Equation 2

(2)

PREG12= VS−VREG12 ⋅IREG12

where VS is the voltage supply of the STDRIVE101, VREG12 is the regulator output voltage and IREG12 is the

average current consumption described previously.

Note:

The size of the external power MOSFETs and the switching frequency increase the gate driver’s

consumption, thus reducing the current availability for external loads.

In addition to the average value IREG12 dynamic contributions must be considered: these are the currents needed

to charge the bootstrap capacitors, e.g. after the high-side MOSFET is kept on for a considerable amount of time

or after leaving the stand-by condition. In these cases, the amount of current is higher than IREG12lim, so it is

mainly provided by the bypass capacitor CREG12 connected to REG12 pin. In this situation, a voltage drop

ΔVREG12 occurs on REG12, while the LDO maximum current IREG12lim mainly recharges CREG12. During this

recharge time (tcharge,REG12 expressed in Equation 3), the availability of current for external loads is reduced.

Equation 3

(3)

tcℎarge,REG12>CREG∆VREG12

IREG12lim

5.2.1

Bootstrap section

The bootstrap circuitry allows to generate a voltage higher than the supply VS 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 VS.

Therefore, the gate must be driven at a voltage higher than VS. 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 VS (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 ( Section 5.5 ). A limitation in a

bootstrap architecture is that the high-side MOSFET cannot be kept 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 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 REG12 pin, a proper bypass capacitor is required. Even using an external supply

connected to the REG12 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 REG12 pin must provide the charge for the three bootstrap capacitors: the bigger the

bootstrap capacitors are, the bigger should be the REG12 capacitor (refer to Equation 4 in Section 5.2.1.2 ).

5.2.1.1

Power-up and wake-up

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

these cases, the drivers 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 12 V LDO regulator is used, its current is limited at IREG12lim (Section 5.2 ).

5.2.1.2

Charging time and external bootstrap diodes

The charging time required to charge 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 11.

STDRIVE101

12 V LDO linear regulator

DS13472 - Rev 1

page 15/32