TD340

10/21

MOS drivers

Output drivers are designed to drive MOS with gate capacitance of up to 4 nF. A small resistor in serial

with gate input is recommended to prevent spurious oscillations due to parasitic inductance in conjunction

with gate capacitance. Typical value of these resistors are from 10 to 100 ohms, depending on the MOS

characteristics.

Charge pump

To drive the high side MOS, the TD340 has to provide a voltage of about 10V higher that the power supply

voltage. The TD340 provides an internal charge pump which acts as a voltage tripling generator clamped

to 12V and allows the output of correct gate voltage with power voltage level as low as 6.5V. Its double

balanced structure ensures low EMI Ground Noise. The internal charge pump is used to achieve correct

voltage level at startup or static states.

An 5.6k resistor needs to be connected between OSC and GND for proper operation.

Bootstrap capacitors

To achieve dynamic driving up to 25kHz, it is necessary to support the internal charge pump with

bootstrap capacitors.

Bootstrap capacitors are charged from Vbat when the lower MOS is ON. When the lower MOS is switched

off and the upper one is switched ON, the bootstrap capacitor provides the necessary current to the driver

in order to charge the gate capacitor to the right voltage level.

A design rule to select the bootstrap capacitor value is to choose ten times the gate capacitance.

For example, MOS with 4 nF gate capacitance will require bootstrap capacitors of about 47nF.

MOS gate discharge

The high side MOS are switched off with internal Gate to Source discharge (not Gate to Ground

discharge) to prevent the Gates from negative transient voltages.

Figure 4 : Typical waveforms on low and high side MOS gates.

Upper trace : High side MOS gate

Lower trace : Low side MOS gate