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LT3495EDDB-1-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LT3495EDDB-1-PBF Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page LT3495/LT3495B/ LT3495-1/LT3495B-1 8 3495b1b1f OPERATION The LT3495 series utilizes a variable peak current, variable off-time control scheme to provide high efficiency over a wide range of output current. The operation of the part can be better understood by referring to the Block Diagram. The part senses the output voltage by monitoring the voltage on the FB pin. The user sets the desired output voltage by choosing the value of the external top feedback resistor. The parts incorporate a precision 76k bottom feedback resistor. Assuming that output voltage adjustment is not used (CTRL pin is tied to 1.5V or greater), the internal reference (VREF = 1.235V) sets the voltage at which FB will servo to during regulation. The Switch Control block senses the output of the ampli- fier and adjusts the switching frequency as well as other parameters to achieve regulation. During the start-up of the circuit, special precautions are taken to ensure that the inductor current remains under control. For the LT3495/LT3495-1, the switching frequency is never allowed to fall below approximately 45kHz. Because of this, a minimum load must be present to prevent the output voltage from drifting too high. For most applications, this minimum load is automatically generated within the part via the Shunt Control block. The level of this current is adaptable, removing itself when not needed to improve efficiency at higher load levels. However when the input Table 1. Difference Between LT3495 and LT3495B/LT3495-1/LT3495B-1 PART SWITCH CURRENT LIMIT (mA) MINIMUM SWITCHING FREQUENCY (kHz) MINIMUM OUTPUT LOAD REQUIREMENT LT3495 650 45 Required under certain conditions LT3495B 650 0 Not Required LT3495-1 350 45 Required under certain conditions LT3495B-1 350 0 Not Required voltage and output voltage are close, the internal shunt current may not be large enough. Under this condition, a minimum output load is required to prevent the output voltage from drifting too high. For the LT3495B/B-1, the minimum switching frequency feature is disabled and the switching frequency can be as low as zero. As a result, the output voltage will never drift high and no minimum output load is required. The LT3495 series also has a PMOS output disconnect switch. The PMOS switch is turned on when the part is enabled via the SHDN pin. When the parts are in shutdown, the PMOS switch turns off, allowing the VOUT node to go to ground. This type of disconnect function is often required in power supplies. The LT3495 series also sets a maximum switch on time of 10μs. This feature guarantees that the parts can continue to deliver energy to the output even if the input supply impedance becomes so large that the commanded peak switch current is never reached. The difference between the LT3495/LT3495B and LT3495-1/ LT3495B-1 is the level of the current limit. LT3495/LT3495B have a typical peak current limit of 650mA while the LT3495-1/LT3495B-1 have a typical peak current limit of 350mA. The differences between the LT3495 and LT3495B/ LT3495-1/LT3495B-1 are listed in Table 1. |
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