STSPIN32G4 datasheet(22/48 Pages) STMICROELECTRONICS

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5.2.4

Bootstrap section

The bootstrap circuitry allows to generate a voltage higher than the supply VM and it is used to supply the

high-side drivers. When one high-side MOSFET is turned on, its source voltage (OUTx pin) increases up to VM.

Therefore, the gate must be driven at a voltage higher than VM.

The bootstrap capacitor is referred to the OUTx pin:

•

When the OUTx pin is forced to GND (i.e. the respective low-side MOSFET is on), the bootstrap

capacitor is charged through the bootstrap diode.

•

When the OUTx is forced to VM (i.e. the respective high-side MOSFET is on), the bootstrap capacitor

supplies the respective high-side driver and discharges.

The voltage drop on the bootstrap capacitors corresponds to the supply of the high-side drivers. Each bootstrap

capacitor must be charged after the corresponding high-side is turned on, otherwise its voltage falls below the

VBO(On) - VBO(Hyst) threshold, causing the turning-off of the respective driver (refer to Section 5.2.6 ).

A limitation in a bootstrap architecture is that the high-side MOSFET cannot stay on for an indefinite amount of

time. In fact, when the high-side is on, the respective bootstrap capacitor starts discharging. If not recharged, the

bootstrap capacitor voltage falls below the VBO(On) - VBO(Hyst) (i.e. the UVLO on the BOOTx pin). For this reason,

working at 100% duty cycle is possible, but only for a limited number of PWM periods. The bigger the bootstrap

capacitor, the longer the time the high-side MOSFET can be kept on.

To avoid excessive drop on the VCC pin, a proper bypass capacitor is required. Even using an external supply

connected to the VCC pin, it is important to have a bypass capacitor with low ESR providing fast current

transients when required by the bootstrap capacitors.

The bypass capacitor on the VCC pin must provide the charge for the three bootstrap capacitors: the bigger the

bootstrap capacitors, the bigger the VCC capacitor should be (refer to Equation 1 in Section 5.2.4.2 ).

5.2.4.1

Power-up and wake-up

During the power-up or after leaving the standby condition, there may be no charge in the bootstrap capacitors.

In these cases, the driver cannot start immediately with normal operation, but the bootstrap capacitor should be

charged turning on the low-side MOSFET.

At the beginning of this procedure, a large amount of current could be required. If the internal VCC buck regulator

is used, its current is limited at ICC (refer to Table 2).

5.2.4.2

Charging time and external bootstrap diodes

The charging time of the bootstrap capacitors depends on their value but also on the resistance of the bootstrap

diode (RDS_diode), which limits the current flow. In order to reduce the minimum time for bootstrap recharge

(i.e. the minimum time the low-side MOSFET must be on), external bootstrap diodes can be used as shown in

Figure 18.

Each diode is in parallel with the corresponding internal bootstrap diode. An external series resistor, smaller

than RDS_diode, can still be used together with each diode to reduce the maximum charging current and helps

to limit the voltage drop on the VCC pin. The maximum drop on the COUT capacitor occurs when the three

bootstrap capacitors must be recharged, and no series resistor is used with the external diodes. This drop can be

approximated as:

Equation 1

(1)

ΔVCOUT≈VCC∙ 3∙CBOOT

COUT+3∙CBOOT

STSPIN32G4

Gate drivers

DS13630 - Rev 1

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