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LTC3802 数据表(PDF) 21 Page - Linear Technology |
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LTC3802 数据表(HTML) 21 Page - Linear Technology |
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21 / 28 page  LTC3802 21 3802f floating, it sits at around 2V and the internal phase-locked loop synchronizes TG1’s falling edge to the falling edge of the PLLIN signal. When PHASEMD is high, these two signals are 90 ° out of phase. TG1 and TG2 remains 180° out of phase independent of PHASEMD input. The PHASEMD signal together with the PLL circuit can be used to synchronize an additional LTC3802 power supply circuit to provide a 4-phase, 4-output solution. Compared to an in-phase multiple controller solution, the LTC3802’s 4-phase design reduces the input capacitor ripple current requirements and efficiency losses because the peak current drawn from the input capacitor is spaced out within the switching cycle. EXTERNAL COMPONENTS SELECTION VCC and PVCC Power Supplies Power for the top and bottom MOSFET drivers is derived from the PVCC pin; the internal controller circuitry is de- rived from the VCC pin. Under typical operating conditions, the total current consumption at these two pins should be well below 100mA. Hence, PVCC and VCC can be connected to an external auxiliary 5V power supply. If an auxiliary supply is not available, a simple zener diode and a darlington NPN buffer can be used to power up these two pins as shown in Figure 7. To prevent switching noise from cou- pling to the sensitive analog control circuitry, VCC should have a 10 µF bypassed capacitor close to the device. The BiCMOS process that allows the LTC3802 to include large on-chip MOSFET drivers also limits the maximum PVCC and VCC voltage to 7V. This limits the practical maximum auxiliary supply to a loosely regulated 7V rail. If VCC drops below 2.5V or PVCC drops below VCC by more than 1V, the LTC3802 goes into undervoltage lockout and prevents the power switches from turning on. Top MOSFET Driver Supply An external bootstrap capacitor, CCP, connected to the BOOST pin supplies the gate drive voltage for the topside MOSFET. This capacitor is charged through diode DCP from PVCC when the switch node is low. When the top MOSFET turns on, the switch node rises to VIN and the BOOST pin rises to approximately VIN + PVCC. The boost capacitor needs to store about 100 times the gate charge required by the top MOSFET. In most applications a 0.1 µF to 1 µF, X5R or X7R dielectric capacitor is adequate. Power MOSFET Selection The LTC3802 requires two external N-channel power MOSFETs, one for the top (main) switch and one for the bottom (synchronous) switch. Important parameters for the power MOSFETs are the threshold voltage V(GS)TH, breakdown voltage V(BR)DSS, maximum current IDS(MAX), on-resistance RDS(ON) and input capacitance. The gate drive voltage is set by the 5V PVCC supply. Consequently, logic-level threshold MOSFETs must be used in LTC3802 applications. If the PVCC voltage is expected to drop below 5V, then sub-logic level threshold MOSFETs should be considered. Pay close attention to the V(BR)DSS specification, because most logic-level MOSFETs are limited to 30V or less. The MOSFETs selected should have a V(BR)DSS rating greater than the maximum input voltage and some margin should be added for transients and spikes. The MOSFETs selected should also have an IDS(MAX) rating of at least two times the maximum power stage output current. Still, this may not be a sufficient margin so it is advisable to calculate the MOSFET’s junc- tion temperature to ensure that it is not exceeded. The LTC3802 uses the bottom MOSFET as the current sense element, particular attention must be paid to its APPLICATIO S I FOR ATIO Figure 7. LTC3802 Power Supply Inputs + + BOOST TG SW BG PVCC PGND VCC VINFF VIN CCP 0.1 µF 0.1 µF DCP COUT VOUT VZ 5.6V Q1: ZETEX FZT603 VZ: MM5Z6V2ST1 QT QB D1 L LTC3802 SGND 3802 F07 10 µF + CIN RZ 2k + 10 µF 10 Ω 100 Ω Q1 |
类似零件编号 - LTC3802 |
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类似说明 - LTC3802 |
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