Datasheet LTC6906 (Analog Devices) - 10

LTC6906
APPLICATIONS INFORMATION
LS R LTC6906 S 330nH 1MHz LTC6906 100Ω 1MHz 3.3V V+ OUT 3.3V V+ OUT GND GRD CLOAD GND GRD CLOAD DIV SET DIV SET RSET RSET 100k 100k 6906 F12 6906 F14
Figure 12. Test Circuit with an Inductive Power Supply Figure 14. Test Circuit with a Resistive Power Supply
3.5 3.5 3 3 2 2 (V) (V) OUT OUT V V 1 1 CLOAD = 5pF CLOAD = 5pF CLOAD = 10pF CLOAD = 10pF CLOAD = 20pF CLOAD = 20pF CLOAD = 50pF CLOAD = 50pF 0 0 4.65 4.75 4.85 4.95 5.05 5.15 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 TIME (s) TIME (s) 6906 F13 6906 F15
Figure 13. Output Waveforms with Figure 15. Output Waveforms with an Inductive Supply (See Figure 12) a Resistive Supply (See Figure 14) Start-Up Time
jitter. The risk increases when the fundamental frequency or harmonics of the noise fall near the master oscillator When the LTC6906 is powered up, it holds the OUT pin frequency. It is relatively easy to filter the LTC6906 power low. After the master oscillator has settled, the OUT pin supply because of the very low supply current. For ex- is enabled and the first output cycle is guaranteed to be ample, an RC filter with R = 160Ω and C = 10μF provides within specification. The time from power-up to the first a 100Hz lowpass filter while dropping the supply voltage output transition is given approximately by: only about 10mV. tSTART ≅ 64 • tOSC + 100μs
Operating the LTC6906 with Supplies Higher Than 3.6V
The digital divider ratio, N, does not affect the start-up time. The LTC6906 may also be used with supply voltages
Power Supply Rejection
between 3.6V and 5.5V under very specific conditions. The LTC6906 has a very low supply voltage coefficient, To ensure proper functioning above 3.6V, a filter circuit meaning that the output frequency is nearly insensitive must be attached to the power supply and located within to the DC power supply voltage. In most cases, this error 1cm of the device. A simple RC filter consisting of a 100Ω term can be neglected. resistor and 1μF capacitor (Figure 16) will ensure that sup- ply resonance at higher supply voltages does not induce High frequency noise on the power supply (V+) pin has the unpredictable oscillator behavior. Accuracy under higher potential to interfere with the LTC6906’s master oscilla- supplies may be estimated from the typical Frequency vs tor. Periodic noise, such as that generated by a switching Supply Voltage curves in the Typical Performance Char- power supply, can shift the output frequency or increase acteristics section of this data sheet. 6906fc 10