Just before Christmas, Maxim introduced the MAX22701 isolated gate driver intended to drive the gates of fast-switching silicon carbide and gallium nitride transistors.

Noise immunity is high, offering a typical common-mode transient immunity (CMTI) of 300kV/µs (fully functioning), plus an isolation withstand voltage of 1.2kVpeak (continuous, 3kVrms for 60s).

Devices are available in an 8pin, narrow-body SOIC package (3.9 x 4.9mm) with 4mm of creepage and clearance – the package material, according to Maxim,  has a minimum comparative tracking index (CTI) of 600V, which gives it a group I rating in creepage tables.

On the speed front, a minimum pulse width of 20ns is supported, with maximum pulse width distortion of 2ns.

Part-to-part propagation delay is matched within 2ns (max) at +25°C ambient, or 5ns (max) over the -40°C to +125°C operating temperature range – this tight matching allows dead-time to be minimised when switching multiple external power transistors using several chips – such as in a half-bridge converter.

The output is an internal half-bridge running from an external output-side supply between 13 and 36V. When fed with 20V, the output typically switches to within a few mV of the rail or secondary ground.

The high-side output transistor arrangement is a little unusual, with an n-channel fet supplementing the p-fet to during the output low-to-high transition to add current and accelerate the turn-on of the external device. The p-mos transistor has a 4.5Ω maximum Rds(on).

The n-channel low-side output transistor is more conventional, with an Rds(on) max of 2.5Ω.

However, a second low-side n-channel fet has been included that can work with the one just mentioned. This is an automatic clamp that can prevent spurious turn-on of the external transistor (always n-channel) at its moment of switch-off, when its rising drain voltage can get to its gate through its drain-gate (‘Miller’) capacitance.

It is particularly relevant when the external transistor is being driven though external components to modify its switching behaviour (diagram left).

The extra pull-down transistor (2.5Ωmax) turns on when the clamp output voltage drops below 2V, providing a low-impedance path to ground for the Miller current, preventing the gate voltage from rising again.

The datasheet includes plenty of advice on the combination of external components required between driver and external gate for both SiC and GaN external transistors.

The MAX22701 is not the only device in this driver range, there are two other devices:

MAX22700E-02EMAX22700 (diagram right) looses the Miller clamp, and gains an output-side common connection on that pin.

MAX22702 has neither a Miller clamp nor output common, but the pin is used to provide an adjustable output-side under-voltage lock-out. In the other two types, it is fixed at ~12.5V.

Although the ’00 and ’02 do not have a Miller clamp fet, they do have a larger pull-down transistor – offering 1.25Ω Rds(on) maximum instead of 2.5Ω.

There are actually six variants in all, as each of the three types comes in an ‘E’ version – for example MAX22701E, which has an input pin and an enable pin (all diagrams above), or there are ‘D’ versions which have differential inputs (diagram below left).

MAX22701DIn the case of differential inputs, when the inputs are in the undefined state (both identical) the output is held at the state it was last time the inputs were valid (different).

Input-side and output-side logic thresholds are separately set by the rail voltages supplied. Input-side under-voltage lock-out is set to trip at around 2.77V.

Both input-side and output-side power systems are completely separate and the device is specified with any combination of valid input-side and output-side voltages.

Power supply decoupling appears to be critical, with a combination of 1nF, 0.1µF and 1µF in parallel on the input-side, and 1nF, 0.1µF, 1µF and 22µF – again, see data sheet.

The parts have UL and cUL approval (UL1577 and CSA Bulletin 5A respectively.

There is an evaluation kit: MAX22701EVKIT#.

Applications are foreseen in inverters, motor drives, uninterruptible PSUs and solar power inverters.