SNVS787I January 2012 – August 2021 LM5018
The LM5018 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with the output voltage feedback (FB) compared to an internal reference (1.225 V). If the FB voltage is below the reference, the internal buck switch is turned on for the one-shot timer period, which is a function of the input voltage and the programming resistor (RON). Following the on-time, the switch remains off until the FB voltage falls below the reference, but never before the minimum off-time forced by the minimum off-time one-shot timer. When the FB pin voltage falls below the reference and the minimum off-time one-shot period expires, the buck switch is turned on for another on-time one-shot period. This continues until regulation is achieved and the FB voltage is approximately equal to 1.225 V (typ).
In a synchronous buck converter, the low-side (sync) FET is 'on' when the high-side (buck) FET is 'off.' The inductor current ramps up when the high-side switch is ‘on’ and ramps down when the high-side switch is ‘off’. There is no diode emulation feature in this IC, therefore, the inductor current can ramp in the negative direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of the output loading. The operating frequency remains relatively constant with load and line variations. The operating frequency can be calculated as shown in Equation 1.
The output voltage (VOUT) is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated as shown in Equation 2.
This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor (COUT). A minimum of 25 mV of ripple voltage at the feedback pin (FB) is required for the LM5018. In cases where the capacitor ESR is too small, additional series resistance can be required (RC in Figure 7-1).
For applications where lower output voltage ripple is required, the output can be taken directly from a low ESR output capacitor, as shown in Figure 7-1. However, RC slightly degrades the load regulation.