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LTC6908IDCB-2 Datasheet(PDF) 7 Page - Linear Technology |
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LTC6908IDCB-2 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  LTC6908-1/LTC6908-2 7 690812fa IRES (µA) 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.1 10 100 1000 690812 F01 1 V+ = 5V V+ = 3V TA = 25°C OPERATIO As shown in the Block Diagram, the LTC6908’s master oscillator is controlled by the ratio of the voltage between the V+ and SET pins and the current entering the SET pin (IMASTER). When the spread spectrum frequency modula- tion (SSFM) is disabled, IMASTER is strictly determined by the (V+ – VSET) voltage and the RSET resistor. When SSFM is enabled, IMASTER is modulated by a filtered pseudorandom noise (PRN) signal. Here the IMASTER current is a random value uniformly distributed between (ISET – 10%) and (ISET + 10%). In this way the frequency of the master oscillator is modulated to produce an approximately flat frequency spectrum that is centered at the frequency set by the ISET current, with a bandwidth equal to approximately 20% of the center frequency. The voltage on the SET pin is forced to approximately 1.1V below V+ by the PMOS transistor and its gate bias volt- age. This voltage is accurate to ±5% at a particular input current and supply voltage (see Figure 1). The LTC6908 is optimized for use with resistors between 20k and 400k, corresponding to output frequencies between 250kHz and 5MHz. Accurate frequencies up to 10MHz (RSET = 10k) are attainable if the supply voltage is greater than 4V. The RSET resistor, connected between the V + and SET pins, locks together the (V+ – VSET) voltage and the current ISET. This allows the parts to attain excellent frequency accuracy regardless of the precision of the SET pin voltage. The master oscillation frequency is: fMASTER = 20MHz • 10k/RSET The master oscillator signal is divided by 2 before driving the output pins, resulting in the simple formula for the Figure 3. Output Waveforms for LTC6908-1, LTC6908-2 Figure 2. RSET vs Desired Output Frequency Figure 1. V+ – VSET Variation with IRES OUT1 OUT2 OUT1 LTC6908-1 (COMPLEMENTARY) LTC6908-2 (QUADRATURE) OUT2 690812 F03 output frequency, fOUT, below (see Figure 2): fOUT = 10MHz • 10k/RSET When the spread spectrum frequency modulation (SSFM) is disabled, the frequency fOUT is the final output fre- quency. When SSFM is enabled, 0.9 • fOUT is the minimum output frequency and 1.1 • fOUT is the maximum output frequency. Both outputs are nominally 50% duty cycle. There are 2 possible output configurations for the LTC6908, shown in Figure 3. Output Configurations The only difference between the two versions of the LTC6908 is the phase relationship between the two outputs. The LTC6908-1 outputs are 180 degrees out of phase and the LTC6908-2 outputs are 90 degrees out of phase. These convenient output options are useful in synchronizing the clocking of multiple phase switching regulator designs. In very high current applications, a significant improvement DESIRED OUTPUT FREQUENCY (Hz) 10k 10k 100k 1M 10M 100k 1M 10M 690812 F02 |
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Similar Description - LTC6908IDCB-2 |
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