6.4.3

Bootstrap diode

The STDRIVEG611 integrates a bootstrap diode structure connected between VCC and BOOT pins, to supply the

high-side floating supply voltage VBOOT-OUT (or shortly VBO). A very low ESR ceramic capacitor (CBOOT to be

selected between 47 nF and 3.3 µF, X7R, 50 V) must be placed as close as possible between the BOOT and

OUT pins.

The internal bootstrap structure is synchronous with the low-side to reduce the voltage drop between VCC and

VBO to almost zero volts. Its turn-on resistance is reported in the electrical characteristics table as RDBOOT. This

integrated structure is characterized by a very low recovery charge and current.

The bootstrap DC characteristics are detailed in Figure 13.

Figure 13. Typical bootstrap diode characteristics

0

100

50

150

200

250

300

Temperature (°C)

-50

-10

30

70

110

150

The final voltage drop between VCC and VBO depends on multiple factors like duty cycle, operating frequency,

GaN gate charge, GaN leakage, temperature, etc. In case the voltage on VBO is not sufficient for a proper VCCH

regulator operation, either increasing the VCC value or using an external bootstrap diode can obtain a correct

VBO voltage.

Motor control applications typically do not require an external bootstrap diode but it is allowed in case of specific

needs as fast high-side start-up or large bootstrap capacitor to allow very long high side turn-on time.

It is recommended to place a resistor RBOOT ≥ 2.2 Ω in series with the above mentioned external bootstrap diode

to limit the amount of peak charging current. In cases of high side not-zero-current hard switching applications (as

motor control), the external bootstrap diode could be subject to high peak current during dead time and peak

recovery current at high side turn-on, leading to strong emissions. A higher RBOOT or using the internal bootstrap

structure with no recovery current is preferable in these cases. Refer to Section 7.1.1 for further details.

Even if a low RBOOT resistor is allowed, it is know that a low value resistor could increase radiated noise; so it is

worth properly selecting the CBOOT capacitor based on applicative requirements. With the same start-up time, the

smaller CBOOT capacitor the higher RBOOT minimum resistance or even no external diode requirement.

Moreover, the internal bootstrap diode has better dynamic characteristics compared to a discrete one.

6.4.4

VHS supply structure and relevant UVLO protection

The energy stored in the CBOOT capacitor supplies the input circuitry of the high-side driver.

The integrated fast startup VCCH regulator reduces the input VBO voltage to a regulated VHS (VCCH-OUT = 6 V

typ.) to drive the gate with a stable and optimized VGS voltage.

The fast startup features a minimized wake-up time especially during intermittent operation (burst mode).

STDRIVEG611

Device description

DS14457 - Rev 2

page 18/33