7.3.5 N-Channel Buck Switch and Driver

The LM25011 integrates an N-Channel buck switch and associated floating high-voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor (C_BST) and an internal high-voltage diode. A 0.1-µF capacitor connected between BST and SW provides the supply voltage for the driver during the on-time. During each off-time, the SW pin is at approximately –1 V, and C_BST is recharged from the internal 5-V regulator for the next on-time. The minimum off-time ensures a sufficient time each cycle to recharge the bootstrap capacitor.

In applications with relatively high output voltage and low minimum load current, the internal pre-charge device of the LM25011 may not pull the SW pin sufficiently low during the off-time to maintain the voltage on the bootstrap capacitor. If the bootstrap capacitor (C_BST) discharges during the long off-times, and the regulator will cycle on and off at a low frequency. Decreasing the values of the feedback resistors R_FB1 and R_FB2 to provide a minimum load of typically 1mA at nominal V_OUT will increase the minimum switching frequency and maintain sufficient bootstrap capacitor voltage.