Datasheet LTC3525L-3 (Analog Devices) - 7

LTC3525L-3
OPERATION
During start-up, the synchronous rectifier is not enabled, The LTC3525L-3 has been designed for low output voltage and the internal P-channel synchronous rectifier acts ripple. The output voltage ripple is typically only 20mV as a follower, causing the peak voltage on SW to reach peak-to-peak at light load and 60mV peak-to-peak at (VIN + 1V) typical. This limits inrush current by maintaining full load using the minimum recommended 10µF output control of the inductor current when VOUT is less than VIN. capacitor. An anti-ring circuit damps any oscillation at the To reduce power dissipation in the P-channel synchronous switch node when the inductor current falls to zero. rectifier when the output is shorted, a foldback feature is incorporated that reduces the peak inductor current when
Power Adjust Feature
VIN is more than 1.7V greater than VOUT. The LTC3525L-3 incorporates a feature that maximizes efficiency at light load while providing increased power
Normal Operation
capability at heavy load by adjusting the peak and valley Once VOUT has increased more than 0.2V typical above of the inductor current as a function of load. Lowering the VIN, and either voltage is above 1.8V, normal operation peak inductor current to 150mA at light load optimizes begins, with synchronous rectification enabled. In this efficiency by reducing conduction losses in the internal mode, the internal N-channel MOSFET connected between MOSFET switches. As the load increases, the peak inductor SW and GND stays on until the inductor current reaches a current is automatically increased to a maximum of 400mA. maximum peak value, after which it is turned off and the At intermediate loads, the peak inductor current may vary P-channel synchronous rectifier is turned on. It stays on, from 150mA to 400mA. Figure 2 shows an example of delivering current to the output, until the inductor current how the inductor current changes as the load increases. has dropped below a minimum value at which point it Please note that output capacitor values greater than 47µF turns off and the cycle repeats. When the output voltage will result in higher peak currents than necessary at light reaches its regulated value both switches are turned off load. This will lower the light load efficiency. and the LTC3525L-3 goes to sleep, during which time the The valley of the inductor current is automatically adjusted output capacitor supplies current to the load. Once the as well, to maintain a relatively constant inductor ripple output voltage drops below the regulation value the IC current. This keeps the switching frequency relatively leaves sleep mode and switching is resumed. constant. INDUCTOR CURRENT 100mA/DIV LOAD CURRENT 50mA/DIV 3525l F02 10µs/DIV
Figure 2. Inductor Current Changing as a Function of Load
3525lfa 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Package Description Revision History Typical Applications Related Parts