AN486 Datasheet, PDF (1/10 Pages) Silicon Laboratories – High-Side Bootstrap Design Using ISODrivers in Power Delivery Systems

AN486: High-Side Bootstrap Design Using
ISODrivers in Power Delivery Systems
Silicon Labs ISOdrivers are isolated gate drivers that combine low latency, high-drive-
strength gate drive circuits with on-chip isolation.
For robust and safe operation, designers of ac-dc and isolated dc-dc switch mode pow-
er supplies (SMPS), uninterruptible power supplies (UPS), solar inverters, and elec-
tronic lighting ballasts must rely not only on a properly selected high side/low-side gate
driver IC, but also on an external bootstrap circuit and its PCB layout.
The ISOdriver high-side drive channel(s) require(s) a bootstrap circuit when the high-
side switch has a drain voltage greater than the ISOdriver's VDDA supply.
The bootstrap capacitor, C_B (shown in Figure 1), charges when the low-side driver is
active, then supplies driver bias to the high side driver when active. At first glance, the
bootstrap appears as a simple, low-cost circuit consisting only of diode D1 and capaci-
tor C_B (and sometimes resistor R_B). But beneath this humble facade lurks potential de-
sign challenges. This application note discusses the operation and design of the high
side bootstrap circuit and presents a methodology for bootstrap design.
KEY POINTS
• Bootstrap tutorial
• Step-by-step ISOdriver bootstrap design
guide and example
• Summary of best layout practices for
ISOdriver with bootstrap circuitry
• Bootstrap calculator available
Figure .1. ISODriver with High-Side Bootstrap Circuit
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