Datasheet LT3502, LT3502A (Analog Devices) - 10

LT3502/LT3502A
applications inForMation FB Resistor Network
Note that this is a restriction on the operating input volt- The output voltage is programmed with a resistor divider age for fixed frequency operation; the circuit will tolerate between the output and the FB pin. Choose the 1% resis- transient inputs up to the absolute maximum ratings of tors according to: the VIN and BOOST pins. The input voltage should be limited to the VIN operating range (40V) during overload ⎛ V R1 ⎞ = R2 OUT ⎜ – 1⎟ conditions. ⎝ 0.8V ⎠
Minimum On-Time
R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram. The LT3502/LT3502A will still regulate the output at input voltages that exceed VIN(MAX) (up to 40V), however, the
Input Voltage Range
output voltage ripple increases as the input voltage is increased. The input voltage range for the LT3502/LT3502A applica- tions depends on the output voltage and on the absolute As the input voltage is increased, the part is required to maximum ratings of the V switch for shorter periods of time. Delays associated with IN and BOOST pins. turning off the power switch dictate the minimum on-time The minimum input voltage is determined by either the of the part. The minimum on-time for the LT3502/LT3502A LT3502/LT3502A’s minimum operating voltage of 3V, or is 60ns (Figure 1). 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: VSW V 20V/DIV DC = OUT + VD VIN – VSW + VD IL where V 500mA/DIV D is the forward voltage drop of the catch diode (~0.4V) and V VOUT SW is the voltage drop of the internal switch 100mV/DIV (~0.45V at maximum load). This leads to a minimum input 1µs/DIV 3502 F01 voltage of: VIN = 33V, VOUT = 3.3V L = 6.8µH, COUT = 10µF, IOUT = 250mA V V OUT + VD – V
Figure 1. Continuous Mode Operation Near
IN(MIN) = DC D + VSW MAX
Minimum On-Time of 60ns
with DCMAX = 0.80 for the LT3502A and 0.90 for the When the required on-time decreases below the mini- LT3502. mum on-time of 60ns, instead of the switch pulse width The maximum input voltage is determined by the becoming narrower to accommodate the lower duty cycle absolute maximum ratings of the V requirement, the switch pulse width remains fixed at IN and BOOST pins. For fixed frequency operation, the maximum input voltage is 60ns. The inductor current ramps up to a value exceed- determined by the minimum duty cycle DC ing the load current and the output ripple increases. The MIN: part then remains off until the output voltage dips below V V OUT + VD the programmed value before it begins switching again IN(MAX) = – VD + VSW DCMIN (Figure 2). DCMIN = 0.15 for the LT3502A and 0.08 for the LT3502. Provided that the load can tolerate the increased output voltage ripple and that the components have been properly selected, operation above VIN(MAX) is safe and will not damage the part. 3502fd 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts