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LT3686IDDPBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3686IDDPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  LT3686 3686fa ApplicAtions inForMAtion Figure 3. Continuous Mode Operation Near Minimum On Time When the required on time decreases below the typical minimum on time of 100ns, instead of the switch pulse width becoming narrower to accommodate the lower duty cycle requirement, the switch pulse width remains fixed at 100ns. The inductor current ramps up to a value exceed- ing the load current and the output ripple increases. The part then remains off until the output voltage dips below the programmed value before it begins switching again (Figure 4). Provided that the load can tolerate the increased output voltagerippleandthatthecomponentshavebeenproperly selected,operationwhilepulseskippingissafeandwillnot damagethepart.Astheinputvoltageincreases,theinductor current ramps up quicker, the number of skipped pulses increases, and the output voltage ripple increases. Inductor current may reach current limit when operating in pulse skip mode with small valued inductors. In this case, the LT3686 will periodically reduce its frequency to keep the inductor valley current to 1.7A (Figure 5). Peak Figure 4. Pulse Skip Occurs When Required On Time Is Below 100ns 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 de- termined by the minimum duty cycle DCMIN: VIN(MAX) = VOUT + VD DCMIN – VD + VSW DCMIN can be adjusted with frequency. Note that this is a restrictionontheoperatinginputvoltageforfixedfrequency operation; the circuit will tolerate transient inputs up to the absolute maximum ratings of the VIN and BOOST pins. Minimum On Time As the input voltage is increased, the LT3686 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 LT3686 is 100ns (Figure 3). VIN = 18V VOUT = 3.3V L = 6.8µH COUT = 22µF ILOAD = 1.2mA VOUT 100mV/DIV AC VSW 10V/DIV IL 500mA/DIV 500ns/DIV 3686 F03 VIN = 35V VOUT = 3.3V L = 6.8µH COUT = 22µF IOUT = 300mA VSW 20V/DIV IL 500mA/DIV VOUT 100mA/DIV AC 2µs/DIV 3686 F04 |
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