9.2.1 Design Requirements

Table 9-1 lists the example design input parameters for system design.

Table 9-1. Design Parameters

EXAMPLE DESIGN PARAMETER

REFERENCE

EXAMPLE VALUE

Gate Supply Voltage

12 V

Gate Charge

50 nC

Switching frequency

25 kHz

Slew Rate of MOSFET

100 nS

9.2.2 Detailed Design Procedure

Bootstrap Capacitor and GVDD Capacitpr Selection

The bootstrap capacitor must be sized to maintain the bootstrap voltage above the undervoltage lockout for

normal operation. Equation 1 calculates the maximum allowable voltage drop across the bootstrap capacitor:

(1)

=12V – 0.85V – 4.5V = 6.65V

where

In this example the allowed voltage drop across bootstrap capacitor is 6.65 V. It is generally recommended that

ripple voltage on both the bootstrap capacitor and VDD capacitor should be minimized as much as possible.

Many of commercial, industrial, and automotive applications use ripple value of 0.5 V.

The total charge needed per switching cycle can be estimated with Equation 2:

(2)

=50nC + 220uA/20kHz = 50nC + 11nC = 61nC

where

The minimum bootstrap capacitor can then be estimated as below:

W

(3)

= 111nC / 4.5V = 24.7nF

The calculated value of minimum bootstrap capacitor is 24.7 nF. It should be noted that, this value of

capacitance is needed at full bias voltage. In practice, the value of the bootstrap capacitor must be greater than

calculated value to allow for situations where the power stage may skip pulse due to various transient conditions.

It is recommended to use a 100-nF bootstrap capacitor in this example. It is also recommenced to include

enough margin and place the bootstrap capacitor as close to the BSTx and SHx pins as possible. For this

application, choose a CBOOT capacitor that has the following specifications: 0.1 µF, 25 V, X7R As a general rule

the local VDD bypass capacitor must be greater than the value of bootstrap capacitor value (generally 10 times

DRV8300

SLVSFG5 – SEPTEMBER 2020

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