11.3.1 Drive Power Requirement Calculations in LM5111

The LM5111 dual low side MOSFET driver is capable of sourcing/sinking 3A/5A peak currents for short intervals to drive a MOSFET without exceeding package power dissipation limits. High peak currents are required to switch the MOSFET gate very quickly for operation at high frequencies.

Figure 16. Driver Output Stage and Load

The schematic above shows a conceptual diagram of the LM5111 output and MOSFET load. Q1 and Q2 are the switches within the gate driver. R_G is the gate resistance of the external MOSFET, and C_IN is the equivalent gate capacitance of the MOSFET. The gate resistance Rg is usually very small and losses in it can be neglected. The equivalent gate capacitance is a difficult parameter to measure since it is the combination of C_GS (gate to source capacitance) and C_GD (gate to drain capacitance). Both of these MOSFET capacitances are not constants and vary with the gate and drain voltage. The better way of quantifying gate capacitance is the total gate charge Q_G in coulombs. Q_G combines the charge required by C_GS and C_GD for a given gate drive voltage V_GATE.

Assuming negligible gate resistance, the total power dissipated in the MOSFET driver due to gate charge is approximated by

For example, consider the MOSFET MTD6N15 whose gate charge specified as 30 nC for V_GATE = 12 V.

The power dissipation in the driver due to charging and discharging of MOSFET gate capacitances at switching frequency of 300 kHz and V_GATE of 12 V is equal to

If both channels of the LM5111 are operating at equal frequency with equivalent loads, the total losses will be twice as this value which is 0.216 W.

In addition to the above gate charge power dissipation, transient power is dissipated in the driver during output transitions. When either output of the LM5111 changes state, current will flow from V_CC to V_EE for a very brief interval of time through the output totem-pole N and P channel MOSFETs. The final component of power dissipation in the driver is the power associated with the quiescent bias current consumed by the driver input stage and Under-voltage lockout sections.

Characterization of the LM5111 provides accurate estimates of the transient and quiescent power dissipation components. At 300-kHz switching frequency and 30-nC load used in the example, the transient power will be 8 mW. The 1-mA nominal quiescent current and 12-V V_GATE supply produce a 12-mW typical quiescent power.

Therefore the total power dissipation

We know that the junction temperature is given by

Or the rise in temperature is given by

For SOIC package, θ_JA is estimated as 170°C/W for the conditions of natural convection. For MSOP-PowerPAD, θ_JA is typically 60°C/W.

Therefore for SOIC T_RISE is equal to

11.3.2 Continuous Current Rating of LM5111

The LM5111 can deliver pulsed source/sink currents of 3 A and 5 A to capacitive loads. In applications requiring continuous load current (resistive or inductive loads), package power dissipation, limits the LM5111 current capability far below the 5-A sink and 3-A source capability. Rated continuous current can be estimated both when sourcing current to or sinking current from the load. For example when sinking, the maximum sink current can be calculated as:

Equation 8.

where

• R_DS(on) is the on resistance of lower MOSFET in the output stage of LM5111

Consider T_J(max) of 125°C and θ_JA of 170°C/W for an SO-8 package under the condition of natural convection and no air flow. If the ambient temperature (T_A) is 60°C, and the R_DS(on) of the LM5111 output at T_J(max) is 2.5 Ω, this equation yields I_SINK(max) of 391 mA which is much smaller than 5-A peak pulsed currents.

Similarly, the maximum continuous source current can be calculated as

Equation 9.

where

• V_DIODE is the voltage drop across hybrid output stage which varies over temperature and can be assumed to be about 1.1 V at T_J(max) of 125°C

Assuming the same parameters as above, this equation yields I_SOURCE(max) of 347 mA.