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LTC1430ACS Datasheet(PDF) 10 Page - Linear Technology |
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LTC1430ACS Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  10 LTC1430A S APPLICATI I FOR ATIO the output load current. This current is always flowing through either Q1 or Q2 with the power dissipation split up according to the duty cycle: DC (Q1) = VOUT VIN DC (Q2) = 1 – VOUT VIN = (VIN – VOUT) VIN The RON required for a given conduction loss can now be calculated by rearranging the relation P = I2R: RON (Q1) = = PMAX(Q1) DC(Q1)(IMAX2) VIN(PMAX)(Q1) VOUT(IMAX2) RON (Q2) = = PMAX(Q2) DC(Q2)(IMAX2) VIN(PMAX)(Q2) (VIN – VOUT)(IMAX2) PMAX should be calculated based primarily on required efficiency. A typical high efficiency circuit designed for 5V in, 3.3V at 10A out might require no more than 3% efficiency loss at full load for each MOSFET. Assuming roughly 90% efficiency at this current level, this gives a PMAX value of (3.3V)(10A/0.9)(0.03) = 1.1W per FET and a required RON of: RON (Q1) = = 0.017 Ω (5V)(1.1W) (3.3V)(10A2) RON (Q2) = = 0.032 Ω (5V)(1.1W) (5V – 3.3V)(10A2) Note that the required RON for Q2 is roughly twice that of Q1 in this example. This application might specify a single 0.03 Ω device for Q2 and parallel two more of the same devices to form Q1. Note also that while the required RON values suggest large MOSFETs, the dissipation numbers scheme. In 5V input designs where an auxiliary 12V supply is available to power PVCC1 and PVCC2, standard MOSFETs with RDS(ON) specified at VGS = 5V or 6V can be used with good results. The current drawn from this supply varies with the MOSFETs used and the LTC1430A’s operating frequency, but is generally less than 50mA. LTC1430A designs that use a doubler charge pump to generate gate drive for Q1 and run from PVCC voltages below 7V cannot provide enough gate drive voltage to fully enhance standard power MOSFETs. When run from 5V, a doubler circuit may work with standard MOSFETs, but the MOSFET RON may be quite high, raising the dissipation in the FETs and costing efficiency. Logic level FETs are a better choice for 5V PVCC systems; they can be fully enhanced with a doubler charge pump and will operate at maximum efficiency. Doubler designs running from PVCC voltages near 4V will begin to run into efficiency problems even with logic level FETs; such designs should be built with tripler charge pumps (see Figure 7) or with newer, super low threshold MOSFETs. Note that doubler charge pump designs running from more than 7V and all tripler charge pump designs should include a zener clamp diode DZ at PVCC1 to prevent transients from exceeding the absolute maximum rating at that pin. Once the threshold voltage has been selected, RON should be chosen based on input and output voltage, allowable power dissipation and maximum required output current. In a typical LTC1430A buck converter circuit operating in continuous mode, the average inductor current is equal to DZ 12V 1N5242 1N5817 1N5817 LTC1430A PVCC1 PVCC2 0.1 µF 10 µF Q1 L1 Q2 G1 G2 PVCC COUT VOUT 1430 • F07 0.1 µF 1N5817 + Figure 7. Tripling Charge Pump |
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