ASIC's Supply Pin connected to a car battery

ASIC Mix signal Specification

Automotive application: ASIC's Supply Pin connected to a car battery

ASIC Mix signal Specification

Automotive application: ASIC's Supply Pin connected to a car battery

Maximum Rating

Normal operating conditions

Error handling, Overvoltage and Undervoltage faults

Undervoltage  fault (VBAT_uv) , detection  hysteresis ( VBAT_uv_hys)  and filter time (t_VBAT_uv)

Undervoltage  fault

Undervoltage detection filter time

Implementation Note and specification requirement of hysteresis

Undervoltage detection hysteresis

Overvoltage  fault (VBAT_ov) , detection  hysteresis ( VBAT_ov_hys)  and filter time (t_VBAT_ov)

Overvoltage  fault

Implementation Note and specification requirement of hysteresis

Overvoltage detection hysteresis

October 4, 2011

Here is one detail of a possible specification of an ASIC Mix signal for automotive applications, that has an analog pin: VBAT, directly connected to a car battery.

An  implementation detail related to the specification outlined in the other blog entry: How to implement an ASIC Mix signal for an Automotive application: ASIC's Supply Pin connected to a car battery .

First assumption made here is that ASIC is working together with a microprocessor (MPU) in the system where it is used. MPU (master) and ASIC (slave) are exchanging data using one of common ( or "standard") interfaces of communication ( e.g. automotive ASIC's use a lot Serial Peripheral Interface Bus or SPI ).

In the master/slave configuration MPU can write and read registers in the ASIC using SPI bus.

Possible functionality related to the pin VBAT could include:

• supply of some ASIC's internal functions:  such as internal charge pump etc.etc.
• monitoring and reporting Overvoltage and Undervoltage faults on a car battery voltage

Here we will assume that there is already existing another ASIC's analog pin: GND, and all references of voltages in this specification are referenced to it.

Maximum Rating

Maximum rating describes that our ASIC will not get damaged ( but there is no guarantie that would be opertationel either) even if car battery voltage goes to extremes: minimum -0.3 and maximum 40V.

Pin name	Value= Min, Max
VBAT	-0.3V, 40V

Note: It is very likely that a car battery voltage connected to VBAT  with a reverse diode as protection. That is why in VBAT Maximum Rating it is assumed that negative voltage will never get more negative then -0.3V.

Normal operating conditions

Pin name	Value= Min, Typ,  Max
VBAT	6V, 14V, 40V

Note: During normal operation for the purpose of the ASIC functionality here is specified that the battery will always produce a voltage in the range of 6-40V, and a typical average value of the voltage should be 14V.

Error handling, Overvoltage and Undervoltage faults

The ASIC should monitor and report Overvoltage and Undervoltage faults on the car battery voltage ( or VBAT pin).
In the case of either Overvoltage and Undervoltage faults, a register flag vbat_ouv is set  and accessible by MPU( e.g. using SPI register read).
Once set, the register flag vbat_ouv stays 1 until there is MPU read and in the same time the value of the flag is clear by the read.
After the "clear-on-read" of the flag vbat_ouv, evaluation of Overvoltage and Undervoltage faults starts again.

Undervoltage  fault (VBAT_uv) , detection  hysteresis ( VBAT_uv_hys)  and filter time (t_VBAT_uv)

The VBAT Undervoltage fault is detected ( and the register flag vbat_ouv is set ) if VBAT voltage is LESS then  VBAT_uv, min=5.0V and max=5.5V ( typical value 5.2V ) , and this event lasts uninterrupted t_VBAT_uv time, between 250 and 350us ( typical value 300us ).

Parameter description	Parameter Symbol	Value: min, typ, max V	Comment
Undervoltage  fault	VBAT_uv	5.0, 5.2, 5.5 V	Measured on falling edge.

The time range ( between 250 and 350us, typical value 300us ) of detection/cancelation of VBAT Undervoltage fault is called Undervoltage detection filter time.

Parameter description	Parameter Symbol	Value: min, typ, max us	Comment
Undervoltage detection filter time	t_VBAT_uv	250, 300, 350 us

Implementation Note and specification requirement of hysteresis

VBAT Undervoltage fault  is going to be decided by a comparator.
Comparator should satisfy a hysteresis requirement: when VBAT is rising a voltage canceling VBAT Undervoltage fault should be higher of a voltage setting VBAT Undervoltage fault  when VBAT is falling.
Difference of the two voltages is called Undervoltage detection hysteresis.

The hysteresis is necessary to avoid that noise on VBAT cause oscillation on comparators output.   

Parameter description	Parameter Symbol	Value: min, typ, max mV	Comment
Undervoltage detection hysteresis	VBAT_uv_hys	50, 150, 250 mV

Overvoltage  fault (VBAT_ov) , detection  hysteresis ( VBAT_ov_hys)  and filter time (t_VBAT_ov)

The VBAT Overvoltage fault is detected ( and the register flag vbat_ouv is set ) if VBAT voltage is MORE then  VBAT_ov, min=30.V and max=34V ( typical value 32V ) , and this event lasts uninterrupted Undervoltage detection filter time, t_VBAT_ov= t_VBAT_uv.

Parameter description	Parameter Symbol	Value: min, typ, max V	Comment
Overvoltage  fault	VBAT_ov	32, 34, 38 V

Implementation Note and specification requirement of hysteresis

VBAT Overvoltage fault  is going to be decided by a comparator.
Comparator should satisfy a hysteresis requirement: when VBAT is rising a voltage canceling VBAT Undervoltage fault should be higher of a voltage setting VBAT Undervoltage fault  when VBAT is falling.
Difference of the two voltages is called Undervoltage detection hysteresis.

The hysteresis is necessary to avoid that noise on VBAT cause oscillation on comparators output. 

 

Parameter description	Parameter Symbol	Value: min, typ, max V	Comment
Overvoltage detection hysteresis	VBAT_ov_hys	0.5, 1, 2 V

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