Datasheet LT3506, LT3506A (Analog Devices) - 8

LT3506/LT3506A
APPLICATIONS INFORMATION FB Resistor Network
maximum input voltage is ~8V with VOUT = 0.8V. Note that this is a restriction on the operating input voltage; The output voltage is programmed with a resistor divider the circuit will tolerate transient inputs up to the absolute between the output and the FB pin. Choose the 1% resis- maximum rating. tors according to: R1 = R2(VOUT/0.8 – 1)
Inductor Selection and Maximum Output Current
The parallel combination of R1 and R2 should be 10k or A good fi rst choice for the inductor value is: less to avoid bias current errors. Reference designators L = 2 • (VOUT + VD) for the LT3506 refer to the Block Diagram in Figure 2. L = (VOUT + VD) for the LT3506A
Input Voltage Range
where VD is the voltage drop of the catch diode (~0.4V) The minimum input voltage is determined by either the and L is in μH. With this value the maximum load current LT3506’s minimum operating voltage of ~3.6V, or by its will be ~1.6A, independent of input voltage. The inductor’s maximum duty cycle. The duty cycle is the fraction of RMS current rating must be greater than your maximum time that the internal switch is on and is determined by load current and its saturation current should be about 30% the input and output voltages: higher. To keep effi ciency high, the series resistance (DCR) should be less than 0.1Ω. Table 1 lists several vendors and DC = (VOUT + VD)/(VIN – VSW + VD) types that are suitable. Of course, such a simple design where VD is the forward voltage drop of the catch diode guide will not always result in the optimum inductor for (~0.4V) and VSW is the voltage drop of the internal switch your application. A larger value provides a slightly higher (~0.3V at maximum load). This leads to a minimum input maximum load current, and will reduce the output volt- voltage of: age ripple. If your load is lower than 1.6A, then you can V decrease the value of the inductor and operate with higher IN(MIN) = (VOUT + VD)/DCMAX – VD + VSW ripple current. This allows you to use a physically smaller with DCMAX = 0.89 (0.78 for the LT3506A). inductor, or one with a lower DCR resulting in higher ef- A more detailed analysis includes inductor loss and the fi ciency. Be aware that if the inductance differs from the dependence of the diode and switch drop on operating simple rule above, then the maximum load current will current. A common application where the maximum duty depend on input voltage. There are several graphs in the cycle limits the input voltage range is the conversion of 5V Typical Performance Characteristics section of this data to 3.3V. The maximum load current that the LT3506 can sheet that show the maximum load current as a function deliver at 3.3V depends on the accuracy of the 5V input of input voltage and inductor value for several popular supply. With a low loss inductor (DCR less than 80mΩ), output voltages. Also, low inductance may result in dis- the LT3506 can deliver 1.2A for V continuous mode operation, which may be acceptable, IN > 4.7V and 1.6A for V but further reduces maximum load current. For details of IN > 4.85V. The maximum input voltage is determined by the absolute maximum ratings of the V maximum output current and discontinuous mode opera- IN and BOOST pins and by the minimum duty cycle DC tion, see Linear Technology Application Note 44. Finally, MIN = 0.08 (0.15 for the LT3506A): for duty cycles greater than 50%(VOUT/VIN < 0.5), there is a minimum inductance required to avoid subharmonic VIN(MAX) = (VOUT + VD)/DCMIN – VD + VSW. oscillations. See Application Note 19 for detailed informa- This limits the maximum input voltage to ~21V with VOUT tion on subharmonic oscillations. The following discussion = 1.2V and ~15V with VOUT = 0.8V. For the LT3506A the assumes continuous inductor current. 3506afc 8