LTC7813 datasheet(42/59 Pages) AD

42 / 59 page

Data Sheet

LT8210-1

analog.com

Rev. A

42 of 59

power dissipation for switches A, B, C, and D as a function of the input and output voltages and switching region. The

purpose is to guide MOSFET selection by determining where the majority of power is being dissipated. In the

following equations, ρτ is a normalization factor (unity at 25°C) accounting for the significant variation in on-

resistance with temperature, typically 0.4%/°C, as shown in Figure 67. For a maximum junction temperature of 125°C,

using a value of ρτ = 1.5 is reasonable. QSW is the switching charge and can be approximated as QSW = QGD + QGS/2 if not

explicitly stated in the MOSFET data sheet.

Figure 67. Normalized RDS(ON) vs. Temperature

The constant k is empirically derived to equal 1.3 and is a function of driver resistance, MOSFET threshold, and gate

resistance.

Switch A:

The power dissipation in switch A is due to both conduction and switching losses and typically reaches a maximum

at either VIN(MIN)in the boost region or VIN(MAX)in the buck region.

Table 6. Switch A Power Dissipation

REGION

POWER DISSIPATION

Buck

I OUT2 • (VOUT /VIN) • ρτ • RDS(ON) + k • IOUT • VIN • f SW • QSW

Buck-Boost

I OUT2 • (VOUT /VIN) • ρτ • RDS(ON) + k • IOUT • VIN • f SW • QSW

Boost

I OUT2 • (VOUT /VIN) 2 • ρτ • RDS(ON)

Pass-Thru (Non-Switching)

I OUT2 • ρτ • RDS(ON)

Switch B:

Switch B power dissipation is due mainly to conduction losses and reaches a maximum in the buck region at VIN(MAX).

Table 7. Switch B Power Dissipation

REGION

POWER DISSIPATION

Buck

IOUT2 • (1 - VOUT/VIN) • ρτ • RDS(ON)

Buck-Boost

IOUT 2 • (1 - VOUT/VIN) • ρτ • RDS(ON)

Boost

Pass-Thru (Non-Switching)

JUNCTION TEMPERATURE (°C)

–50

1.0

1.5

150

0.5

100

2.0

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