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LTC3531 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3531 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LTC3531/ LTC3531-3.3/LTC3531-3 10 3531fb Buck Mode If VIN is ~800mV above VOUT, the LTC3531 operates in buck or step-down mode. The higher offset between VIN and VOUT (800mV) is required to ensure sufficient mag- netizing voltage across the inductor when the RDSONS are taken into account. At the beginning of a buck mode cycle (Figure 1 right side) switches A and D are turned on (VIN – VOUT is applied across the inductor), current is delivered to the output and ramped up until IPEAK is detected. When this occurs, A is turned off, B is turned on and inductor current falls (–VOUT across the induc- tor) until an IVALLEY is detected. This AD then BD switch sequence is repeated until the output is pumped above its regulation voltage, a final IZERO is detected, and the part returns to sleep mode. Start-Up Mode Before VOUT reaches approximately 1.6V, the D switch is disabled and its body diode is used to transfer current to the output capacitor. In start-up mode, the IVALLEY/IZERO sense circuit is disabled and an alternate algorithm is used to control inductor current. When the LTC3531 is brought out of shutdown (assuming VOUT is discharged) switches A and C are turned on until the inductor current reaches IPEAK. The AC switches are then turned off and inductor current flows to the output through the B switch and D body diode. The period for the B switch/D body diode is controlled by the tOFF timer to ~800nS. This sequence of AC switch-on to IPEAK then B switch and D body diode for ~800ns is repeated until VOUT reaches ~1.6V. Once this OPERATIO threshold is reached, the LTC3531 will transfer through the required modes until VOUT is brought into regulation. Due to propagation delays in the sense circuitry, the magnitudes of the IPEAK, IVALLEY, and IZERO currents may shift depending on VIN, VOUT and operating mode. OTHER LTC3531 FEATURES Shutdown: The part is shut down by pulling SHDN below 0.4V, and made active by pulling the pin up to VIN or VOUT. Note that SHDN an be driven above VIN or VOUT, as long as it is limited to less than 6V. Output Disconnect and Inrush Limiting: The LTC3531 is designed to allow true output disconnect by opening both P-channel MOSFET rectifiers. This allows VOUT to go to zero volts during shutdown, drawing no current from the input source. It also provides inrush current limiting at turn-on, minimizing surge currents seen by the input supply. Thermal Shutdown: If the die temperature reaches ap- proximately 150°C, the part will go into thermal shut- down and all switches will be turned off. The part will be enabled again when the die temperature has dropped by 10°C (nominal). To deliver the power that the LTC3531 is capable of, it is imperative that a good thermal path be provided to dissipate the heat generated within the pack- age. It is recommended that multiple vias in the printed circuit board be used to conduct heat away from the IC and into a copper plane with as much area as possible. Soldering the Exposed Pad to the GND plane (DFN version) is recommended to improve thermal performance. |
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Similar Description - LTC3531 |
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