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MAX6499 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX6499 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page • The overvoltage waveform period (tOV) • The power dissipated across the package (PDISS) During an initial overvoltage occurrence, the discharge time ( ∆t1) of COUT, caused by IOUT and IGATEPD. The discharge time is approximately: where VOV is the overvoltage threshold, IOUT is the load current, and IGATEPD is the GATE’s 100mA pulldown current. Upon OUT falling below the threshold point, the MAX6495/MAX6496/MAX6499s’ charge-pump current must recover and begins recharging the external GATE voltage. The time needed to recharge GATE from -VD to the MOSFET’s gate threshold voltage is: where CISS is the MOSFET’s input capacitance, VGS(TH) is the MOSFET’s gate threshold voltage, VD is the internal clamp (from OUTFB to GATE) diode’s for- ward voltage (1.5V, typ) and IGATE is the charge-pump current (100µA typ). During ∆t2, COUT loses charge through the output load. The voltage across COUT (∆V2) decreases until the MOSFET reaches its VGS(TH) threshold and can be approximated using the following formula: Once the MOSFET VGS(TH) is obtained, the slope of the output-voltage rise is determined by the MOSFET Qg charge through the internal charge pump with respect to the drain potential. The new rise time needed to reach a new overvoltage event can be calculated using the following formula: where QGD is the gate-to-drain charge. The total period of the overvoltage waveform can be summed up as follows: ∆tOV = ∆t1 + ∆t2 + ∆t3 The MAX6495/MAX6496/MAX6499 dissipate the most power during an overvoltage event when IOUT = 0. The maximum power dissipation can be approximated using the following equation: The die-temperature increase is related to θJC (8.3°C/W and 8.5°C/W for the MAX6495/MAX6496/MAX6499, respectively) of the package when mounted correctly with a strong thermal contact to the circuit board. The MAX6495/MAX6496/MAX6499 thermal shutdown is governed by the equation: TJ = TA + PDISS ( θJC +θCA) < +170°C Based on these calculations, the parameters of the MOSFET, the overvoltage threshold, the output load current, and the output capacitors are external vari- ables affecting the junction temperature. If these para- meters are fixed, the junction temperature can also be affected by increasing ∆t3, which is the time the switch is on. By increasing the capacitance at the GATE pin, ∆t3 increases as it increases the amount of time required to charge up this additional capacitance (75µA gate current). As a result, ∆tOV increases, there- by reducing the power dissipated (PDISS). PV I t t DISS OV GATEPD OV . =× × × 0 975 1 ∆ ∆ ∆ ∆ t Q V V I GD GS OUT GATE 3 ≅ ∆ ∆ VI t C OUT OUT 2 2 = ∆tC VV I ISS GS TH D GATE 2 () = + ∆tC V II OUT OV OUT GATEPD 1 005 . ( ) = × + 72V, Overvoltage-Protection Switches/ Limiter Controllers with an External MOSFET ______________________________________________________________________________________ 11 ∆t2 ∆t1 GATE OUTFB ∆tOV ∆t3 Figure 5. MAX6495/MAX6496/MAX6499 Timing |
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