3 outputs to settle.  Could include clocked latch on counter output for cleanliness,  but for simplicity and because runt pulses are 10X shorter than downstream enable times, keep it simple: drive HC138 directly with HC4060.

Ripple counter driving decoder will generate runt pulses which are unlikely to affect muxes (LTC4314 0.5-1 usec enable, ideal diodes probably several microseconds), however the reset pulse one-shot would be triggered.  Therefore us HC237 latch/decoder; HC4060 count advances on falling transition of CTC which is rising transition of RTC, HC237 latches inputs while /LE is high and propagates them after count has settled and RTC goes low.

I2C Mux

Two approaches considered
1) LTC4314 4-channel I2C Mux/Buffer (2 ea).  

Diode/Load Switches

Charge bus requires load switch per battery stick, to enable/disable charge current in sequence.  The charge bus needs at least diode functionality at the charger input, to prevent reverse current flow out the charge port or in to the charger.  It seems safer to have diode functionality on each load switch to each stick, to prevent sticks reverse-driving the charge bus (although there’s normally nowhere for that current to go except through an enabled switch into another stick).  

Discharge bus requires diode functionality to prevent battery sticks from back-driving each other in event of charge imbalance, or more importantly in event of shorted cell.  Discharge bus also can use load switch functionality to enable power control of the whole platform.

Simplest solution would adopt a common ideal-diode/load-switch circuit for each stick charge input and discharge output.  An alternate (not pursued) would be an ideal diode at charger input, charge bus load switches at each stick, discharge bus ideal diodes at each stick, and a load switch at the platform power output.