Datasheet LTC1799 (Analog Devices) - 8

LTC1799
applicaTions inForMaTion POwER SUPPLY REJECTION START-UP TIME Low Frequency Supply Rejection (Voltage Coefficient)
The start-up time and settling time to within 1% of the final value can be estimated by tSTART ≅ RSET(2.8µs/kΩ) Figure 5 shows the output frequency sensitivity to power + 20µs. Note the start-up time depends on RSET and it supply voltage at several different temperatures. The is independent from the setting of the divider pin. For LTC1799 has a conservative guaranteed voltage coeffi- instance with RSET = 50k, the LTC1799 will settle with 1% cient of 0.1%/V but, as Figure 5 shows, the typical supply of its 200kHz final value (N = 10) in approximately 160µs. sensitivity is lower. Figure 6 shows start-up times for various RSET resistors. 0.15 Figure 7 shows an application where a second set resistor RSET = 31.6k PIN 4 = FLOATING (÷10) RSET2 is connected in parallel with set resistor RSET1 via switch S1. When switch S1 is open, the output frequency 0.10 of the LTC1799 depends on the value of the resistor RSET1. 25°C When switch S1 is closed, the output frequency of the –40°C 0.05 LTC1799 depends on the value of the parallel combination 85°C of RSET1 and RSET2. 0 FREQUENCY DEVIATION (%) The start-up time and settling time of the LTC1799 with switch S1 open (or closed) is described by tSTART shown –0.05 above. Once the LTC1799 starts and settles, and switch 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V) S1 closes (or opens), the LTC1799 will settle to its new 1799 F05 output frequency within approximately 25µs.
Figure 5. Supply Sensitivity
60 TA = 25°C
High Frequency Power Supply Rejection
V+ = 5V 50 The accuracy of the LTC1799 may be affected when its 40 power supply generates significant noise with frequency 30 contents in the vicinity of the programmed value of fOSC. If a switching power supply is used to power up the LTC1799, 20 200k and if the ripple of the power supply is more than a few 10 FREQUENCY ERROR (%) 10k tens of millivolts, make sure the switching frequency and 31.6k 0 its harmonics are not related to the output frequency of the LTC1799. Otherwise, the oscillator may show an ad- –10 0 100 200 300 400 500 600 ditional 0.1% to 0.2% of frequency error. TIME AFTER POWER APPLIED (µs) 1799 F06 If the LTC1799 is powered by a switching regulator and
Figure 6. Start-Up Time
the switching frequency or its harmonics coincide with the output frequency of the LTC1799, the jitter of the 3V OR 5V oscillator output may be affected. This phenomenon will 1 5 ( 10k N • R ) V+ OUT fOSC = 10MHz • SET1 become noticeable if the switching regulator exhibits S1 LTC1799 OR 2 R V+ 10k ripples beyond 30mV. SET1 GND fOSC = 10MHz • (N • RSET1//RSET2) R ÷100 SET2 3 4 SET DIV ÷10 ÷1 1799 F07
Figure 7
1799fd 8 For more information www.linear.com/LTC1799 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Theory of Operation Applications Information Typical Application Package Description Revision History Typical Applications