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LT3493IDCB-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LT3493IDCB-PBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  LT3493 8 3493fb 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 0.78V –1 R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram. An optional phase lead capacitor of 22pF between VOUT and FB reduces light-load output ripple. Input Voltage Range The input voltage range for LT3493 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 LT3493’s minimum operating voltage of 3.6V, 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 = V OUT + VD V IN –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 = 0.91 (0.88 over temperature). The maximum input voltage is determined by the absolute maximum ratings of the VIN and BOOST pins. For con- tinuous mode operation, the maximum input voltage is determined by the minimum duty cycle DCMIN = 0.10: VIN(MAX) = VOUT + VD DCMIN –VD + VSW Note that this is a restriction on the operating input voltage for continuous mode operation; the circuit will tolerate transient inputs up to the absolute maximum ratings of the VIN and BOOST pins. The input voltage should be limited to the VIN operating range (36V) during overload conditions (short-circuit or start-up). Minimum On Time The part will still regulate the output at input voltages that exceed VIN(MAX) (up to 40V), however, the output voltage ripple increases as the input voltage is increased. Figure 1 illustrates switching waveforms in continuous mode for a 3V output application near VIN(MAX) = 33V. As the input voltage is increased, the part is required to switch for shorter periods of time. Delays associated with turning off the power switch dictate the minimum on time of the part. The minimum on time for the LT3493 is ~120ns. Figure 2 illustrates the switching waveforms when the input voltage is increased to VIN = 35V. VSW 20V/DIV VOUT 200mV/DIV AC COUPLED COUT = 10μF VOUT = 3V VIN = 30V ILOAD = 0.75A L = 10μH 2μs/DIV 3493 F01 IL 0.5A/DIV VSW 20V/DIV VOUT 200mV/DIV AC COUPLED COUT = 10μF VOUT = 3V VIN = 35V ILOAD = 0.75A L = 10μH 2μs/DIV 3493 F02 IL 0.5A/DIV Figure 1 Figure 2 |
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