Datasheet LTC6907 (Analog Devices) - 9

LTC6907
U U W U APPLICATIO S I FOR ATIO
1000
Power Supply Rejection
CLOAD = 5pF T = 25°C The LTC6907 has a very low supply voltage coefficient, µA) 3.3V, –1 : meaning that the output frequency is nearly insensitive to 3.3V, –3 : the DC power supply voltage. In most cases, this error 3.3V, –10 : 100 term can be neglected. High frequency noise on the power supply (V+) pin has the SUPPLY CURRENT ( potential to interfere with the LTC6907’s master oscillator. Periodic noise, such as that generated by a switching power supply, can shift the output frequency or increase 1010 100 1000 10000 jitter. The risk increases when the fundamental frequency OUTPUT FREQUENCY (kHz) or harmonics of the noise fall near the master oscillator 6907 F09
Figure 9. Supply Current vs Frequency over DIV Settings
frequency. It is relatively easy to filter the LTC6907 power supply because of the very low supply current. For ex- layout that uses the GRD pin and a “guard ring” to absorb ample, an RC filter with R = 160Ω and C = 10µF provides leakage currents. The guard ring surrounds the SET pin a 100Hz lowpass filter while dropping the supply voltage and the end of RSET to which it is connected. The guard ring only about 10mV. must have no solder mask covering it to be effective. The GRD pin voltage is held within a few millivolts of the SET
Operating the LTC6907 with Supplies Higher
pin voltage, so any leakage path between the SET pin and
Than 3.6V
the guard ring generates no leakage current. The LTC6907 may also be used with supply voltages between 3.6V and 5.5V under very specific conditions. To
Start-Up Time
ensure proper functioning above 3.6V, a filter circuit must When the LTC6907 is powered up, it holds the OUT pin be attached to the power supply and located within 1cm of low. After the master oscillator has settled, the OUT pin is the device. A simple RC filter consisting of a 100Ω resistor enabled and the first output cycle is accurate. The time and 1µF capacitor (Figure 11) will ensure that supply from power-up to the first output transition is given resonance at higher supply voltages does not induce approximately by: unpredictable oscillator behavior. Accuracy under higher t supplies may be estimated from the typical Frequency vs START ≅ 64 • tOSC + 100µs Supply Voltage curves in the Typical Performance Charac- The digital divider ratio, N, does not affect the start- teristics section of this data sheet. up time. V+ LTC6907 3.6V TO 5.5V DC 1 OUT V+ 6 NO SOLDER MASK OVER THE GUARD RING 100Ω LTC6907 GRD V+ OUT 2 GND 5 1µF GND GRD GUARD RING DIV SET 3 DIV SET 4 RSET RSET NO LEAKAGE 6907 F11 CURRENT
Figure 11. Using the LTC6907 at Higher Supply Voltages
LEAKAGE CURRENT 6907 F10
Figure 10. PC Board Layout with Guard Ring
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