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Monday 1 December 2008
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How to improve mosfet driver performance
by Steve Bush
Thursday 31 July 2008
Microcontrollers increasingly include pulse-width modulators which will happily drive the gate of a mosfet if switching speed is not an issue.
But when a bit more performance is required, or n-channel high-side switching is necessary, a driver is needed ‘twixt I/O pin and gate.
Microchip has recently introduced a pair of single output mosfet drivers, the MCP1415 and the MCP1416, which are rated at 1.5A and operated from 4.5 to 18V.
The difference between the two is that MCP1415 is inverting and the MCP1416 and non-inverting.
The firm claims “excellent” latch-up immunity and, packaged in 5pin SOT-23s, “because of their small size, designers can position them close to the mosfet’s physical gate connection which minimises gate bounce caused by the parasitic PCB layout”, said Microchip. “This also minimises gate rise-and-fall times, propagation-delay times and shoot-through current, all of which help to increase system efficiency and reduce power dissipation.”
Linear Technology has introduced the LTC4447, a mosfet driver that produces gate signals for upper and lower power n-channel devices in synchronous rectified power converters.
Bootstrapping from the output with an external capacitor means both transistor gates are supplied, with respect to their sources, over a voltage range of 4 to 6.5V depending on the driver chip supply voltage.
The mosfets can operate from a supply up to 38V as the bootstrap circuit tops out at 42V above ground.
Parallel mosfets and bipolar transistors inside the chip can sink up to 5A and source up to 3.2A.
Sinking current from the bottom devices gate is particularly important as its drain will be rising in voltage, trying to charge the gate through the drain-gate capacitance. “The LTC4447’s powerful parallel combination of the n-channel mosfet and NPN on the bottom gate pull-down generates a phenomenal 4ns fall time on the bottom gate while driving a 3nF load,” said Linear. “Similarly, the 0.8ohm pull-down mosfet on the top gate results in a 7ns fall time with a 3nF load.”
Climbing there voltage scale a little higher, Maxim recently introduced what it claims is the “industry’s highest voltage 3A half-bridge mosfet drivers pin compatible with industry standard HIP2100IB and HIP2101IB”.
At 125V, they are 20V above competing products said the firm.
The MAX15018 and MAX15019 provide 3A peak source and sink current, 35ns typical propagation delay, and guaranteed 2ns maximum difference in propagation delay between drivers “making them ideal for high-frequency power supplies,” said Maxim.
Aimed at telecom PSUs, several versions are available with the choice of TTL or CMOS compatible inputs in various inverting and non-inverting combinations. “The logic inputs are protected against voltage spikes up to +15V, regardless of the V+ supply voltage.”
But when a bit more performance is required, or n-channel high-side switching is necessary, a driver is needed ‘twixt I/O pin and gate.
Microchip has recently introduced a pair of single output mosfet drivers, the MCP1415 and the MCP1416, which are rated at 1.5A and operated from 4.5 to 18V.
The difference between the two is that MCP1415 is inverting and the MCP1416 and non-inverting.
The firm claims “excellent” latch-up immunity and, packaged in 5pin SOT-23s, “because of their small size, designers can position them close to the mosfet’s physical gate connection which minimises gate bounce caused by the parasitic PCB layout”, said Microchip. “This also minimises gate rise-and-fall times, propagation-delay times and shoot-through current, all of which help to increase system efficiency and reduce power dissipation.”
Linear Technology has introduced the LTC4447, a mosfet driver that produces gate signals for upper and lower power n-channel devices in synchronous rectified power converters.
Bootstrapping from the output with an external capacitor means both transistor gates are supplied, with respect to their sources, over a voltage range of 4 to 6.5V depending on the driver chip supply voltage.
The mosfets can operate from a supply up to 38V as the bootstrap circuit tops out at 42V above ground.
Parallel mosfets and bipolar transistors inside the chip can sink up to 5A and source up to 3.2A.
Sinking current from the bottom devices gate is particularly important as its drain will be rising in voltage, trying to charge the gate through the drain-gate capacitance. “The LTC4447’s powerful parallel combination of the n-channel mosfet and NPN on the bottom gate pull-down generates a phenomenal 4ns fall time on the bottom gate while driving a 3nF load,” said Linear. “Similarly, the 0.8ohm pull-down mosfet on the top gate results in a 7ns fall time with a 3nF load.”
Climbing there voltage scale a little higher, Maxim recently introduced what it claims is the “industry’s highest voltage 3A half-bridge mosfet drivers pin compatible with industry standard HIP2100IB and HIP2101IB”.
At 125V, they are 20V above competing products said the firm.
The MAX15018 and MAX15019 provide 3A peak source and sink current, 35ns typical propagation delay, and guaranteed 2ns maximum difference in propagation delay between drivers “making them ideal for high-frequency power supplies,” said Maxim.
Aimed at telecom PSUs, several versions are available with the choice of TTL or CMOS compatible inputs in various inverting and non-inverting combinations. “The logic inputs are protected against voltage spikes up to +15V, regardless of the V+ supply voltage.”
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