Functional Description

TD352

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4 Functional Description

4.1 Input stage

TD352 IN input is clamped at about 5V to 7V. The input is triggered by the signal edge. When using an

open collector optocoupler, the resistive pull-up resistor can be connected to either VREF or VH.

Recommended pull-up resistor value with VH=16V are from 4.7k to 22k.

4.2 Voltage reference

A voltage reference is used to create accurate timing for the turn-on delay with external resistor and

capacitor. The same circuitry is also used for the two-level turn-off delay.

A decoupling capacitor (10nF to 100nF) on VREF pin is required to ensure good noise rejection.

4.3 Active Miller clamp:

The TD352 offers an alternative solution to the problem of the Miller current in IGBT switching

applications. Instead of driving the IGBT gate to a negative voltage to increase the safety margin, the

TD352 uses a dedicated CLAMP pin to control the Miller current. When the IGBT is off, a low impedance

path is established between IGBT gate and emitter to carry the Miller current, and the voltage spike on

the IGBT gate is greatly reduced.

During turn-off, the gate voltage is monitored and the clamp output is activated when gate voltage goes

below 2V (relative to VL). The clamp voltage is VL+4V max for a Miller current up to 500mA. The clamp

is disabled when the IN input is triggered again.

The CLAMP function doesn’t affect the turn-off characteristic, but only keeps the gate to the low level

throughout the off time. The main benefit is that negative voltage can be avoided in many cases, allowing

a bootstrap technique for the high side driver supply.

4.4 Turn-on delay

is approximately given by:

The turn-on delay can be disabled by connecting the CD pin to VREF with a 4.7k resistor.

4.5 Desaturation protection

Desaturation protection ensures the protection of the IGBT in the event of overcurrent. When the DESAT

voltage goes higher than VH-2V, the TD352 OUT pin is driven low. The fault state is only exit after power-

down and power-up.

A programmable blanking time is used to allow enough time for IGBT saturation. Blanking time is

by:

4.6 Output stage

The output stage is able to sink/source 1.7A/1.3A typical at 25°C and 1.0A/0.75A min. over the full

temperature range. This current capability is specified near the usual IGBT Miller plateau.