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LT3844 Datasheet(PDF) 10 Page - Linear Technology |
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LT3844 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  LT3844 10 3844fb OPERATION (Refer to Functional Diagram) the CSS pin voltage then the current threshold set by the DC control voltage, VC, is decreased and the inductor cur- rent is lowered. This in turn decreases the output voltage slew rate allowing the CSS pin voltage ramp to catch up to the VFB pin voltage. An internal 100mV offset is added to the VFB pin voltage relative to the to CSS pin voltage so that at start-up the soft-start circuit will discharge the VC pin voltage below the DC control voltage equivalent to zero inductor current. This will reduce the input supply inrush current. The soft-start circuit is disabled once the CSS pin voltage has been charged to 200mV above the internal reference of 1.231V. During a VIN UVLO, VCC UVLO or SHDN UVLO event, the CSS pin voltage is discharged with a 50μA current source. In normal operation the CSS pin voltage is clamped to a diode above the VFB pin voltage. Therefore, the value of the CSS capacitor is relevant in how long of a fault event will retrigger a soft-start. In other words, if any of the above UVLO conditions occur, the CSS pin voltage will be discharged with a 50μA current source. There is a diode worth of voltage headroom to ride through the fault before the CSS pin voltage enters its active region and the soft- start function is enabled. Also, since the CSS pin voltage is clamped to a diode above the VFB pin voltage, during a short circuit the CSS pin volt- age is pulled low because the VFB pin voltage is low. Once the short has been removed the VFB pin voltage starts to recover. The soft-start circuit takes control of the output voltage slew rate once the VFB pin voltage has exceeded the slowly ramping CSS pin voltage, reducing the output voltage overshoot during a short-circuit recovery. Slope/Antislope Compensation The IC incorporates slope compensation to eliminate potential subharmonic oscillations in the current control loop. The IC’s slope compensation circuit imposes an artificial ramp on the sensed current to increase the rising slope as duty cycle increases. Typically, this additional ramp affects the sensed current value, thereby reducing the achievable current limit value by the same amount as the added ramp represents. As such, the current limit is typically reduced as the duty cycle increases. The LT3844, however, contains antislope com- pensation circuitry to eliminate the current limit reduction associated with slope compensation. As the slope compen- sation ramp is added to the sensed current, a similar ramp is added to the current limit threshold. The end result is that the current limit is not compromised so the LT3844 can provide full power regardless of required duty cycle. Shutdown The LT3844 includes a shutdown mode where all the internal IC functions are disabled and the VIN current is reduced to less than 10μA. The shutdown pin can be used for undervoltage lockout with hysteresis, micropower shut- down or as a general purpose on/off control of the converter output. The shutdown function has two thresholds. The first threshold, a precision 1.23V threshold with 120mV of hysteresis, disables the converter from switching. The second threshold, approximately a 0.7V referenced to SGND, completely disables all internal circuitry and reduces the VIN current to less than 10μA. See the Application Information section for more information. APPLICATIONS INFORMATION The basic LT3844 step-down (buck) application, shown in the Typical Application on the front page, converts a larger positive input voltage to a lower positive or negative output voltage. This Application Information section assists selection of external components for the requirements of the power supply. RSENSE Selection The current sense resistor, RSENSE, monitors the inductor current of the supply (See Typical Application on front page). Its value is chosen based on the maximum required output load current. The LT3844 current sense amplifier has a maximum voltage threshold of, typically, 100mV. |
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