Datasheet LTC4364-1, LTC4364-2 (Analog Devices) - 15

LTC4364-1/LTC4364-2
APPLICATIONS INFORMATION
gate and source of each MOSFET for extra protection thermal mass. This is analyzed by simulation, using the (Figures 8 to 10). MOSFET’s thermal model.
Transient Stress in the MOSFET
For short duration transients of less than 100ms, MOS- FET survival is increasingly a matter of SOA, an intrinsic The SOA of the MOSFET must encompass all fault condi- property of the MOSFET. SOA quantifies the time required tions. In normal operation the pass devices are fully on, at any given condition of VDS and ID to raise the junction dissipating very little power. But during either overvoltage or temperature of the MOSFET to its rated maximum. MOSFET overcurrent faults, the HGATE pin is controlled to regulate SOA is expressed in units of watt-squared-seconds (P2t), either the output voltage or the current through MOSFET which is an integral of P(t)2dt over the duration of the M1. Large current and high voltage drop across M1 can transient. This figure is essentially constant for intervals coexist in these cases. The SOA curves of the MOSFET of less than 100ms for any given device type, and rises must be considered carefully along with the selection of to infinity under DC operating conditions. Destruction the fault timer capacitor. mechanisms other than bulk die temperature distort the During an overvoltage event, the LTC4364 drives the pass lines of an accurately drawn SOA graph so that P2t is not MOSFET M1 to regulate the output voltage at an acceptable the same for all combinations of ID and VDS. In particular level. The load circuitry may continue operating throughout P2t tends to degrade as VDS approaches the maximum this interval, but only at the expense of dissipation in the rating, rendering some devices useless for absorbing MOSFET pass device. MOSFET dissipation or stress is a energy above a certain voltage. function of the input voltage waveform, regulation voltage and load current. The MOSFET must be sized to survive
Calculating Transient Stress
this stress. To select a MOSFET suitable for any given application, Most transient event specifications use the model shown the SOA stress of M1 must be calculated for each input in Figure 5. The idealized waveform comprises a linear transient which shall not interrupt operation. It is then ramp of rise time t a simple matter to choose a device which has adequate r, reaching a peak voltage of VPK and exponentially decaying back to V SOA to survive the maximum calculated stress. P2t for a IN with a time constant of τ. A typical automotive transient specification has prototypical transient waveform is calculated as follows constants of t (Figure 6). r = 10μs, VPK = 80V and τ = 1ms. A surge condition known as “load dump” has constants of tr = Let: 5ms, VPK = 60V and τ = 200ms. a = VREG – VIN MOSFET stress is the result of power dissipated within b = V the device. For long duration surges of 100ms or more, PK – VIN stress is increasingly dominated by heat transfer; this is where VIN = Nominal Input Voltage. a matter of device packaging and mounting, and heat sink V V PK PK = 80V τ τ = 1ms VREG = 16V V V IN IN = 12V tr = 10µs t 436412 F06 r 436412 F05
Figure 5. Prototypical Transient Waveform Figure 6. Safe Operating Area Required to Survive Prototypical Transient Waveform
436412f 15 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts