Datasheet LT8705 (Analog Devices) - 26
| Manufacturer | Analog Devices |
| Description | 80V VIN and VOUT Synchronous 4-Switch Buck-Boost DC/DC Controller |
| Pages / Page | 44 / 26 — applicaTions inForMaTion. Switch M2:. Switch M3:. Figure 9. Normalized … |
| File Format / Size | PDF / 508 Kb |
| Document Language | English |
applicaTions inForMaTion. Switch M2:. Switch M3:. Figure 9. Normalized MOSFET RDS(ON) vs Temperature
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LT8705
applicaTions inForMaTion
are greatest or in the boost region when VIN is smallest, Since the switching power (PSWITCHING) often dominates, VOUT is highest and M1 is always on. Switch M1 power look for MOSFETs with lower CRSS or consider operating consumption can be approximated as: at a lower frequency to minimize power loss and increase P efficiency. M1 = PI2R + PSWITCHING
Switch M2:
In most cases the switching power dissipa- V ≅ OUT •I 2 •R tion in the M2 switch is quite small and I2R power losses V OUT DS(ON) • ρτ IN dominate. I2R power is greatest in the buck region where +(V the switch operates as the synchronous rectifier. Higher IN • IOUT • f • tRF1)W B ( UCK REGION) VIN and lower VOUT causes the M2 switch to be “on” for V 2 the most amount of time, leading to the highest power ≅ OUT •I •R V OUT DS(ON) • ρτ consumption. The M2 switch power consumption in the IN buck region can be approximated as: +0W (BOOST REGION) where: P 2 (M2,BUCK) ≅ VIN – VOUT •IOUT(MAX) •RDS(ON) •ρτ W VIN the PSWITCHING term is 0 in the boost region
Switch M3:
Switch M3 operates in the boost and buck-boost tRF1 is the average of the SW1 pin rise and fall times. regions as a control switch. Similar to the M1 switch, the Typical values are 20ns to 40ns depending on the power dissipation comes from I2R power and switching MOSFET capacitance and VIN voltage. power. The maximum power dissipation is when VIN is ρτ is a normalization factor (unity at 25°C) accounting the lowest and VOUT is the highest. The following expres- for the significant variation in MOSFET on-resistance sion approximates the power dissipation in the M3 switch with temperature, typically about 0.4%/°C, as shown under those conditions: in Figure 9. For a maximum junction temperature of P 125°C, using a value ρ M3 =PI2R +PSWITCHING ≅ τ = 1.5 is reasonable. (V OUT – VIN ) • VOUT •I 2.0 V OUT2 •RDS(ON) •ρτ IN2 1.5 +VOUT2 •IOUT • f• tRF2 W VIN 1.0 where the total power is 0 in the buck region. 0.5 NORMALIZED ON-RESISTANCE (Ω) ρ τ 0–50 0 50 100 150 JUNCTION TEMPERATURE (°C) 8705 F09
Figure 9. Normalized MOSFET RDS(ON) vs Temperature
8705ff 26 For more information www.linear.com/LT8705 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
applicaTions inForMaTion
are greatest or in the boost region when VIN is smallest, Since the switching power (PSWITCHING) often dominates, VOUT is highest and M1 is always on. Switch M1 power look for MOSFETs with lower CRSS or consider operating consumption can be approximated as: at a lower frequency to minimize power loss and increase P efficiency. M1 = PI2R + PSWITCHING
Switch M2:
In most cases the switching power dissipa- V ≅ OUT •I 2 •R tion in the M2 switch is quite small and I2R power losses V OUT DS(ON) • ρτ IN dominate. I2R power is greatest in the buck region where +(V the switch operates as the synchronous rectifier. Higher IN • IOUT • f • tRF1)W B ( UCK REGION) VIN and lower VOUT causes the M2 switch to be “on” for V 2 the most amount of time, leading to the highest power ≅ OUT •I •R V OUT DS(ON) • ρτ consumption. The M2 switch power consumption in the IN buck region can be approximated as: +0W (BOOST REGION) where: P 2 (M2,BUCK) ≅ VIN – VOUT •IOUT(MAX) •RDS(ON) •ρτ W VIN the PSWITCHING term is 0 in the boost region
Switch M3:
Switch M3 operates in the boost and buck-boost tRF1 is the average of the SW1 pin rise and fall times. regions as a control switch. Similar to the M1 switch, the Typical values are 20ns to 40ns depending on the power dissipation comes from I2R power and switching MOSFET capacitance and VIN voltage. power. The maximum power dissipation is when VIN is ρτ is a normalization factor (unity at 25°C) accounting the lowest and VOUT is the highest. The following expres- for the significant variation in MOSFET on-resistance sion approximates the power dissipation in the M3 switch with temperature, typically about 0.4%/°C, as shown under those conditions: in Figure 9. For a maximum junction temperature of P 125°C, using a value ρ M3 =PI2R +PSWITCHING ≅ τ = 1.5 is reasonable. (V OUT – VIN ) • VOUT •I 2.0 V OUT2 •RDS(ON) •ρτ IN2 1.5 +VOUT2 •IOUT • f• tRF2 W VIN 1.0 where the total power is 0 in the buck region. 0.5 NORMALIZED ON-RESISTANCE (Ω) ρ τ 0–50 0 50 100 150 JUNCTION TEMPERATURE (°C) 8705 F09
Figure 9. Normalized MOSFET RDS(ON) vs Temperature
8705ff 26 For more information www.linear.com/LT8705 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