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LTC3114-1 Arkusz danych(PDF) 15 Page - Analog Devices |
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LTC3114-1 Arkusz danych(HTML) 15 Page - Analog Devices |
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15 / 34 page  LTC3114-1 15 Rev. D For more information www.analog.com OPERATION maximum average inductor current is approximately 1.7A (typical). In buck mode, the output current is approxi- mately equal to the inductor current, IL. IOUT(BUCK) ≈ IL • 0.9 The SW1/SW2 forced low time on each switching cycle briefly disconnects the inductor from VOUT and VIN result- ing in slightly less output current in either buck or boost mode for a given inductor current. In boost mode, the output current is related to average inductor current and duty cycle by: IOUT(BOOST) ≈ IL • (1 – D) where D is the converter duty cycle. Since the output current in boost mode is reduced by the duty cycle (D), the output current rating in buck mode is always greater than in boost mode. Also, because boost mode operation requires a higher inductor cur- rent for a given output current compared to buck mode, the efficiency in boost mode will be lower due to higher IINDUCTOR2 • RDS(ON) losses in the power switches. This will further reduce the output current capability in boost mode. In either operating mode, however, the inductor peak-to-peak ripple current does not play a major role in determining the output current capability, unlike peak current mode control. With peak current mode control, the maximum output current capability is reduced by the magnitude of inductor ripple current because the peak inductor current level is the control variable, but the average inductor current is what determines the output current. The LTC3114 -1 measures and controls average inductor current, and therefore, the inductor ripple current magnitude has little effect on the maximum current capability in contrast to an equivalent peak current mode converter. Under most conditions in buck mode, the LTC3114 -1 is capable of providing 1A to the load. Under certain conditions, more output current is possible, refer to the Typical Performance Characteristics section for more details. In boost mode, as described previously, the output current capability is related to the boost ratio or duty cycle (D). For a 3.6V VIN to 5V output application, the LTC3114-1 can provide up to 500mA to the load. Refer to the Typical Performance Characteristics section for more detail on output current capability. At VC levels below 135mV, the LTC3114-1 will not com- mand any current because the internal current sense sig- nal has a built-in 135mV offset. Therefore, the active range of VC is between approximately 135mV (zero current) and 1V (full current). In some applications, an external circuit may be used to control the VC voltage level. Any such circuit needs to have the capability to sink or source the approximate 12µA provided by the internal error amplifier and to pull below 135mV to disable the current command, if necessary. OVERLOAD CURRENT LIMIT AND ZERO CURRENT COMPARATOR The internal current sense waveform is also used by the peak overload current (IPEAK) and zero current (IZERO) comparators. The IPEAK current comparator monitors ISENSE and halts converter operation if the inductor cur- rent level exceeds its maximum internal threshold, which is approximately 50% above the normal maximum current level commanded by the current control loop. An inductor current level of this magnitude will only occur during a fault, such as an output short circuit or a fast VIN (line) transient. If the IPEAK comparator is engaged, the PWM is halted for the remainder of the switching cycle with SW1 and SW2 held low. If VOUT is less than approximately 1.8V when the peak limit occurs, then a soft-start cycle is initiated. In the event that the current overload is the result of an output short-circuit condition, the LTC3114-1 will remain in a low frequency restart mode, keeping the on-chip power dissipation to very low levels. If the short circuit is removed, the LTC3114-1 will restart in the normal fashion. The LTC3114-1 exhibits discontinuous inductor current operation at light output loads by virtue of the IZERO com- parator circuit under most operating conditions. This improves efficiency at light output loads if PWM mode operation compared to continuous conduction mode. If the internal current sense waveform transitions below the internally set zero current threshold, the LTC3114-1 will disconnect the inductor from VOUT, by shutting off switch D, to prevent discharge of the output capacitor. The IZERO circuitry is reset by the oscillator clock at the end of the switching cycle. The IZERO comparator threshold is set |
Podobny numer części - LTC3114-1 |
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Podobny opis - LTC3114-1 |
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