SM74101 Datasheet, PDF (10/23 Pages) Texas Instruments – SM74101 Tiny 7A MOSFET Gate Driver

SM74101
SNOSBA2B – JULY 2011 – REVISED MAY 2015
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7.5 Thermal Considerations
The primary goal of the thermal management is to maintain the integrated circuit (IC) junction temperature (Tj)
below a specified limit to ensure reliable long term operation. The maximum T_J of IC components should be
estimated in worst case operating conditions. The junction temperature can be calculated based on the power
dissipated on the IC and the junction to ambient thermal resistance θ_JA for the IC package in the application
board and environment. The θ_JA is not a given constant for the package and depends on the PCB design and the
operating environment.
7.5.1 Drive Power Requirement Calculations In SM74101
SM74101 is a single low side MOSFET driver capable of sourcing / sinking 3A / 7A 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.
V_GATE
V_TRIG
Q1
R_G
V_HIGH
C_IN
Q2
Figure 13.
The schematic above shows a conceptual diagram of the SM74101 output and MOSFET load. Q1 and Q2 are
the switches within the gate driver. Rg is the gate resistance of the external MOSFET, and Cin is the equivalent
gate capacitance of the MOSFET. The equivalent gate capacitance is a difficult parameter to measure as it is the
combination of Cgs (gate to source capacitance) and Cgd (gate to drain capacitance). The Cgd is not a constant
and varies with the drain voltage. The better way of quantifying gate capacitance is the gate charge Qg in
coloumbs. Qg combines the charge required by Cgs and Cgd for a given gate drive voltage Vgate. The gate
resistance Rg is usually very small and losses in it can be neglected. The total power dissipated in the MOSFET
driver due to gate charge is approximated by:
P_DRIVER = V_GATE x Q_G x F_SW
Where
• F_SW = switching frequency of the MOSFET.
(1)
For example, consider the MOSFET MTD6N15 whose gate charge specified as 30 nC for V_GATE = 12V.
Therefore, 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 12V is equal to
P_DRIVER = 12V x 30 nC x 300 kHz = 0.108W.
(2)
In addition to the above gate charge power dissipation, - transient power is dissipated in the driver during output
transitions. When either output of the SM74101 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 SM74101 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 12V V_GATE supply produce a 12 mW typical quiescent power.
Therefore the total power dissipation
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