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LTC1429CS Datasheet(PDF) 10 Page - Linear Technology |
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LTC1429CS Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LTC1429 going into very high frequency oscillation under such condi- tions. It also creates an output error as the feedback loop regulates based on the top of the spike, not the average value of the output (Figure 7). The resulting output voltage behaves asifaresistorofvalueCESR×(IPK/IAVE)Ωwasplacedinseries with the output. To minimize this effect, output capacitor ESR should be as low as possible or smaller value high frequency bypass (typically a 0.1 µFceramic)shouldbeaddedinparallel with the output capacitor. APPLICATIONS INFORMATION Figure 7. Output Ripple with Low and High ESR Caps LOW ESR OUTPUT CAP CLOCK VOUT AVERAGE VSET COMP1 OUTPUT VOUT HIGH ESR OUTPUT CAP VOUT AVERAGE VSET COMP1 OUTPUT VOUT LTC1429 • F07 Note that ESR in the flying caps will not cause the same condition; in fact, it may actually improve the situation by cutting the peak currents and lowering the amplitude of the spike. More flying cap ESR is not necessarily better, however; as soon as the RC time constant approaches half of a clock period (the time the capacitors have to share charge at full duty cycle) the output current capability of the LTC1429 will begin to diminish. For 0.1 µF flying capacitors and typical 700kHz external clock, this gives a maximum total series resistance of: = / 0.1 µF = 7.14Ω 1 2 ) ) tCLK CFLY 1 2 ) ) 1 700kHz Most of this resistance is already provided by the internal switches in the LTC1429 (especially in tripler mode). More than 1 Ω or 2Ω of ESR on the flying caps will start to affect the regulation at maximum load. RESISTOR SELECTION Resistor selection is easy with the fixed output versions of the LTC1429; no resistors are needed! Selecting the right resistors for the adjustable parts is only a little more difficult. A resistor divider should be used to divide the signal at the output to give 1.24V at the ADJ pin with respect to VOUT (Figure 8). The LTC1429 uses a positive reference with respect to VOUT, not a negative reference with respect to ground (Figure 4 shows reference connec- tion). Be sure to keep this in mind when connecting the resistors! If the initial output is not what you expected, try swapping the two resistors. The 14-pin adjustable parts include a built-in resistor string which can provide an assortment of output voltages by using different pin-strapping options at the RO, R1 and RADJ pins (Table 3). The internal resistors are roughly 124k, 226k, 100k and 50k (see Figure 4) giving output options of – 3.5V, – 4V, – 4.5V and – 5V. The resistors are carefully matched to provide accurate divider ratios, but the absolute values can vary substantially from part to part. It’s not a good idea to create a divider using an external resistor and one of the internal resistors unless the output voltage accuracy is not critical. Table 3. Output Voltages Using the Internal Resistor Divider PIN CONNECTIONS OUTPUT VOLTAGE ADJ - RADJ – 5.0V ADJ - RADJ, RO - GND – 4.5V ADJ - RADJ, R1 - RO – 4.0V ADJ - RADJ, R1-GND – 3.5V ADJ - R1 – 1.77V ADJ - R0 – 1.38V ADJ - GND – 1.24V There are some oddball output voltages available as well. They are obtained by connecting ADJ to R0 or R1 and shorting out some of the internal resistors. If one of them gives you the output voltage you want, by all means use it! LTC1429 GND R1 6 (4)* 10 (5)* 11 (6)* LTC1429CS *(LTC1429CS8) LTC1429 • F07 VOUT = –1.24V R2 (R1 + R2) R2 ADJ OUT Figure 8. External Resistor Connections |
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