Datasheet LT3504 (Analog Devices) - 10

LT3504
applicaTions inForMaTion FB Resistor Network
At input voltages that exceed VIN(PS) the part will continue The output voltage is programmed with a resistor divider to regulate the output voltage up to 40V. However the connected from the output and the FB pin. Choose the 1% part will skip pulses (see Figure 1) resulting in unwanted resistor according to: harmonics, increased output voltage ripple, and increased peak inductor current. Provided that the inductor does not ⎛ V R1 ⎞ = R2 • OUT − 1 saturate and that the switch current remains below 2A, ⎝⎜ 0.8V ⎠⎟ operation above VIN(PS) is safe and will not damage the part. For a more detailed discussion on minimum on-time A good value for R2 is 10kΩ, R2 should not exceed 20kΩ and pulse-skipping, refer to the Applications Information to avoid bias current error. section of the LT3505 data sheet.
Input Voltage Range
Avoid starting up the LT3504 at input voltages greater The input voltage range for LT3504 applications depends than 36V, as the LT3504 must simultaneously conduct on the output voltage and on the absolute maximum rat- maximum currents at high VIN. The maximum operating ing of the V junction temperature of 125°C may be exceeded due to IN pin. the high instantaneous power dissipation. The minimum input voltage to regulate the output gener- ally has to be at least 400mV greater than the greatest programmed output voltage. The only exception is when the largest programmed output voltage is less than 2.8V. IL 0.5A/DIV In this case the minimum input voltage is 3.2V. The absolute maximum input voltage of the LT3504 is 40V and the part will regulate output voltages as long as the input voltage remains less than or equal to 40V. VSW However for constant-frequency operation (no pulse- 10V/DIV skipping) the maximum input voltage is determined by 2µs/DIV 3504 F01a the minimum on-time of the LT3504 and the programmed switching frequency. The minimum on-time is the shortest
Figure 1a: The LT3504 Operating in Constant-Frequency Operation (Below V
period of time that it takes the switch to turn on and off.
IN(PS)), VIN = 26.5V, VOUT = 3.3V, fSW = 2MHz, tON(MIN) = 74ns at T = 25°C
Therefore the maximum input voltage to operate without pulse-skipping is: VIN(PS) = [ (VOUT + VD)/(fSW • tON(MIN)) ] + VSW – VD IL where: 0.5A/DIV • VIN(PS) is the maximum input voltage to operate in constant frequency operation without skipping pulses. • VOUT is the programmed output voltage VSW 10V/DIV • VSW is the switch voltage drop, at IOUT = 1A, VSW = 0.4V 2µs/DIV 3504 F01b • VD is the catch diode forward voltage drop, for an ap- propriately sized diode, V
Figure 1b.The LT3504 Operating in Pulse-Skipping Mode
D = 0.4V
(Above VIN(PS)), VIN = 27V, VOUT = 3.3V, fSW = 2MHz,
• f
t
SW is the programmed switching frequency
ON(MIN) = 74ns at T = 25°C
• tON(MIN) is the minimum on-time, worst-case over temperature = 110ns (at T = 125°C) 3504fa 10 For more information www.linear.com/LT3504 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts