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LT3475 Datasheet(PDF) 9 Page - Linear Technology |
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LT3475 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LT3475/LT3475-1 9 3475fb Table 1 shows suggested RT selections for a variety of switching frequencies. Operating Frequency Selection The choice of operating frequency is determined by several factors. There is a tradeoff between efficiency and component size. A higher switching frequency allows the use of smaller inductors at the cost of increased switching losses and decreased efficiency. Another consideration is the maximum duty cycle. In certain applications, the converter needs to operate at a high duty cycle in order to work at the lowest input voltage possible. The LT3475 has a fixed oscillator off time and a variable on time. As a result, the maximum duty cycle increases as the switching frequency is decreased. Input Voltage Range The minimum operating voltage is determined either by the LT3475’s undervoltage lockout of 4V, 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 + VF () VIN –VSW + VF () where VF 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: V IN MIN () = V OUT + VF DC MAX –V F + VSW with DCMAX = 1–tOFF(MIN) • f where t0FF(MIN) is equal to 167ns and f is the switching frequency. Example: f = 600kHz, VOUT = 4V DC MAX = 1− 167ns • 600kHz = 0.90 V IN MIN () = 4V + 0.4V 0.9 – 0.4V + 0.4V = 4.9V The maximum operating voltage is determined by the absolute maximum ratings of the VIN and BOOST pins, and by the minimum duty cycle. V IN MAX () = V OUT + VF DC MIN –V F + VSW with DCMIN = tON(MIN) • f where tON(MIN) is equal to 140ns and f is the switching frequency. Example: f = 750kHz, VOUT = 3.4V DC MIN = 140ns • 750kHz = 0.105 V IN MAX () = 3.4V + 0.4V 0.105 – 0.4V + 0.4V = 36V The minimum duty cycle depends on the switching fre- quency. Running at a lower switching frequency might allow a higher maximum operating voltage. Note that this is a restriction on the operating input voltage; 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 LT3475 will regulate the output current at input volt- ages greater than VIN(MAX). For example, an application with an output voltage of 3V and switching frequency of 1.2MHz has a VIN(MAX) of 20V, as shown in Figure 3. Figure 4 shows operation at 35V. Output ripple and peak inductor VSW 20V/DIV IL 1A/DIV VOUT 500mV/DIV (AC COUPLED) 3475 F03 Figure 3. Operation at VIN(MAX) = 20V. VOUT = 3V and fSW = 1.2MHHz APPLICATIONS INFORMATION |
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