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LTC1530CS8 bảng dữ liệu(PDF) 11 Page - Linear Technology |
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LTC1530CS8 bảng dữ liệu(HTML) 11 Page - Linear Technology |
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11 / 24 page  11 LTC1530 MOSFET whose RDS(ON) is rated at VGS = 4.5V does not necessarily have a logic level MOSFET GATE threshold voltage. Logic level FETs are the recommended choice for 5V-only systems. Logic level FETs can be fully enhanced with a doubler charge pump and will operate at maximum efficiency. Note that doubler charge pump designs run- ning from supplies higher than 6.5V should include a Zener diode clamp at PVCC to prevent transients from exceeding the absolute maximum rating of the pin. After the MOSFET threshold voltage is selected, choose the RDS(ON) based on the input voltage, the output voltage, allowable power dissipation and maximum output cur- rent. In a typical LTC1530 buck converter circuit, operat- ing in continuous mode, the average inductor current is equal to the output load current. This current flows through either Q1 or Q2 with the power dissipation split up accord- ing to the duty cycle: DC Q V V DC Q V V VV V OUT IN OUT IN IN OUT IN () () 1 21 = =− = − () The RDS(ON) required for a given conduction loss can now be calculated by rearranging the relation P = I2R. R P DC Q I VP V I R P DC Q I VP VV I DS ON Q MAX Q MAX IN MAX Q OUT MAX DS ON Q MAX Q MAX IN MAX Q IN OUT MAX () () () () () () () () 1 1 2 1 2 2 2 2 2 2 1 2 = [] = ()[] () = [] = ()[] − () + + 0.22 µF 10 µF + CO CIN LO MBR0530T1 MBR0530T1 OPTIONAL FOR VIN > 6.5V LTC1530 PVCC G1 VOUT 1530 F07 VIN 13V 1N5243B Q1 Q2 G2 Power MOSFETs Two N-channel power MOSFETs are required for synchro- nous LTC1530 circuits. They should be selected based primarily on threshold voltage and on-resistance consid- erations. Thermal dissipation is often a secondary con- cern in high efficiency designs. The required MOSFET threshold should be determined based on the available power supply voltages and/or the complexity of the gate drive charge pump scheme. In 5V input designs where a 12V supply is used to power PVCC, standard MOSFETs with RDS(ON) specified at VGS = 5V or 6V can be used with good results. The current drawn from the 12V supply varies with the MOSFETs used and the LTC1530’s operat- ing frequency, but is generally less than 50mA. LTC1530 applications that use a 5V VIN voltage and a doubling charge pump to generate PVCC do not provide enough gate drive voltage to fully enhance standard power MOSFETs. Under this condition, the effective MOSFET RDS(ON) may be quite high, raising the dissipa- tion in the FETs and reducing efficiency. In addition, power supply start-up problems can occur with standard power MOSFETs. These start-up problems can occur for two reasons. First, if the MOSFET is not fully enhanced, the higher effective RDS(ON) causes the LTC1530 to acti- vate current limit at a much lower level than the desired trip point. Second, standard MOSFETs have higher GATE threshold voltages than logic level MOSFETs, thereby increasing the PVCC voltage required to turn them on. A Figure 7. Doubling Charge Pump APPLICATIO S I FOR ATIO |
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