Datasheet LT8705 (Analog Devices) - 25

LT8705
applicaTions inForMaTion
The peak inductor current when operating in the buck that might damage the devices. For most buck-boost ap- region is: plications the M1 and M3 switches will have the highest I power dissipation where M2 will have the lowest unless L(MAX,BUCK) ≅IOUT(MAX) the output becomes shorted. In some cases it can be  DC  helpful to use two or more MOSFETs in parallel to reduce  V (MAX,M2,BUCK power dissipation in each device. This is most helpful when OUT(MIN) •    100% +  power is dominated by I2R losses while the MOSFET is  A 2•L • f  “on”. The additional capacitance of connecting MOSFETs   in parallel can sometimes slow down switching edge rates   and consequently increase total switching power losses. where DC(MAX,M2,BUCK) is the maximum duty cycle percent- The following sections provide guidelines for calculating age of the M2 switch in the buck region given by: power consumption of the individual MOSFETs. From a  V  known power dissipation, the MOSFET junction tempera- DC OUT(MIN)   ture can be obtained using the following formula: M ( AX,M2,BUCK) ≅ 1–  V •100%  IN(MAX)  TJ = TA + P • RTH(JA) Note that the inductor current can be higher during load where: transients and if the load current exceeds the expected T maximum I J is the junction temperature of the MOSFET OUT(MAX). It can also be higher during start- up if inadequate soft-start capacitance is used or during TA is the ambient air temperature output shorts. Consider using the output current limiting P is the power dissipated in the MOSFET to prevent the inductor current from becoming excessive. Output current limiting is discussed later in the Input/ RTH(JA) is the MOSFET’s thermal resistance from the Output Current Monitoring and Limiting section. Care- junction to the ambient air. Refer to the manufacturer’s ful board evaluation of the maximum inductor current data sheet. is recommended. RTH(JA) normally includes the RTH(JC) for the device plus the thermal resistance from the case to the ambient tem-
Power MOSFET Selection and Efficiency
perature R
Considerations
TH(JC). Compare the calculated value of TJ to the manufacturer’s data sheets to help choose MOSFETs The LT8705 requires four external N-channel power MOS- that will not overheat. FETs, two for the top switches (switches M1 and M4, shown
Switch M1:
The power dissipation in switch M1 comes in Figure 3) and two for the bottom switches (switches from two primary components: (1) I2R power when the M2 and M3, shown in Figure 3). Important parameters for switch is fully turned “on” and inductor current is flowing the power MOSFETs are the breakdown voltage, VBR,DSS, through the drain to source connections and (2) power threshold voltage, VGS,TH, on-resistance, RDS(ON), reverse- dissipated while the switch is turning “on” or “off”. As the transfer capacitance, CRSS (gate-to-drain capacitance), and switch turns “on” and “off” a combination of high current maximum current, IDS(MAX). The gate drive voltage is set and high voltage causes high power dissipation in the by the 6.35V GATEVCC supply. Consequently, logic-level MOSFET. Although the switching times are short, the aver- threshold MOSFETs must be used in LT8705 applications. age power dissipation can still be significant and is often It is very important to consider power dissipation when the dominant source of power in the MOSFET. Depending selecting power MOSFETs. The most efficient circuit will on the application, the maximum power dissipation in use MOSFETs that dissipate the least amount of power. the M1 switch can happen in the buck region when VIN Power dissipation must be limited to avoid overheating is highest, VOUT is highest, and switching power losses 8705ff For more information www.linear.com/LT8705 25 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts