Datasheet LT1616 (Analog Devices) - 7

LT1616
U U W U APPLICATIO S I FOR ATIO Inductor Selection and Maximum Output Current
If your application calls for output current less than 400mA, you may be able to relax the value of the inductor The duty cycle of the internal switch is: and operate with higher ripple current. This may allow you DC = (VOUT + VD)/(VIN – VSW + VD) to pick a physically smaller inductor or one with a lower DC where V resistance. Be aware that these equations assume con- D is the forward voltage drop of the catch diode (D1) and V tinuous inductor current. If the inductor value is low or the SW is the voltage drop of the internal switch. Usually one is interested in DC at full load current, so you load current is light, then the inductor current may become can use V discontinuous. This occurs when ∆I D = VSW = 0.4V. Note that the LT1616 has a L = 2IOUT. For details maximum guaranteed duty cycle of 0.8. This will limit the of discontinuous mode operation, see Linear Technology minimum input voltage for a particular output voltage. Application Note AN44. Also, high duty cycle operation may require slightly higher inductor values to avoid sub- When the switch is off, the inductor sees the output harmonic oscillations. See AN19. voltage plus the catch diode drop. This gives the peak-to- peak ripple current in the inductor: The maximum load current as a function of input voltage is plotted in the Typical Performance Characteristics sec- ∆IL = (1 – DC)(VOUT + VD)/(L • f) tion of this data sheet. Maximum load current for 3.3V and where f is the switching frequency of the LT1616 and L is 5V outputs is shown for several values of L. At the highest the value of the inductor. The average inductor current is input voltages, the load current is limited by power dissi- equal to the output current, so the peak inductor current pation in the LT1616. will be the output current plus one half of the ripple Choose an inductor that is intended for power applica- current: tions. Table 1 lists several manufacturers and inductor I series. The saturation current of the inductor should be LPK = IOUT + ∆IL/2. above 0.5A. The RMS current rating should be equal to or To maintain output regulation, this peak current must be greater than output current. For indefinite operation into a less than the LT1616’s switch current limit ILIM. ILIM is at short circuit, the RMS current rating should be greater least 630mA at low duty cycles, decreasing to 430mA at than 0.7A. The DC resistance should be less than 0.5Ω in 80% duty cycle. The maximum output current is a function order maintain circuit efficiency. of the chosen inductor value: I
Capacitor Selection
OUT(MAX) = ILIM – ∆IL/2. If the inductor value is chosen so that the ripple current is A Buck regulator draws from its input a square wave of small, then the available output current will be near the current with peak-to-peak amplitude as high as the switch switch current limit. A good approach is to choose the current limit. The input capacitor (C1) must supply the AC inductor so that the peak-to-peak inductor ripple is equal component of this current. An RMS current rating of to one third of the switch current limit. This leads to: 250mA is adequate for LT1616 circuits. The input capaci- tor must bypass the LT1616 internal control circuitry and L = 3(1 – DC)(VOUT + VD)/(ILIM • f) any other circuitry that operates from the input source. A and 1µF ceramic capacitor will satisfy both of these require- ments. If the impedance of the input source is high (due to IOUT(MAX) = (5/6)ILIM. long wires or filter components), additional bulk input These expressions depend on duty cycle and therefore on capacitance may be required. In high duty cycle applica- input voltage. Pick a nominal input voltage to calculate L, tions (5VIN to 3.3VOUT, for example), increase the input then check the maximum available output current at the capacitor to 2.2µF. It may be possible to achieve lower cost minimum and maximum input voltages. by using an electrolytic capacitor (tantalum or aluminum) 7