SNVSCU3 October 2025 LM5137
PRODMIX
An integrated 20μA charge pump is always active in the LM5137. As VIN approaches the VOUT setpoint, the duty cycle extends towards 100%. When the control loop attempts to set an off-time less than tOFF(min), the LM5137 skips the off-time interval for that particular switching cycle and enters 100% duty cycle mode. Under these conditions, CBOOT cannot charge by the usual path from VCC through the bootstrap diode, since the low-side MOSFET is not turned on. Instead, the 20μA charge pump charges CBOOT and keeps the high-side MOSFET on, thus achieving true 100% duty cycle – at least until the control loop again demands an off-time greater than tOFF(min). This action allows for the lowest possible dropout voltage from input to output. Note that adding a gate-to-source resistor at the high-side MOSFET can discharge CBOOT and thus affect 100% duty-cycle operation.