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LTC3441 Datasheet(PDF) 7 Page - Linear Technology |
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LTC3441 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 20 page  LTC3602 7 3602fb OPERATION Main Control Loop The LTC3602 is a monolithic, constant-frequency, current- mode step-down DC/DC converter. During normal opera- tion, the internal top power switch (N-channel MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor increases until the current comparator trips and turns off the top power MOSFET. The peak inductor current at which the current comparator shuts off the top power switch is controlled by the voltage on the ITH pin. The error amplifier adjusts the voltage on the ITH pin by comparing the feedback signal from a resistor divider on the VFB pin with an internal 0.6V reference. When the load current increases, it causes a reduction in the feedback voltage relative to the reference. The error amplifier raises the ITH voltage until the average inductor current matches the new load current. When the top power MOSFET shuts off, the synchronous power switch (N-channel MOSFET) turns on until either the bottom current limit is reached or the beginning of the next clock cycle. The bottom current limit is set at –2.5A for forced continuous mode and 0A for Burst Mode operation. The operating frequency is externally set by an external resistor connected between the RT pin and ground. The practical switching frequency can range from 300kHz to 3MHz. Overvoltage and undervoltage comparators will pull the PGOOD output low if the output voltage comes out of regulation by ±7.5%. In an overvoltage condition, the top power MOSFET is turned off and the bottom power MOSFET is switched on until either the overvoltage condition clears or the bottom MOSFET’s current limit is reached. Forced Continuous Mode Connecting the SYNC/MODE pin to INTVCCwilldisableBurst Mode operation and force continuous current operation. At light loads, forced continuous mode operation is less efficient than Burst Mode operation, but may be desirable in some applications where it is necessary to keep switching harmonics out of a signal band. The output voltage ripple is minimized in this mode. Burst Mode Operation Connecting the SYNC/MODE pin to a voltage in the range of 0.42V to 1V enables Burst Mode operation. In Burst Mode operation, the internal power MOSFETs operate intermittently at light loads. This increases efficiency by minimizing switching losses. During Burst Mode opera- tion, the minimum peak inductor current is externally set by the voltage on the SYNC/MODE pin and the voltage on the ITH pin is monitored by the burst comparator to determine when sleep mode is enabled and disabled. When the average inductor current is greater than the load current, the voltage on the ITH pin drops. As the ITH voltage falls below 330mV, the burst comparator trips and enables sleep mode. During sleep mode, the top power MOSFET is held off and the ITH pin is disconnected from the output of the error amplifier. The majority of the internal circuitry is also turned off to reduce the quiescent current to 75μA while the load current is solely supplied by the output capacitor. When the output voltage drops, the ITH pin is reconnected to the output of the error amplifier and the top power MOSFET along with all the internal circuitry is switched back on. This process repeats at a rate that is dependent on the load demand. Pulse-skipping opera- tion is implemented by connecting the SYNC/MODE pin to ground. This forces the burst clamp level to be at 0V. As the load current decreases, the peak inductor current will be determined by the voltage on the ITH pin until the ITH voltage drops below 330mV. At this point, the peak inductor current is determined by the minimum on-time of the current comparator. If the load demand is less than the average of the minimum on-time inductor current, switching cycles will be skipped to keep the output volt- age in regulation. Frequency Synchronization The internal oscillator of the LTC3602 can be synchronized to an external clock connected to the SYNC/MODE pin. The frequency of the external clock can be in the range of 300kHz to 3MHz. For this application, the oscillator timing resistor should be chosen to correspond to a frequency that is 25% lower than the synchronization frequency. When synchronized, the LTC3602 will operate in pulse- skipping mode. |
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Similar Description - LTC3441 |
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