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LT1933 Datasheet(PDF) 7 Page - Linear Technology |
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LT1933 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 20 page  LT1933 7 1933fe OPERATION (Refer to Block Diagram) The LT1933 is a constant frequency, current mode step down regulator. A 500kHz oscillator enables an RS flip- flop, turning on the internal 750mA power switch Q1. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC node. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less cur- rent is delivered. An active clamp (not shown) on the VC node provides current limit. The VC node is also clamped to the voltage on the SHDN pin; soft-start is implemented by generating a voltage ramp at the SHDN pin using an external resistor and capacitor. An internal regulator provides power to the control cir- cuitry. This regulator includes an undervoltage lockout to prevent switching when VIN is less than ~3.35V. The SHDN pin is used to place the LT1933 in shutdown, dis- connecting the output and reducing the input current to less than 2μA. The switch driver operates from either the input 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 fully saturate the internal bipolar NPN power switch for efficient opera- tion. The oscillator reduces the LT1933’s operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the output current during startup and overload. APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resis- tors according to: R1 = R2(VOUT/1.245 – 1) R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram. Input Voltage Range The input voltage range for LT1933 applications depends on the output voltage and on the absolute maximum rat- ings of the VIN and BOOST pins. The minimum input voltage is determined by either the LT1933’s minimum operating voltage of ~3.35V, 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 + VD)/(VIN – VSW + VD) where VD is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch (~0.4V at maximum load). This leads to a minimum input voltage of: VIN(MIN) = (VOUT + VD)/DCMAX – VD + VSW with DCMAX = 0.88 The maximum input voltage is determined by the absolute maximum ratings of the VIN and BOOST pins and by the minimum duty cycle DCMIN = 0.08 (corresponding to a minimum on time of 130ns): VIN(MAX) = (VOUT + VD)/DCMIN – VD + VSW Note that this is a restriction on the operating input voltage; the circuit will tolerate transient inputs up to the absolute maximum ratings of the VIN and BOOST pins. Inductor Selection and Maximum Output Current A good first choice for the inductor value is: L = 5 (VOUT + VD) where VD is the voltage drop of the catch diode (~0.4V) and L is in μH. With this value the maximum load current will be above 500mA. The inductor’s RMS current rating must be greater than your maximum load current and its |
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