Tethys LRAUV
Load Control System
Description  (Draft)

28 May 09	ecm	initial draft
  8 Jun 09	ecm	revise to released board spec
  5 Nov 10	ecm	revise to 3.1
15 May 12	ecm	revise to 3.3
14 Jul 14	ecm	add serial, micro, and config outline
22 May 17	ecm	add parts sub notes for Rev 3.4

Summary

The Load Control System distributes and controls serial I/O and power to peripheral devices within the Tethys vehicle.  The overall system consists of up to four 8-slot Backplanes, each containing up to eight Load Control Boards (total 32).  The vehicle main computer communicates with the Load Control Boards (LCBs) via SPI (Serial Peripheral Interconnect).  The vehicle Motherboard supplies bulk DC power for distribution, at nominal voltages of 3.3, 5, 12, 24, and Raw Battery (12-17V).  

Each LCB contains a UART, level shifters for RS-232 or RS-485, and a power switch with soft start, circuit breaker, and isolation functions.   Each board is configured at assembly time for one of three serial formats and one of the five power distribution voltages.  A PIC microcontroller on each board operates the power switch section based on commands from the main vehicle computer, and returns telemetry of current, voltage, and switch state.  Individual peripheral devices throughout the vehicle plug into output connectors on each LCB to receive switched power and serial comms.

The Load Control System is designed with micropower operation as a design priority, and operates in standby (power on, no serial traffic) on a few mW per channel.

Specifications

Power Voltages:	Nominally 3.3V, 5V, 12V, Raw Battery (10-17V), and 24V
			(any combination up to five voltages can be configured)

Current Rating:		3A; nominal 2.7A current limit

On Resistance:		0.05 ohm

Circuit Breaker:		Hardware current limit at 2.7A
			Hardware circuit breaker after 70 msec in current limit
			Sortware circuit breaker with (TBD) scan times
			Separate Peak and Average trip values
			5A fuse

Soft Start:		Load inrush charge 0.2 Coulomb max (approx)
			e.g. up to 6800 uF at 24V, up to 33,000 uF at 5V, etc

Isolation:		dual coil latching relays
			DPST power, 4PST comms

Serial Comms:		9600 - 115k bits/sec
			RS-232, RS485, or TTL (configuration option)

Telemetry:		0 - 4A, 0 - 25V, 10 bit resolution
			Switch Closed, CB Tripped, and Output-On flags

Board Power:		3.3V logic supply @ ~1 mA
			3.3V serial-comms supply, current depends on serial format
			12V or raw battery supply, 25 mA for 10 msec (relay coils)
			Power switch circuit draws 0.25 mA from power rail when active

Dimensions:		5”w x 2”h x 0.45”t, fits 0.5” backplane pitch

Description

1) Power Switch

a) Switch/Current Limiter:   

The power switch section functions as an on/off switch, up to a current limit which is programmed by a current command from the micro (U12).    The switch acts as a current limiter, holding current to the programmed value, for short durations (a few dozen milliseconds) after which it is shut down to avoid thermal limits.  The current limit provides a soft-start function for capacitive loads, and the shutdown provides a circuit breaker function for load and wiring faults.
.   
The core of the power switch is the current-limit circuit formed by Q7, U8, and R3.  The current-limit level is set by the voltage on U8’s (+) input, which in turn is set by drawing a small command current from the positive rail, through R20, and down through Q2.  

The R26/R21 divider establishes a millivolt-level bias, in the Off direction, that overcomes any offset in U8 and guarantees that Q7 is turned hard off (U8’s output swings high) when the current command is zero.  Zener D15 protects Q7’s gate, and C13/R24 compensate Q7’s gate capacitance.

b) Current Command

The current command sets the current limit of the main switch Q7.   The command current is drawn by Q2 and R16, in proportion to the input voltage on Q2’s base which is the signal labelled SwitchOnCurrent.  In the simplest case this input is a 0V/3.3V logic signal that turns the main switch either off, or on with a 2.7A current limit.  This input voltage can also be varied linearly to generate a programmable current limit, in which case R13 and C11 can serve as a lowpass filter for a >1kHz PWM signal from the micro.  The command current can be re-scaled by changing R16.

c) Phantom Ground:  

The power switch is designed to work with power voltages between 3.3V and 24V (or higher, with suitable choice of Q7).  To accommodate this wide range, the power switch circuits (eg U6-7-8) all draw operating current from the positive rail, and return it to a ‘phantom ground’ rail that is held 7.5 volts below the positive rail.  This ‘ground’ current then flows through Q1 and down to actual ground via Q3.    This arrangement provides a controlled, low impedance gate swing for Q7 (via U8), and a low voltage environment for the rest of the power switch circuits, regardless of power voltage.     As an incidental feature, the power switch circuits can be powered down completely by opening Q3, which allows the phantom ground rail to drift upward to the positive rail, and reduces overall consumption from 250uA to <1 uA.

The phantom ground’s voltage is regulated by Q1.  For power voltages greater than 10V, and assuming Q3 is on, zener D4 holds the gate of Q1 at 10V below the input rail.   Q1 then turns on any time its source, which is the phantom ground rail, rises Vgs(th) (about 2.5V) above this (which is 7.5V below the positive rail), and conducts phantom ground current down to the common rail.   Q1 transconductance is high enough to keep Vphantom within a few tenths of a volt even with milliamps of current flowing, such as during Q7 gate excursions.

For power voltages of 3.3V or 5V, the phantom ground is not required and the switch circuits can operate between the positive rail and the power common rail.  This is accomplished by bypassing Q1 with JMP2.

d) Current Sense

Shunt R3 is already present to generate the feedback signal for current limit amplifier U8.  The shunt signal, nominally 20mV per amp, is also used to drive current sense amplifier U7.     R25 and Q5 convert the shunt voltage into a current signal with a nominal value of 10uA per amp.  Q5 also translates the signal down to the common rail, where it is scaled to a voltage by R15 and then buffered by U3.  C10 provides lowpass filtering and R18 limits fault current which could flow from the positive rail through R25, Q5, and U3 (via ESD diodes) onto the logic supply LCB3.3+.

e) Current-Limit Detect

During normal operation, the power switch Q7 is either off or fully on, and U8’s rail-to-rail output swings Q7’s gate all the way to either the positive or phantom ground rails respectively.  When load current reaches the current-limit setpoint, U8 drives Q7 into the linear region, with gate voltage typically 2-3V below the positive rail.  Q7 will overheat rapidly while limiting current, potentially dissipating as much as Vin x Ilim which can be dozens of watts.  To signal this condition, U6 plus U18 form a window comparator that drive opto U19 whenever the gate drive to Q7 is potentially in the linear region, defined as more than 0.25V away from either rail.  The microprocessor should respond to the resulting CL_Active signal by turning off the switch command (SwitchOnCurrent) within 50-75 msec, to limit the temperature rise of Q7 to survivable levels.

f) Discussion

The LT6003 op amps used for current sense and current limit detect are ~2uA devices that are very slow (2 KHz GBW, 0.2V/msec SR).  If more speed is needed for any of these functions, the LT6000 should be able to drop in and provide 10x speed, at the cost of 10x supply current.

The “floating” switch section, which operates referenced to the positive rail, is interfaced to logic and power common by high-voltage devices (Q1, Q2, Q3, Q5, Q6).   With a suitable choice for Q7 (and possible re-scaling of R10), this should allow the switch circuit to operate to at least 48V, and possibly quite a bit higher.

The input current command (SwitchOnCurrent) and current sense output (CurrSense) functions are subject to offset and gain errors, due to several 1% resistors, op amp offsets, etc.  For best accuracy these errors can be calibrated out by checking at (say) 10% and 90% of full scale, and storing correction values in the microprocessor.

The current rating of the switch, and the circuit breaker trip time, represent conservative values based on worst-case voltage (24V).  There is margin in the other current-carrying parts of the circuit to explore higher current ratings, or longer trip times, once the stock values are proven reliable and when applications emerge that require higher ratings.

2) Serial Comms

a)	 SPI UART U10
b)	RS-232 Level Shifter U11
c)	RS-485 Level Shifter U13 and Transmit Enable timer U9
d)	Blinky light U14, D2
e)	Comms isolation relay K1
f)	Serial config options: 232, 485, TTL, or Offboard Serial

3) Discrete I/O

a)	Pull-up Q8, Pull-down (open drain) Q10, current limiter RT1
b)	Input scaling R41/R42; filter C29

4) Microcontroller and Bus Interface

a)	PIC Micro U12
b)	Programmer interface J1-D12
c)	SPI selection logic U15
d)	SPI bus buffers U16

5) Assembly and Configuration

a)	Power switch voltage selection
b)	Serial format
c)	

Revision Notes

Rev 3.4, for “Build-Five”

NLA Parts Subs:

1)	C12, C17	.01uF/630V	Panasonic ECJ-3FB2J103K 	TDK CGA5K4X7R2J223K130AE
2) 	D8		ESD Array	Cal Micro CM1216-06SM	Cal Micro CM1216-06MR
3)	K1		4P Relay	Panasonic DS4E-SL2-DC12V	2 ea Panasonic DS2E-SL2-DC12V 
4)	Q7		P-FET		Infineon IPB100P03P3L-04	Infineon IPB120P04P4L03ATMA1