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LTC3872EDDB Datasheet(PDF) 9 Page - Linear Technology |
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LTC3872EDDB Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LTC3872 9 3872fa APPLICATIO S I FOR ATIO the maximum drain current (ID(MAX)) and the MOSFET’s thermal resistances (RTH(JC)andRTH(JA)).Logic-level(4.5V VGS-RATED) threshold MOSFETs should be used when input voltage is high, otherwise if low input voltage operation is expected (e.g., supplying power from a lithium-ion battery or a 3.3V logic supply), then sublogic-level (2.5V VGS-RATED) threshold MOSFETs should be used. Pay close attention to the BVDSS specifications for the MOSFETs relative to the maximum actual switch voltage in the application. Many logic-level devices are limited 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 (not just on a lab breadboard!) for excessive ringing. During the switch on-time, the control circuit limits the maximum voltage drop across the power MOSFET to about 270mV, 100mV and 170mV at low duty cycle with IPRG tied to VIN, GND, or left floating respectively. The peak inductor current is therefore limited to (270mV, 170mV and 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: RV D I DS ON SENSE MAX MAX O MAX T () ( ) () • – •• ≤ + ⎛ ⎝⎜ ⎞ ⎠⎟ 1 1 2 χ ρ VSENSE(MAX) is the maximum voltage drop across the power MOSFET. VSENSE(MAX) is typically 270mV, 170mV and 100mV. It is reduced with increasing duty cycle as shown in Figure 3. The ρTtermaccountsforthetemperature coefficient 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), since MOSFET on-resistances are available in discrete values. IV D R O MAX SENSE MAX MAX DS ON T () () () • – •• = + ⎛ ⎝⎜ ⎞ ⎠⎟ 1 1 2 χ ρ 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. Calculating Power MOSFET Switching and Conduction Losses and Junction Temperatures In order to calculate the junction temperature of the power MOSFET,thepowerdissipatedbythedevicemustbeknown. This power dissipation is a function of the duty cycle, the load current and the junction temperature itself (due to the positive temperature coefficient of its RDS(ON)). As a 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 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 |
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