Datasheet LTC6908-1, LTC6908-2 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | Resistor Set SOT-23 Oscillator with Spread Spectrum Modulation |
| Pages / Page | 16 / 7 — OPERATION. Output Configurations. Figure 2. RSET vs Desired Output … |
| Revision | B |
| File Format / Size | PDF / 582 Kb |
| Document Language | English |
OPERATION. Output Configurations. Figure 2. RSET vs Desired Output Frequency. Figure 1. V+ – VSET Variation with IRES
Model Line for this Datasheet
- LTC6908CDCB-1#TRMPBF LTC6908CDCB-1#TRPBF LTC6908CDCB-2#TRMPBF LTC6908CDCB-2#TRPBF LTC6908CS6-1#TRMPBF LTC6908CS6-1#TRPBF LTC6908CS6-2#TRMPBF LTC6908CS6-2#TRPBF LTC6908HS6-1#TRMPBF LTC6908HS6-1#TRPBF LTC6908HS6-2#TRMPBF LTC6908HS6-2#TRPBF LTC6908IDCB-1#TRMPBF LTC6908IDCB-1#TRPBF LTC6908IDCB-2#TRMPBF LTC6908IDCB-2#TRPBF LTC6908IS6-1#TRMPBF LTC6908IS6-1#TRPBF LTC6908IS6-2#TRMPBF LTC6908IS6-2#TRPBF
Text Version of Document
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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 IRES Figure 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
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 IRES Figure 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
