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LTC3531 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3531 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC3531/ LTC3531-3.3/LTC3531-3 9 3531fb The LTC3531, LTC3531-3.3 and LTC3531-3 synchro- nous buck-boost converters utilize a Burst Mode control technique to achieve high efficiency over a wide dynamic range of load currents. A 2% accurate comparator is used to monitor the output voltage. If VOUT is above its programmed reference threshold no switching occurs and only quiescent current is drawn from the power source (sleep mode). When VOUT drops below the reference threshold the IC “wakes up”, switching commences, and the output capacitor is charged. The value of the output capacitor, the load current, and the comparator hysteresis (~1%) determines the number of current pulses required to pump-up the output capacitor before the part returns to sleep. In order to determine the best operating mode for the converter, the LTC3531 contains a second comparator that monitors the relative voltage difference between VIN and VOUT. Input and output voltages in the various modes as well as typical inductor currents are shown in Figure 1. Regions of the current waveforms where switches A and D are on provide the highest efficiency since energy is transferred directly from the input source to the output. Boost Mode If VIN is ~400mV below VOUT, the LTC3531 operates in boost or step-up mode. Referring to Figure 1 (left side) when VOUT falls below its regulation voltage, switches A and C are turned on (VIN is applied across the inductor) and current is ramped until IPEAK is detected. When this occurs, C is turned off, D is turned on and current is deliv- ered to the output capacitor (VIN – VOUT is applied across the inductor). Inductor current falls when D is on, until an IVALLEYisdetected.TerminatingatIVALLEY,ratherthanIZERO, results in an increased load current capability for a given peak current. This AC then AD 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 (IVALLEY is ignored and IZERO is used in all modes once VOUT is above its programmed value). 4-Switch Mode If (VOUT – 400mV) < ~VIN < (VOUT + 800mV), the LTC3531 operates in 4-switch step-up/down mode. Returning to Figure 1 (center) when VOUT falls below its regulation volt- age, switches A and C are turned on and current is ramped until IPEAK is detected. As with Boost Mode operation, C is then turned off, D is turned on and current is delivered to the output. When A and D are on, the inductor current slope is dependant on the relationship between VIN, VOUT, and the RDSON of the switches. In 4-switch mode, a tOFF timer (approximately 3μs) is used to terminate the AD pulse. Once the tOFF timer expires, switch A is turned off, B is turned on and inductor current is ramped down (VOUT is applied across the inductor) until IVALLEY is detected. This sequence is repeated until the output is regulated, BD switches are turned on, and a final IZERO is detected. Anticross conduction circuitry in all modes ensures the P-channel MOSFET and N-channel MOSFET switch pairs (A and B or D and C) are never turned on simultaneously. OPERATIO Figure 1. Voltage and Current Waveforms VOUT IMAX BOOST MODE 4SW MODE BUCK MODE IPEAK IVALLEY IZERO VIN VIN tOFF tOFF tOFF AC AC AC AD AD AD AD AD AD AD AC AC BD BD BD BD BD 3531 F01 VIN VOUT A BC D SW1 SW2 L |
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Similar Description - LTC3531 |
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