Purpose

Enables the ability to switch endpoints on/off, provides redundant power from AUX/DCDC to all loads, and protects the LV bus from endpoint short circuits by disconnected damaged endpoints.

Pin Mappings

Load Switches

Diagnostic mode is always pulled high for current sensing. Toggling the sense select pin high reads output 1, and low reads output 0.

Expander 1

Address: 0x21

Expander 0

Address: 0x20

Power Input

Transient Suppression

Each power input is coming from a long wire originating from the battery box and going to the front of the car. These wires will have a lot of inductance, causing voltage spikes which may damage components. A TVS diode is placed at each input to clamp the rail voltage in the event of any transients.

Current Sensing

There is current sensing for monitoring/logging purposes. An INA180 will be used due to it having good performance being the cheapest current sense amplifier available. The sense resistor will be placed on the low side due to the INA180 have much lower input offset with no common mode DC bias.

Resistor and Gain

Using a gain of 100V/V, the input measurement range is 33mV for a 3.3V output. Using a 6mR sense resistor, the current measurement range is 5.5A at 600mV/A. This is above the maximum current in the power budget of 3.5A with some margin.

Power Supply

The input PSRR is typically 8uV/V. Assuming 3V3 from controller board has a ripple of 50mV, the error caused by that would be 0.4uV. This would amount to an additional error of 0.07mA, which is negligible. Thus an LDO is not required to power the IC.

Input Capacitance

Input capacitors are placed before the current shunt to reduce the common mode swings across the sense resistor caused by the ESL of the source during changing loads.

Power Path

LTC4418 - Dual Channel Prioritized PowerPath Controller (ciiva.com)

Enables dual powering of loads, where if the primary supply (V1) is no longer available (voltage not within specified OV/UV range for a certain amount of time), it will switch automatically to a secondary supply.

Shutdown

Shutting down the power path ICs provides the ability to fully de-power the vehicle. The shutdown pin of both ICs are tied together and connected to the switch on the dash.

Inrush

LTC4418 minimized startup inrush by soft starting the gate drive of the first input to connect the output. However, during supply switchover excessive inrush due to the voltage difference between the 2 supplies can lead to undesirable UV fault.

When DCDC switches to AUX:

• Vout Min = 9V (DCDC supply 10V - 1V droop)

• Vin Max = 15V (Fully charged AUX)

• Rsrc = 400mR (200mR cells, 165mR 6m round trip 2x 22AWG, 35mR misc. fuse, conn, shunt, etc.)

• ESRcout = 10mR

• Rdson = 5mR

Inrush = 14.3A

When AUX switches to DCDC:

• Vout Min = 9V (Depleted AUX 10V - 1V droop)

• Vin Max = 13V (DCDC supply)

• Rsrc = 230mR (30mR DCDC output cap, 165mR 6m round trip 2x 22AWG, 35mR misc.)

• ESRcout = 10mR

• Rdson = 5mR

Inrush = 16A

Given that the two inputs are similar in voltage and the ESR of the source is high due to long interconnects, expected inrush is low and thus slew rate limiting the gate drive as described in Figure 4 of the datasheet is not required. In reality, inrush is likely to be even lower due to ESL of long wires.

Output Capacitance

Sufficient capacitance must be present to minimize the output voltage droop during supply switchover. The highest minimum voltage I could find for a priority load is 9V for the WS22 motor controller. Given the UV threshold is 10V, allowable voltage droop is 1V to keep the LV bus above 9V.

Using the following values:

• Imax = 3.5A (LV Power Budget [MSXV])

• tg = 4us (datsheet worst case)

• Vdroop = 1V

• ESR = 10mR

Cout = 14.6uF

Use of AU caps was avoided since they are prone to damage while reworking nearby components with hot air. A few different MLCC caps were considered below.

As expected, a larger package size and more stable dielectric contributes significantly to the voltage derating of the part, leaving 1210 X7R parts as the clear option. The capacitance at 12V between 22uF and 10uF parts is very similar, with the 10uF 35V part having surprisingly low voltage derating so it is chosen. Two of them will be used to satisfy the capacitance requirement.

Input Capacitance

Input capacitance is added to decouple the long ESL of the power wires, and reduce the voltage droop during input switchover. 20uF of input capacitance, the same as the output, is added which should theoretically cut the voltage droop in half.

Operating Thresholds

The operating threshold is determined by the voltage range of the aux battery which is from 10-15V. This allows the aux battery to be disconnected automatically when it becomes depleted. Some margin will be added so that, accounting for resistor divider and comparator error, resulting in the following thresholds:

• UV = 10.5V

• OV = 15.5V

• Hysteresis = 500mV

An input will be considered valid given that it is between 11V and 15V with 500mV of hysteresis to allow for voltage droop due to inrush or transients due to ESL of input.

Hysteresis will be set with a external current of 500nA, giving Rhys to be 126k. We will use 127k as the closest common value, yielding Iext = 496nA.

Using Iext from above and 500mV desired hysteresis, R3 = 1.008M, 1M is used as the closest value.

The comparator threshold is 1V, so R1 = 71.3k, with 71.5k used.

R2 = 33.8k, with 34k used.

Actual configuration, not accounting for error is as follows:

• UV = 10.48V

• UVhys = 496mV

• OV = 15.46V

• OVhys = 513mV

Pmos Selection

Given the large amount of inductance that the long power cables would have, it is conceivable that during input switching, there can be large inductive voltage spikes on the power rail which might exceed 20V. Thus, the Vds of the Pmos was chosen to be at least 30V, and the lowest cost option with a Rdson of < 10mV was chosen.

Load Switch

Current Sensing

The IS pin outputs a current proportional to the load current during nominal operation. Sensing voltage to load current calculation can be found in output table above.

Clamping Diodes

During a fault or open load scenario, the ISense voltage can exceed the 4V maximum rating on the MCU ADC pins. No discrete clamp diode is added since the mux clamps inputs to Vcc + 0.5V, where Vcc is 3.3V.

Layout

Given the max steady state current is anticipated to be 3.5A, minimum conductor width will be sized for 4A with a temp rise of 25C. Conductors will be enlarged beyond the minimum where possible to reduce power loss.

Inner Layer 0.5oz: 6mm

Outer Layer 1oz: 3mm

0.3mm/0.5mm via: 1.1A

Thermal Reliefs

Where pads connect directly to very large planes (typically ground) a thermal relief is added. Due to the high density of parts many smaller polygons do not feature thermal reliefs to pads, since they would take up a significant portion of the polygon area.

Mounting Holes

Mounting holes and any attached standoffs are grounded. Test equipment may be clipped onto these points to help with debug.

Component Designators

Component designators have largely been omitted from the layout due to space constraints. The usefulness of designators decreases with increasing board size, as having “C57” among hundreds of 0603 parts does not make it easier to find. Eventually the “Top Designator” layer will be updated with all designators so that they are included in the PDF export.

Change Log

Rev 1.0

Somehow the NC drill file got messed up

• There was an erroneous drill hole at (x, y) (14.75, 58.25)mm, shorting both 12V fused and gnd

  • Resolution: Drill out the via, luckily there are no nearby traces/components

• Missing holes at (13.25, 55.25) and (13.85, 54.4), which are the A and B supplies for DCDC power path U5B

  • Resolution: Solder jumper wires