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LCLV Datasheet(PDF) 10 Page - Linear Technology |
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LCLV Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  LT3502/LT3502A 10 3502fc APPLICATIONS INFORMATION Figure 1. Continuous Mode Operation Near Minimum On Time of 60ns 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 0.8V –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 the LT3502/LT3502A applica- tions depends on the output voltage and on the absolute maximum ratings of the VIN and BOOST pins. The minimum input voltage is determined by either the LT3502/LT3502A’s minimum operating voltage of 3V, 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 (~0.45V at maximum load). This leads to a minimum input voltage of: VIN(MIN) = VOUT + VD DCMAX –VD + VSW with DCMAX = 0.80 for the LT3502A and 0.90 for the LT3502. The maximum input voltage is determined by the absolute maximum ratings of the VIN and BOOST pins. For fixed frequency operation, the maximum input voltage is determined by the minimum duty cycle DCMIN: VIN(MAX) = VOUT + VD DCMIN –VD + VSW DCMIN = 0.15 for the LT3502A and 0.08 for the LT3502. Note that this is a restriction on the operating input volt- age for fixed frequency 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 (40V) during overload conditions. Minimum On Time The LT3502/LT3502A 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. 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 LT3502/LT3502A is 60ns (Figure 1). When the required on time decreases below the minimum on time of 60ns, instead of the switch pulse width becoming narrower to accommodate the lower duty cycle require- ment, the switch pulse width remains fixed at 60ns. The inductor current ramps up to a value exceeding the load cur- rent and the output ripple increases. The part then remains VSW 20V/DIV VOUT 100mV/DIV 1μs/DIV VIN = 33V VOUT = 3.3V L = 6.8μH COUT = 10μF IOUT = 250mA 3502 F01 IL 500mA/DIV |
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