The TPS7H60x5 provides the user several options for charging the bootstrap capacitor. The flexibility is to allow for operation with a wide range of PWM controllers, and also to allow the user to select an option with trade-offs that are most desirable for the specific application. In both instances, a bootstrap resistor is recommended to limit the bootstrap current during initial startup. The bootstrap resistor and capacitor need to be chosen such that sufficient time is allowed for the re-charge of the capacitor for the specific application.

The first option is to allow for charging of the bootstrap capacitor through the internal bootstrap switch of the driver. This switch is internally connected between VIN and BST pins and the bootstrap diode is connected externally between BST (anode) and BOOT (cathode). The bootstrap switch is only on when the low side driver output is on. By disallowing bootstrap charging during the converter dead times, the maximum voltage across the bootstrap capacitor can be reduced. The internal bootstrap switch has a parallel resistance of 1kΩ that allows for slow charging the bootstrap capacitor at start-up before low-side FET turn-on.

Figure 8-1 Internal Switch Bootstrap Charging Configuration

Another option is to charge the bootstrap capacitor directly from VIN. This is a more conventional method used with half-bridge drivers. This option can be considered in a number of use cases, but is particularly helpful in instances where the low-side FET turn-on is not immediate. This is the case when using the TPS7H60x5 with one of the three controllers in the TPS7H500x-SP family that have integrated synchronous rectification outputs. The synchronous rectification outputs are disabled during soft-start, and as such, when implementing a synchronous buck topology the bootstrap capacitor cannot be charged through the internal bootstrap switch of the driver. The bootstrap switch does have the parallel resistor for slow-charging, but sequencing and/or startup requirements for the converter can potientially dictate that the charging need to occur more rapidly. When using the direct VIN charging, the options for preventing overcharging of the bootstrap capacitor are to add a resistor in series with the bootstrap capacitor, to add a Zener diode in parallel with the bootstrap capacitor, or a combination of both. A consideration that must be made if using the Zener diode is that has an associated leakage current during normal operation, which contributes to the overall converter losses.

Figure 8-2 Direct VIN Bootstrap Charging Configuration

Lastly, a dual-charging option can be considered, which is a combination of the bootstrap switch and direct VIN charging methods. This method offers the benefit of circumventing any potential bootstrap charging issues during startup due to the low-side FET not turning on, while also taking advantage of the reduction of bootstrap voltage during normal operation offered by the internal switch. The series resistor used with the bootstrap diode in the direct VIN charging path must be higher than the resistance of the internal bootstrap switch to make sure that the charging is via the bootstrap switch during normal operation. This higher resistor value also effectively reduces the Zener current during normal operation. The trade-off for this configuration is the additional part count.

Figure 8-3 Dual Bootstrap Charging Configuration