7.3.10 HO and LO Output Drivers

The LM25119 contains a high-current, high-side driver and associated high voltage level shift to drive the buck switch of each regulator channel. This gate driver circuit works in conjunction with an external diode and bootstrap capacitor. A 0.1 µF or larger ceramic capacitor, connected with short traces between the HB pin and SW pin, is recommended. During the OFF-time of the high-side MOSFET, the SW pin voltage is approximately 0 V and the bootstrap capacitor charges from VCC through the external bootstrap diode. When operating with a high PWM duty cycle, the buck switch is forced off each cycle for 320 ns to ensure that the bootstrap capacitor is recharged.

The LO and HO outputs are controlled with an adaptive dead-time methodology which insures that both outputs are never enabled at the same time. When the controller commands HO to be enabled, the adaptive dead-time logic first disables LO and waits for the LO voltage to drop. HO is then enabled after a small delay. Similarly, the LO turnon is disabled until the HO voltage has discharged. This methodology insures adequate dead-time for any size MOSFET.

Exercise care in selecting an output MOSFET with the appropriate threshold voltage, especially if VCC is supplied from the regulator output. During start-up at low input voltages the MOSFET threshold must be lower than the 4-V VCC undervoltage lockout threshold. Otherwise, there may be insufficient VCC voltage to completely turn on the MOSFET as VCC undervoltage lockout is released during start-up. If the buck switch MOSFET gate drive is not sufficient, the regulator may not start or it may hang up momentarily in a high power dissipation state. This condition can be avoided by selecting a MOSFET with a lower threshold voltage or if VCC is supplied from an external source higher than the output voltage. If the minimum input voltage programmed by the UVLO pin resistor divider is above the VCC regulation level, this precaution is of no concern.