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LTC6990IS6-TRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC6990IS6-TRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 28 page  LTC6990 10 6990p OPERATION The LTC6990 is built around a master oscillator with a 1MHz maximum frequency. The oscillator is controlled by the SET pin current (ISET) and voltage (VSET), with a 1MHz • 50k conversion factor that is accurate to ±0.8% under typical conditions. fMASTER = 1 tMASTER = 1MHz • 50k • ISET VSET A feedback loop maintains VSET at 1V ±30mV, leaving ISET as the primary means of controlling the output frequency. The simplest way to generate ISET is to connect a resistor (RSET) between SET and GND, such that ISET = VSET/RSET. The master oscillator equation reduces to: fMASTER = 1 tMASTER = 1MHz • 50k RSET From this equation it is clear that VSET drift will not affect the output frequency when using a single program resistor (RSET). Error sources are limited to RSET tolerance and the inherent frequency accuracy ΔfOUT of the LTC6990. RSET values between 50k and 800k (equivalent to ISET between 1.25μA and 20μA) produce the best results, although RSET may be reduced to 25k (ISET = 40μA) with reduced accuracy. The LTC6990 includes a programmable frequency divider which can further divide the frequency by 1, 2, 4, 8, 16, 32, 64 or 128 before driving the OUT pin. The divider ratio NDIV is set by a resistor divider attached to the DIV pin. fOUT = 1 tOUT = 1MHz • 50k NDIV • ISET VSET With RSET in place of VSET/ISET the equation reduces to: fOUT = 1 tOUT = 1MHz • 50k NDIV •RSET DIVCODE The DIV pin connects to an internal, V+ referenced 4-bit A/D converter that monitors the DIV pin voltage (VDIV) to determine the DIVCODE value. DIVCODE programs two settings on the LTC6990: 1. DIVCODE determines the output frequency divider setting, NDIV. 2. DIVCODE determines the state of the output when disabled, via the Hi-Z bit. VDIV may be generated by a resistor divider between V+ and GND as shown in Figure 1. Figure 1. Simple Technique for Setting DIVCODE 6990 F01 LTC6990 V+ DIV GND R1 R2 2.25V TO 5.5V |
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