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LTC3872 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3872 Datasheet(HTML) 10 Page - Linear Technology |
10 / 22 page LTC3872 10 3872fc For more information www.linear.com/LTC3872 to 30V or less, and the switch node can ring during the turn-off of the MOSFET due to layout parasitics. Check the switching waveforms of the MOSFET directly across the drain and source terminals using the actual PC board layout(notjustonalabbreadboard!)forexcessiveringing. During the switch on-time, the control circuit limits the maximumvoltagedropacrossthepowerMOSFETtoabout 270mV, 100mV and 170mV at low duty cycle with IPRG tied to VIN, GND, or left floating respectively. The peak inductorcurrentisthereforelimitedto(270mV,170mVand 100mV)/RDS(ON) depending on the status of the IPRG pin. The relationship between the maximum load current, duty cycle and the RDS(ON) of the power MOSFET is: RDS(ON) ≤ VSENSE(MAX) • 1–DMAX 1+ χ 2 •IO(MAX) •ρT VSENSE(MAX) is the maximum voltage drop across the powerMOSFET.VSENSE(MAX)istypically270mV,170mVand 100mV. It is reduced with increasing duty cycle as shown in Figure 3. The rT term accounts for the temperature co- efficient of the RDS(ON) of the MOSFET, which is typically 0.4%/°C. Figure 4 illustrates the variation of normalized RDS(ON) over temperature for a typical power MOSFET. Another method of choosing which power MOSFET to use is to check what the maximum output current is for a given RDS(ON),sinceMOSFETon-resistancesareavailable in discrete values. IO(MAX) = VSENSE(MAX) • 1–DMAX 1+ χ 2 •RDS(ON) •ρT It is worth noting that the 1 – DMAX relationship between IO(MAX) and RDS(ON) can cause boost converters with a wide input range to experience a dramatic range of maxi- mum input and output current. This should be taken into consideration in applications where it is important to limit the maximum current drawn from the input supply. Voltage on the NGATE pin should be within –0.3V to (VIN + 0.3V) limits. Voltage stress below –0.3V and above VIN + 0.3V can damage internal MOSFET driver, see Func- tional Diagram. This is especially important in case of driving MOSFETs with relatively high package inductance (DPAK and bigger) or inadequate layout. A small Schottky diode between NGATE pin and ground can prevent nega- tive voltage spikes. Two small Schottky diodes can inhibit positive and negative voltage spikes (Figure 5). JUNCTION TEMPERATURE (°C) –50 1.0 1.5 150 3872 F04 0.5 0 0 50 100 2.0 Figure 4. Normalized RDS(ON) vs Temperature Figure 5 Figure 3. Maximum SENSE Threshold Voltage vs Duty Cycle DUTY CYCLE (%) 1 0 50 100 150 200 250 300 20 40 60 80 3872 G03 100 IPRG = HIGH IPRG = FLOAT IPRG = LOW applicaTions inForMaTion SW GND VIN NGATE 3872 F04 LTC3872 SW GND VIN NGATE LTC3872 |
Similar Part No. - LTC3872_15 |
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Similar Description - LTC3872_15 |
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