link to page 7 link to page 7 link to page 7 LTC6908-1/LTC6908-2 OPERATION As shown in the Block Diagram, the LTC6908’s master The master oscillator signal is divided by 2 before driving oscillator is controlled by the ratio of the voltage between the output pins, resulting in the simple formula for the the V+ and SET pins and the current entering the SET pin output frequency, fOUT, below (see Figure 2): (IMASTER). When the spread spectrum frequency modula- f tion (SSFM) is disabled, I OUT = 10MHz • 10k/RSET MASTER is strictly determined by the (V+ – VSET) voltage and the RSET resistor. When SSFM When the spread spectrum frequency modulation (SSFM) is enabled, IMASTER is modulated by a filtered pseudoran- is disabled, the frequency fOUT is the final output fre- dom noise (PRN) signal. Here the IMASTER current is a quency. When SSFM is enabled, 0.9 • fOUT is the minimum random value uniformly distributed between (ISET – 10%) output frequency and 1.1 • fOUT is the maximum output and (ISET + 10%). In this way the frequency of the master frequency. oscillator is modulated to produce an approximately flat Both outputs are nominally 50% duty cycle. There are 2 frequency spectrum that is centered at the frequency set possible output configurations for the LTC6908, shown by the ISET current, with a bandwidth equal to approxi- in Figure 3. mately 20% of the center frequency. The voltage on the SET pin is forced to approximately 1.1V Output Configurations below V+ by the PMOS transistor and its gate bias volt- The only difference between the two versions of the age. This voltage is accurate to ±5% at a particular input LTC6908 is the phase relationship between the two outputs. current and supply voltage (see Figure 1). The LTC6908 The LTC6908-1 outputs are 180 degrees out of phase and is optimized for use with resistors between 20k and 400k, the LTC6908-2 outputs are 90 degrees out of phase. These corresponding to output frequencies between 250kHz and convenient output options are useful in synchronizing the 5MHz. Accurate frequencies up to 10MHz (RSET = 10k) clocking of multiple phase switching regulator designs. In are attainable if the supply voltage is greater than 4V. The R 10M SET 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 volt- 1M ) age. The master oscillation frequency is: (Ω f R SET MASTER = 20MHz • 10k/RSET 100k 10k 1.4 T 10k 100k 1M 10M A = 25C DESIRED OUTPUT FREQUENCY (Hz) 1.3 690812 F02 Figure 2. RSET vs Desired Output Frequency 1.2 SET V+ = 5V LTC6908-1 (COMPLEMENTARY) + – V 1.1 OUT1 = V V+ = 3V V RES 1.0 OUT2 0.9 LTC6908-2 (QUADRATURE) OUT1 0.80.1 1 10 100 1000 I OUT2 RES (μA) 690812 F01 690812 F03 Figure 1. V+ – VSET Variation with IRESFigure 3. Output Waveforms for LTC6908-1, LTC6908-2 Rev B For more information www.analog.com 7 Document Outline Features Applications Description Absolute Maximum Ratings Electrical Characteristics Pin Functions Operation Applications Information Typical Applications Package Description Typical Application
