ZHCSIM4H April 2009 – November 2014 LM25011 , LM25011-Q1
PRODUCTION DATA.
The LM25011 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 (2.51 V). If the FB voltage is below the reference, the internal buck switch is switched on for the one-shot timer period which is a function of the input voltage and the programming resistor (RT). Following the on-time, the switch remains off until the FB voltage falls below the reference, but never less than the minimum off-time forced by the off-time one-shot timer. When the FB pin voltage falls below the reference and the off-time one-shot period expires, the buck switch is then turned on for another on-time one-shot period.
When in regulation, the LM25011 operates in continuous conduction mode at heavy load currents and discontinuous conduction mode at light load currents. In continuous conduction mode, the inductor current is always greater than zero and the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduction mode is one-half of the ripple current amplitude of the inductor. The approximate operating frequency is calculated as follows:
The buck switch duty cycle is approximately equal to:
When the load current is less than one-half of the ripple current amplitude of the inductor, the circuit operates in discontinuous conduction mode. The off-time is longer than in continuous conduction mode while the inductor current is zero, causing the switching frequency to reduce as the load current is reduced. Conversion efficiency is maintained at light loads because the switching losses are reduced with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows:
where RL = the load resistance, and L1 is the inductor in the circuit.
The output voltage is set by the two feedback resistors (RFB1, RFB2 in the Functional Block Diagram). The regulated output voltage is calculated as follows:
Ripple voltage, which is required at the input of the regulation comparator for proper output regulation, is generated internally in the LM25011, and externally when the LM25011A is used. In the LM25011 the ERM (emulated ripple mode) control circuit generates the required internal ripple voltage from the ripple waveform at the CS pin. The LM25011A, which is designed for higher frequency operation, requires additional ripple voltage which must be generated externally and provided to the FB pin. This is described in the Application and Implementation section.