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LTC3413 Datasheet(PDF) 7 Page - Linear Technology |
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LTC3413 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  LTC3413 7 3413fc OPERATION main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-channel MOSFET and the inductor. Low Supply Operation The LTC3413 is designed to operate down to an SVIN input supply voltage of 2.25V. One important consideration at low input supply voltages is that the RDS(ON) of the P-channel and N-channel power switches increases. The user should calculate the power dissipation when the LTC3413 is used at 100% duty cycle with low input voltages to ensure that thermal limits are not exceeded. Slope Compensation and Inductor Peak Current Slopecompensationprovidesstabilityinconstantfrequency architectures by preventing subharmonic oscillations at duty cycles greater than 50%. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, the maximum inductor peak current is reduced when slope compensation is added. In the LTC3413, however, slope compensation recovery is implemented to keep the maximum inductor peak current constant throughout the range of duty cycles. Short-Circuit Protection When the output is shorted to ground, the inductor cur- rent decays very slowly during a single switching cycle. To prevent current runaway from occurring, a secondary current limit is imposed on the inductor current. If the inductor valley current increases greater than 5A, the top power MOSFET will be held off and switching cycles will be skipped until the inductor current is reduced. Pre-Biased Load It is important to sequence the start-up of the LTC3413 prior to any external circuitry that might drive the VOUT pin. If the VOUT pin is externally driven to a voltage more than 10% (the OV threshold) above the desired VOUT voltage, the LTC3413 may enter a latched state where it no longer switches. To avoid this scenario, the user should ensure there is not a pre-biased load during start-up. This can be accomplished by sequencing the LTC3413’s RUN pin before the load’s supply. APPLICATIONS INFORMATION The basic LTC3413 application circuit is shown in Figure 1a. External component selection is determined by the maximum load current and begins with the selection of the inductor value and operating frequency followed by CIN and COUT. Operating Frequency Selection of the operating frequency is a tradeoff between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by reducing internal gate charge losses but requires larger inductance values and/or capacitance to maintain low output ripple voltage. The operating frequency of the LTC3413 is determined by an external resistor that is connected between pin RT and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator and can be calculated by using the following equation. R f k OSC =Ω () Ω 323 10 10 11 .• – _ Although frequencies as high as 2MHz are possible, the minimum on-time of the LTC3413 imposes a minimum limit on the operating duty cycle. The minimum on-time is typically 110ns. Therefore, the minimum duty cycle is equal to 100 • 110ns • f (Hz). |
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Similar Description - LTC3413 |
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