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LTM4609IVPBF Datasheet(PDF) 9 Page - Linear Technology |
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LTM4609IVPBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LTM4609 9 4609fa OPERATION Power Module Description The LTM4609 is a non-isolated buck-boost DC/DC power supply. It can deliver a wide range output voltage from 0.8V to 34V over a wide input range from 4.5V to 36V, by only adding the sensing resistor, inductor and some external input and output capacitors. It provides precisely regulated output voltage programmable via one external resistor. The typical application schematic is shown in Figure 18. The LTM4609 has an integrated current mode buck-boost control, ultralow RDS(ON) FETs with fast switching speed and integrated Schottky diodes. With current mode control and internal feedback loop compensation, the LTM4609 module has sufficient stability margins and good transient performance under a wide range of operating conditions and with a wide range of output capacitors. The frequency of LTM4609 can be operated from 200kHz to 400kHz by setting the voltage on the PLLFLTR pin. Alternatively, its frequency can be synchronized by the input clock signal from the PLLIN pin. The typical switching frequency is 400kHz. The Burst Mode® and skip-cycle mode operations can be enabled at light loads in the LTM4609 to improve its efficiency, while the forced continuous mode and discon- tinuous mode operations are used for constant frequency applications. Foldback current limiting is activated in an overcurrent condition as VFB drops. Internal overvoltage and undervoltage comparators pull the open-drain PGOOD output low if the output feedback voltage exits the ±7.5% window around the regulation point. Pulling the RUN pin below 1.6V forces the controller into its shutdown state. If an external bias supply is applied on the EXTVCC pin, then an efficiency improvement will occur due to the reduced power loss in the internal linear regulator. This is especially true at the higher input voltage range. Burst Mode is a registered trademark of Linear Technology Corporation. APPLICATIONS INFORMATION The typical LTM4609 application circuit is shown in Figure 18. External component selection is primarily determined by the maximum load current and output voltage. Refer to Table 3 for specific external capacitor requirements for a particular application. Output Voltage Programming The PWM controller has an internal 0.8V±1% reference voltage. As shown in the Block Diagram, a 100k 0.5% internal feedback resistor connects VOUT and VFB pins together. Adding a resistor RFB from the VFB pin to the SGND pin programs the output voltage: VOUT = 0.8V • 100k +RFB RFB Table 1. RFB Resistor (0.5%) vs Output Voltage VOUT 0.8V 1.5V 2.5V 3.3V 5V 6V 8V 9V RFB Open 115k 47.5k 32.4k 19.1k 15.4k 11k 9.76k VOUT 10V 12V 15V 16V 20V 24V 30V 34V RFB 8.66k 7.15k 5.62k 5.23k 4.12k 3.4k 2.74k 2.37k Operation Frequency Selection The LTM4609 uses current mode control architecture at constant switching frequency, which is determined by the internal oscillator’s capacitor. This internal capacitor is charged by a fixed current plus an additional current that is proportional to the voltage applied to the PLLFLTR pin. The PLLFLTR pin can be grounded to lower the frequency to 200kHz or tied to 2.4V to yield approximately 400kHz. When PLLIN is left open, the PLLFLTR pin goes low, forc- ing the oscillator to its minimum frequency. A graph for the voltage applied to the PLLFLTR pin vs frequency is given in Figure 2. As the operating frequency increases, the gate charge losses will be higher, thus the efficiency is low. The maximum switching frequency is approximately 400kHz. FREQUENCY SYNCHRONIZATION The LTM4609 can also be synchronized to an external source via the PLLIN pin instead of adjusting the voltage on the PLLFLTR pin directly. The power module has a phase- |
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