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LT1943 Datasheet(PDF) 11 Page - Linear Technology |
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LT1943 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT1943 11 1943fa The control loop for the four switchers is similar. A pulse from the slave oscillator sets the RS latch and turns on the internal NPN bipolar power switch. Current in the switch and the external inductor begins to increase. When this current exceeds a level determined by the voltage at VC, the current comparator resets the latch, turning off the switch. The current in the inductor flows through the Schottky diode and begins to decrease. The cycle begins again at the next pulse from the oscillator. In this way, the voltage on the VC pin controls the current through the inductor to the output. The internal error amplifier regulates the output voltage by continually adjusting the VC pin voltage. The threshold for switching on the VC pin is 0.8V, and an active clamp of 1.8V limits the output current. The RUN/SS and SS-234 pins also clamp the VC pin voltage. As the internal current source charges the external soft-start capacitor, the current limit increases slowly. Each switcher contains an extra, independent oscillator to perform frequency foldback during overload conditions. This slave oscillator is normally synchronized to the mas- ter oscillator. A comparator senses when VFB is less than 0.5V and switches the regulator from the master oscillator to a slower slave oscillator. The VFB pin is less than 0.5V during startup, short-circuit, and overload conditions. Frequency foldback helps limit switch current and power dissipation under these conditions. The switch driver for SW1 operates either from VIN or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to saturate the internal bipolar NPN power switch for efficient operation. STEP-DOWN CONSIDERATIONS FB Resistor Network The output voltage for switcher 1 is programmed with a resistor divider (refer to the Block Diagram) between the output and the FB pin. Choose the resistors according to: R2 = R1(VOUT/1.25V – 1) R1 should be 10k Ω or less to avoid bias current errors. Input Voltage Range The minimum operating voltage of switcher 1 is deter- mined either by the LT1943’s undervoltage lockout of ~4V, or by its maximum duty cycle. The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: DC = (VOUT + VF)/(VIN – VSW + VF) where VF is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch (~0.3V at maximum load). This leads to a minimum input voltage of VIN(MIN) = (VOUT + VF)/DCMAX – VF + VSW with DCMAX = 0.82. Inductor Selection and Maximum Output Current A good first choice for the inductor value is: L = (VOUT + VF)/1.2 where VF is the voltage drop of the catch diode (~0.4V) and L is in µH. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be at least 30% higher. For highest effi- ciency, the series resistance (DCR) should be less than 0.1 Ω. Table 1 lists several vendors and types that are suitable. The optimum inductor for a given application may differ from the one indicated by this simple design guide. A larger value inductor provides a higher maximum load current, and reduces the output voltage ripple. If your load is lower than the maximum load current, then you can relax the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. Be aware that the maximum load current depends on input voltage. A graph in the Typical Perfor- mance section of this data sheet shows the maximum load current as a function of input voltage and inductor value for VOUT = 3.3V. In addition, low inductance may result in discontinuous mode operation, which further reduces OPERATIO |
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Similar Description - LT1943_15 |
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