Datasheet LTC3525D-3.3 (Analog Devices) - 9

LTC3525D-3.3
operaTion Normal Operation
efficiency by reducing conduction losses in the internal Once V MOSFET switches. As the load increases, the peak inductor OUT has increased more than 0.2V typical above V current is automatically increased to a maximum of 400mA. IN, and either voltage is above 1.8V, normal operation begins, with synchronous rectification enabled. In this At intermediate loads, the peak inductor current may vary mode, the internal N-channel MOSFET connected be- from 150mA to 400mA. Figure 2 shows an example of tween SW and GND stays on until the inductor current how the inductor current changes as the load increases. reaches a maximum peak value, after which it is turned Please note that output capacitor values greater than 47µF off and the P-channel synchronous rectifier is turned will result in higher peak currents than necessary at light on. It stays on, delivering current to the output, until the load. This will lower the light load efficiency. inductor current has dropped below a minimum value at The valley of the inductor current is automatically adjusted which point it turns off and the cycle repeats. When the as well, to maintain a relatively constant inductor ripple output voltage reaches its regulated value both switches current. This keeps the switching frequency relatively are turned off and the LTC3525D goes to sleep, during constant. which time the output capacitor supplies current to the The maximum average load current that can be supported load. Once the output voltage drops approximately 9mV is given by: below the regulation value the IC leaves sleep mode and switching is resumed. 0 3 . • V • η I IN Amps ( ) = The LTC3525D has been designed for low output voltage O MAX VO ripple. The output voltage ripple is typically only 20mV peak-to-peak at light load and 60mV peak-to-peak at where η is the efficiency (see Typical Performance full load using the minimum recommended 10µF output Characteristics). capacitor. An anti-ring circuit damps any oscillation at the The “burst” frequency (how often the LTC3525D delivers switch node when the inductor current falls to zero. a burst of current pulses to the load) is determined by the internal hysteresis (output voltage ripple), the load
Power Adjust Feature
current and the amount of output capacitance. All Burst The LTC3525D incorporates a feature that maximizes Mode operation or hysteretic converters will enter the efficiency at light load while providing increased power audible frequency range when the load is light enough. capability at heavy load by adjusting the peak and valley However, due to the low peak inductor current at light load, of the inductor current as a function of load. Lowering the circuits using the LTC3525D does not typically generate peak inductor current to 150mA at light load optimizes any audible noise. INDUCTOR CURRENT 100mA/DIV LOAD CURRENT 50mA/DIV 3525d33 F02 10µs/DIV
Figure 2. Inductor Current Changing as a Function of Load
3525d33fb For more information www.linear.com/LTC3525D3.3 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Description Revision History Related Parts