# AN554 Note. Datasheet pdf. Equivalent

Recommendation AN554 Datasheet
 Part AN554 Description Application Note Feature AN554; AN554 APPLICATION NOTE CHOICE OF PROTECTION IN AUTOMOTIVE APPLICATIONS (CLASSICAL TOPOLOGY) INTRODUC. Manufacture ST Microelectronics Datasheet Download AN554 Datasheet

 AN554 APPLICATION NOTE CHOICE OF PROTECTION IN AUTOMOTIVE APPLICATIONS (CLASSICAL TOPOLOGY) INTRODUCTION This paper describes a protection schematic based on discrete components, together with a general meth- www.DataSheet4Uo.cdomof choosing the components to suppress the surge effects on automotive modules. Figure 1. General Protection Topology + Vbat Io RS D P Im C – GENERAL PROTECTION SCHEMATIC Positive impulsive overvoltages This type of overvoltage is clamped by the protection component P at maximum voltage VCL. Resistance RS limits the dissipated energy in the protection component without compromising the clamping function. Negative impulsive overvoltages There are two ways to limit these: – Without diode D: the protection component operates as a rectifier diode and clamps the voltage at the unit terminals to approximately 1V. – With diode D: the diode is reverse-biased and therefore protects the unit. One important thing to take into account is the peak reverse voltage limit of D. VRRM = 400V seems a good compromise (see curve N° 6 of the ISO/TC22 standard). Positive continuous overvoltages During this phase, the protection component must be in the stand- by phase (very low current passing through the component). Negative continuous overvoltages This protection is achieved by diode D which is reverse-biased. Impulsive voltage drop During this phase, the unit is fed by capacitor C while diode D prevents C from discharging into the battery circuit. April 2004 REV. D2A - 3584 1/9

 AN554 APPLICATION NOTE THE CHOICE OF COMPONENTS Diode (D) The following parameters will constitute the selection criteria: - The average current used by the electronic module. - The maximum repetitive peak reverse voltage VRRM - The maximum ambient temperature Tamb. The following inequality must apply in all cases: Tamb + Rth P < Tj max where www.DataSheet4U.com P = VTO IF (AV) + rd I2F(RMS) Rth = thermal resistance (Junction - ambient) for the device and mounting in use. Resistance (RS) Its presence allows a "size" (and thus cost) reduction of the protection component. Its value is a function of the following elements: Vbat min: lowest battery voltage which is specified in the technical note issued by the manufacturer. VCC min: minimum voltage needed for the electronic unit in operation. ICC max: maximum supply current of the electronic module. The maximum value of RS will be: RS max = (Vbat min - VCC min)/ICC max Capacitor (C) Its role is to make sure that the voltage at the terminals of the electronic unit is greater than or equal to Vcc min while the starter circuit is active. Its value depends on: Vbat: voltage across the battery before the disturbance VCC min: see “B: Battery voltage”. T: length of the disturbance (130 ms: see application note 4.1, paragraph III.4) The minimum value of C will be: Cmin = (130 * 10-3/Req)/ln (VCC min/Vbat) with Req = equivalent resistance of the electronic unit Req = VCC min/ICC max 2/9

 Protection component (P) - How it works: Figure 2. Transil Behaviour V VP www.DataSheet4U.com Vbat V VCL Vbat I IPP IPP/2 tp AN554 APPLICATION NOTE A t B t C t A: Disturbance B: Voltage across the protection device C: Current through the protection device The role of the protection device is to suppress the destructive effects of the surge (see Figure 2a), the most agressive being the load dump impulse. To achieve this, the TRANSIL clamps the spike at a maximum value VCL (see Figure 2b). A surge current flows through the suppressor during this phase (see Figure 2c). THE CHOICE OF THE PROTECTION DEVICE Parameters to take into account To choose the TRANSIL we have to know the surge parameters and the application requirements. Surge parameters. The surge is defined by the peak value Ip and the duration tp of the current wave flow- ing through the protection device during the clamping. As shown in the ISO/TC22 standard the most energetic impulsive disturbance is the load dump surge. Most car manufacturers recommend the SCHAFFNER NSG 506 generator to synthesise this wave (see Figure 3). 3/9