Q2

Q1

LM5112

SNVS234B – SEPTEMBER 2004 – REVISED APRIL 2006

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The minimum recommended operating voltage between Vcc and IN_REF is 3.5V. An Under Voltage Lock Out

pin is held in the low state. The UVLO hysteresis prevents chattering during brown-out conditions; the driver will

Layout Considerations

Attention must be given to board layout when using LM5112. Some important considerations include:

2. Proper grounding is crucial. The driver needs a very low impedance path for current return to ground

avoiding inductive loops. Two paths for returning current to ground are a) between LM5112 IN_REF pin and

power MOSFET being driven. Both paths should be as short as possible to reduce inductance and be as

wide as possible to reduce resistance. These ground paths should be distinctly separate to avoid coupling

between the high current output paths and the logic signals that drive the LM5112. With rise and fall times in

the range of 10 to 30nsec, care is required to minimize the lengths of current carrying conductors to reduce

their inductance and EMI from the high di/dt transients generated when driving large capacitive loads.

Thermal Performance

INTRODUCTION

The primary goal of the thermal management is to maintain the integrated circuit (IC) junction temperature (Tj)

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

board and environment. The

operating environment.

DRIVE POWER REQUIREMENT CALCULATIONS IN LM5112

LM5112 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.

The schematic above shows a conceptual diagram of the LM5112 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:

8

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