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LT1317 Datasheet(PDF) 8 Page - Linear Technology |
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LT1317 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1317/LT1317B The LT1317B differs from the LT1317 in that the bias point on A1 is set lower than on the LT1317 so that minimum switch current can drop below 50mA. Because A1’s bias point is set lower, there is no Burst Mode operation at light loads and the device continues switching at constant frequency. This results in the absence of low frequency output voltage ripple at the expense of light load efficiency. The difference between the two devices is clearly illus- trated in Figure 2. The top two traces in Figure 2 show an LT1317/LT1317B circuit, using the components indicated in Figure 1, set to a 3.3V output. Input voltage is 2V. Load current is stepped from 2mA to 200mA for both circuits. Low frequency Burst Mode operation voltage ripple is observed on Trace A, while none is observed on Trace B. APPLICATIONS INFORMATION COMPONENT SELECTION Inductors Inductors appropriate for use with the LT1317 must possess three attributes. First, they must have low core loss at 600kHz. Most ferrite core units have acceptable losses at this switching frequency. Inexpensive iron pow- der cores should be viewed suspiciously, as core losses can cause significant efficiency penalties at 600kHz. Sec- ond, the inductor must be able to handle peak switch current of the LT1317 without saturating. This places a lower limit on the physical size of the unit. Molded chokes or chip inductors usually do not have enough core to support the LT1317 maximum peak switch current and are unsuitable for the application. Lastly, the inductor should have low DCR (copper wire resistance) to prevent effi- ciency-killing I2R losses. Linear Technology has identified several inductors suitable for use with the LT1317. This is not an exclusive list. There are many magnetics vendors whose components are suitable for use. A few vendor’s components are listed in Table 1. L CIN COUT D VOUT VIN GND MULTIPLE VIAs GROUND PLANE 1317 F03 1 2 3 4 8 7 6 5 LT1317 Figure 3. Recommended Component Placement. Traces Carrying High Current Are Direct. Trace Area at FB Pin and VC Pin is Kept Low. Lead Length to Battery Should be Kept Short. 1ms/DIV 1317 F02 200mA LT1317 VOUT 100mV/DIV AC COUPLED LT1317B VOUT 100mV/DIV AC COUPLED 2mA TRACE A TRACE B ILOAD Figure 2. LT1317 Exhibits Ripple at 2mA Load During Burst Mode Operation, the LT1317B Does Not LAYOUT HINTS The LT1317 switches current at high speed, mandating careful attention to layout for proper performance. You will not get advertised performance with careless layouts. Figure 3 shows recommended component placement. Follow this closely in your PC layout. Note the direct path of the switching loops. Input capacitor CIN must be placed close (< 5mm) to the IC package. As little as 10mm of wire or PC trace from CIN to VIN will cause problems such as inability to regulate or oscillation. |
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