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Posted on 01 December 2019

Power Management Device for Door Zone Systems

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Battery life is the most important factor in electronic modules for automotive

Vehicle manufacturers call for lower and lower current consumption for every module in the car: severe requirements are common to almost all the ‘over key’ applications in the body segment. Those specific electronic subsystems must also operate in ultra-standby mode, drawing a very low quiescent current, and this minimum sink current maximizes and extends the battery life. The most effective way to reduce current loss is to optimize at the ‘module level’ and not at ‘device level’ as was done for the previous generation of modules.

By Martina Giuffrida, Technical Marketing Engineer, STMicroelectronics and Giovanni Torrisi, Senior Technical Marketing Engineer, APG CAR BODY Division, STMicroelectronics

 

The Door Zone System module has become a widely used solution: this module typically contains a supply reference (voltage regulator), motor controllers (mirror folder and adjustment, power window), bulbs drivers (to drive footwell, exterior, and blinker lamps), special load drivers (camera view supply, electro-chromic mirror glass control, mirror heater) and a transceiver block that is responsible for the communication between all the subsystems in the automobile. The transceiver can be either a CAN or LIN module, according to cost and technical requirements. While CAN provides serial communication with a high level of security and robustness in noisy environments and can reach a bit rate of 1Mbit/s, the creation of the LIN protocol assured the same level of reliability as the CAN solution, with a significant cost reduction, although much slower.

A new device, (named L9952GXP, see Fig. 1) gathers a Power Management Strategy, a Lin Physical Layer, and Power Actuator Drivers in an ‘all-in-one’ solution that provides a sensible improvement, simplifying the Door Zone Applications’ implementation. More specifically, the device embeds several features, as two 5V low drop voltage regulators (the first one for microcontroller supply with a current capability of 250mA and the second one for peripheral supply with a current capability of 100mA), the window watchdog, and “wake-up” logic with cyclic contact monitoring. The device is LIN 2.0 compliant; a 24- bit SPI interface is used for mode control and diagnostics, and all the outputs are short circuit and temperature protected.

A Power Management Strategy, a Lin Physical Layer, and Power Actuator Drivers

Battery life is the most important factor in electronic modules for automotive, and this in turn means that current consumption of electronic devices must be minimized. Because of this, car makers focus on quiescent current when choosing a specific device for a specific application. In this sense, one of the most valuable features of the L9952GXP, also called the companion chip, is its advanced power management: this concept, in applications like the door zone, reduces system quiescent current to a minimum. In order to do this, two independent voltage regulators (V1 and V2) are necessary.

V1

The first voltage regulator provides 5V supply voltage and up to 250mA continuous load current for the external digital logic (micro controller, CAN transceiver, etc.). In addition, regulator V1 drives the internal companion chip’s 5V loads. The voltage regulator must be protected against overload and over-temperature. An external reverse current protection has to be provided by the application circuitry to prevent the output capacitor from being discharged by negative transients or low input voltage. The output voltage precision is better than ±2% (including temperature drift, line regulation and load regulation) for operating mode; respectively ±3% during low current mode. Current limitation of the regulator ensures fast charge of external bypass capacitors. The output voltage is stable for ceramic load capacitors bigger than 220nF (instead of bigger and expensive electrolytic capacitors).

If device junction temperature hits thermal shutdown, all outputs except V1 will be deactivated - high side switches, low side switches, and the other regulator V2, LIN. Hence, the microcontroller has the possibility for interaction or error logging. In case of exceeding a higher thermal shutdown threshold (TSD2), V1 will then be deactivated. A timer is started and the voltage regulator is deactivated for 1 second. During this time, all other wakeup sources (CAN, LIN, and WU1-4) are disabled. After 1 second, the voltage regulator will try to restart automatically. If this higher threshold (TSD2) occurs within one minute and for 8 consecutive times, the device enters a mode named ‘VBAT standby mode’. This standby mode, later described, can also be reached in case of a short to ground of V1 after the initial turn on. In this case, the reactivation (wake-up) of the device can be achieved with signals from CAN, LIN, WU1 to WU4, SPI.

V2

The smaller voltage regulator, V2, supplies additional 5V loads (for example, logic components, external sensors, external potentiometers). The continuous load current is 50mA. The regulator provides accuracy better than ± 3% at 50mA (±4% at 100mA). In case of a short to ground of V2 after the initial turn on, the V2 regulator is switched off. The microprocessor has to send a clear command to reactivate the V2 regulator.

The power-management functionality of the device allows it to reach the quiescent current required for different sleep modes. Typical values are in the range of 100ìA per module. The concept of the power-management functionality of the L9952GXP is depicted by the fact that the microcontroller can be operated in stop or halt mode, or can be disabled even by switching off the 5V supply. The required read-out of the contacts is executed by the power-management device. Thus, the device can be operated in cyclic and in static read-out configuration of the contacts, depending on the carmaker’s specification and target for quiescent current. The setting is programmed via SPI before entering the sleep mode, and then the companion chip operates independently. The time base for the cyclic contact supply and read-out is generated autonomously inside the power management device by using an integrated RC oscillator. An additional filtering strategy (debouncing) for wake-up inputs is realized without any additional use of the microcontroller, using the internal time base. This avoids any wake-up events induced by EMC noise.

VBAT standby mode

To achieve minimum current consumption during VBAT standby mode, all the functions (except the ones for wake-up functionality) are switched off. In VBAT standby mode the current consumption of the L9952GXP is reduced to 7ìA, typical (without cyclic sense feature selected). The transitions from active mode to either V1 standby or VBAT standby are controlled by SPI. VBAT standby mode is dominant; if relevant bits, V1 standby and VBAT standby are set to “1”, the device will enter VBAT standby mode.

V1 standby mode

In this mode, outputs and internal loads are switched off. To supply the microcontroller in a low power mode, the voltage regulator 1 (V1) remains active. The intention of the V1 standby mode is to preserve the ROM contents of the external micro. A cyclic contact supply and wake-up input sense feature (for cyclic monitoring of external contacts) can be activated by SPI.

Two possible standby modes in a door aone module VBAT standby and V1 standby

Cyclic contact monitoring

In addition to a continuous sensing (static contact monitoring) at the wake-up inputs, a cyclic wakeup feature is implemented. This feature allows periodical activations of the wake-up inputs to read the status of the external contacts. The periodical activation can be linked to Timer 1 (0.5sec to 4.0sec in 0.5sec steps) or Timer 2 (50ms). The input signal is filtered with a filter time of 16us after a programmable delay (80us or 800us). A wake-up will be processed if the status has changed versus the previous cycle.

Both standby modes (V1 and VBAT) support cyclic sense of the contacts.

Compared to the static contact monitoring, the main advantage of the cyclic contact supply and sense is a significant reduction of power consumption. For the open active contact (closed in not active state) - for example, when the power consumption in static mode is around 10mA for one contact; with the cyclic contact sense, this contact supply current is reduced to a short time when the contact is checked while the current consumption of the L9952GXP increases by approximately 65uA. Furthermore, when cyclic contact monitoring is adopted, the contact does not have to be powered by an external power source.

Contact monitoring is done during the “On- Time” phase of the timer selected for cyclic sense functionality. During the remaining timer period, the contact is not powered: this is why it is not consuming any power. Of course, this feature does effect some contact configurations like opening contact, contacts which have two stable positions (On/Off), and contacts with parallel resistance or parasitic resistance (due for example to humidity).

However, regardless of the configuration, the cyclic monitoring limits the current consumption in case of a stuck contact switch.

In order to reduce WU input leakage current when the contact is not powered and floating, the current sink or current source configuration is active only during the “Off-Time” phase of the timer for cyclic sense.. When the contact is checked (timer “On-Time” phase), the wake-up input is automatically reconfigured to the setup used in Active mode where an internal pull-down of 200K is activated.

As shown in Figure 3, any of the outputs has to be set to timer 1 or timer 2 mode to be able to power the contact in standby mode. Timer 2 is usually intended for contact sense, but timer 1 can be used as well if its settings are appropriate. The cyclic sense of the contact is based on the selected timer settings. The WAKE_UP input which is used for contact sensing must have the filter configuration relative to the adopted timer settings that means the right period and “Ontime”.

Any of the outputs has to be set to timer 1 or timer 2 mode to be able to power the contact in standby mode

If the configuration of the Output and the WAKE_UP input filter is correct and the device is switched to stand by mode, during every “On-time” phase of the selected timer the contact supply is activated (black contact supply in Figure 3) and the contact status is evaluated at the WAKE_UP input. The input signal is filtered and the WAKE_UP is processed when a level change on the input is detected.

Thermal behavior

The L9952GXP is housed in PowerSSO-36, which is a new power package ST designed to meet the market requirements that continuously call for increasing thermal performance.

Thermal performance is an especially critical factor within the small PCB area of the door zone system, which integrates several electronic devices closely together. For this reason, new power packages such as the PowerSSO family have been designed r to save space PCB real estate and to improve thermal performance as compared to standard “SO” plastic packages. The body dimension of a PowerSSO-12, for example, is the same as an SO-8, while the body dimension of a PowerSSO-24 is the same as an SO-16. The main difference between the “classic” plastic packages and the newer power packages is that in these new ones the heat dissipation from the chip to the PCB is performed through an exposed slug or pad soldered directly onto the PCB. This characteristic improves the overall junction-to-ambient thermal resistance when compared to the “SO” standard packages for integrated circuits, where the heat propagates through leads only. Building on ST’s long experience as a leader in power management ICs and smart-power technology, the L9952GXP reduces the PCB area and Bill of Materials, thereby enabling customers to minimize the complete system cost.

All these features make the L9952GXP a sophisticated but user-friendly power management device ideal for compact and lightweight systems. A very low quiescent current can be achieved with the use of this device. Control and diagnostics are managed through a serial peripheral interface (SPI), and superior thermal performance is guaranteed with the small power packages as the PowerSSO-36, that offers the optimum trade-off between thermal behavior, dimensions, and cost.

 

 

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