ltc6905 1 6905fb , ltc and lt are registered trademarks of linear technology corporation. thinsot is a trademark of linear technology corporation. all other trademarks are the property of their respective owners. protected by u.s. patents, including 6614313, 6342817. one external resistor sets the frequency fast start-up time: 100 s typical frequency range: 17mhz to 170mhz frequency error 0.5% typ 17mhz to 170mhz (t a = 0 c to 70 c, over all settings) 20ppm/ c temperature stability rise time: 0.5ns, c l = 5pf timing jitter: 7.2ps rms at 170mhz 50% 2.5% duty cycle 6ma typical supply current, f osc = 100mhz cmos output drives 500 ? load (v s = 3v) operates from a single 2.7v to 5.5v supply low profile (1mm) thinsot tm package 17mhz to 170mhz resistor set sot-23 oscillator high frequency precision oscillator high speed data bus clock fixed crystal oscillator replacement ceramic oscillator replacement the ltc ? 6905 precision, programmable silicon oscillator is easy to use and occupies very little board space. it requires only a single resistor to set the output frequency from 17mhz to 170mhz with a typical frequency error of 0.5% or less. the ltc6905 operates with a single 2.7v to 5.5v power supply and provides a rail-to-rail, 50% duty cycle square wave output. the cmos output driver ensures fast rise/fall times and rail-to-rail switching. operation is simple: a single resistor, r set , between 10k to 25k is used to set the frequency, and an internal three-state divider (div input) allows for division of the master clock by 1, 2 or 4, providing three frequencies for each r set value. the ltc6905 features a proprietary feedback loop that linearizes the relationship between r set and frequency, eliminating the need for tables to calculate frequency. the oscillator can be easily programmed using the simple formula outlined below: f mhz k r mhz n n open osc set = ? + ? ? ? ? ? ? = ? ? ? ? ? = = = + 168 5 10 15 1 1 2 4 .? .?, , , , div pin v div pin div pin gnd for higher accuracy, fixed frequency versions that include an internal frequency-setting resistor, see the ltc6905- xxx series datasheet. basic connection typical distribution of frequency error, t a = 25 c descriptio u features applicatio s u typical applicatio u v + 1 2 3 5 17.225mhz f osc 170mhz 5v 5v 10k r set 25k 0.1 f 6905 ta01 4 gnd ltc6905 set out div open 2 1 4 % error C0.5 0 units 10 20 30 40 50 60 note: resistor, r set , tolerance will add to the frequency error C0.3 C0.1 0.1 0.3 6905 ta02 0.5 v + = 3v r set = 12k div = 1
ltc6905 2 6905fb supply voltage (v + ) to gnd ........................C 0.3v to 6v div to gnd .................................... C 0.3v to (v + + 0.3v) set to gnd ................................... C 0.3v to (v + + 0.3v) output short-circuit duration (note 6) ........... indefinite operating temperature range (note 7) ltc6905c, i ....................................... C 40 c to 85 c ltc6905h ........................................ C 40 c to 125 c specified temperature range (note 8) ltc6905c ............................................... 0 c to 70 c ltc6905i .............................................C40 c to 85 c ltc6905h .........................................C40 c to 125 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c (note 1) absolute axi u rati gs w ww u package/order i for atio uu w the denotes the specifications which apply over the full specified temperature range, otherwise specifications are at t a = 25 c or as noted. v + = 2.7v to 5.5v, r l = 15k, c l = 5pf, pin 4 = v + unless otherwise noted. all voltages are with respect to gnd. symbol parameter conditions min typ max units ? f frequency accuracy (notes 2, 9) v + = 2.7v, 17.225mhz < f < 170mhz 0.5 1.4 % v + = 5v, 17.225mhz < f < 170mhz 2.2 % ltc6905cs5 v + = 2.7v, 17.225mhz < f < 170mhz 1.7 % v + = 5v, 17.225mhz < f < 170mhz 2.5 % ltc6905hs5 (25 c t 125 c), ltc6905is5 (25 c t 85 c) v + = 2.7v, 17.225mhz < f < 170mhz 1.9 % v + = 5v, 17.225mhz < f < 170mhz 2.9 % ltc6905hs5 (C40 c t 125 c), ltc6905is5 (C40 c t 85 c) v + = 2.7v, 17.225mhz < f < 170mhz 3.5 % v + = 5v, 17.225mhz < f < 170mhz 3.5 % r set frequency-setting resistor range 10 25 k ? f max maximum frequency pin 4 = v + , n = 1 170 mhz f min minimum frequency pin 4 = 0v, n = 4 17.225 mhz ? f/ ? t freq drift over temp (note 2) r set = 10k 20 ppm/ c ? f/ ? v freq drift over supply (notes 2, 9) v + = 2.7v to 5.5v, r set = 10k 0.5 %/v peak-to-peak timing jitter (note 3) 0.8 % long-term stability of output frequency 300 ppm/ khr duty cycle 47.5 50 52.5 % v + operating supply range 2.7 5.5 v i s power supply current r set = 10k, n = 1, r l = ,v + = 5.5v 14 20 ma f osc = 170mhz, c l = 5pf v + = 2.7v 712 ma r set = 20k, n = 4, r l = ,v + = 5.5v 57 ma f osc = 21.44mhz, c l = 5pf v + = 2.7v 35 ma electrical characteristics t jmax = 125 c, ja = 150 c/w order part number consult factory for parts specified with wider operating temperature ranges. order options tape and reel: add #tr lead free: add #pbf lead free tape and reel: add #trpbf lead free part marking: http://www.linear.com/leadfree/ top view s5 package 5-lead plastic sot-23 1 2 3 v + gnd set 5 4 out div s5 part marking ltc6905cs5 ltc6905is5 ltc6905hs5 ltbjc
ltc6905 3 6905fb v ih high level div input voltage v + C 0.15 v v il low level div input voltage 0.2 v i div div input current (note 4) pin 4 = v + v + = 5.5v 15 40 a pin 4 = 0v v + = 5.5v C40 C11 a v oh high level output voltage (note 4) v + = 5.5v, pin 4 = 0v i oh = C 1ma 5.25 5.45 v i oh = C 4ma 5.20 5.30 v v + = 2.7v, pin 4 = 0v i oh = C 1ma 2.5 2.6 v i oh = C 4ma 2.4 2.4 v v ol low level output voltage (note 4) v + = 5.5v, pin 4 = 0v i ol = 1ma 0.05 0.25 v i ol = 4ma 0.2 0.3 v v + = 2.7v, pin 4 = 0v i ol = 1ma 0.1 0.3 v i ol = 4ma 0.4 0.5 v t r , t f out rise/fall time (note 5) 0.5 ns v set voltage at r set pin v + = 5.5v 4.27 4.5 4.73 v v + = 2.7v 1.61 1.7 1.79 v symbol parameter conditions min typ max units the denotes the specifications which apply over the full specified temperature range, otherwise specifications are at t a = 25 c or as noted. v + = 2.7v to 5.5v, r l = 15k, c l = 5pf, pin 4 = v + unless otherwise noted. all voltages are with respect to gnd. note 1: absolute maximum ratings are those values beyond which the life of the device may be impaired. note 2: frequency accuracy is defined as the deviation from the f osc equation. accuracy is tested with div = v + , n = 1 and other divide ratios are guaranteed by design. note 3: jitter is the ratio of the peak-to-peak distribution of the period to the mean of the period. this specification is based on characterization and is not 100% tested. note 4: to conform with the logic ic standard convention, current out of a pin is arbitrarily given as a negative value. note 5: output rise and fall times are measured between the 10% and 90% power supply levels. electrical characteristics typical perfor a ce characteristics uw supply current vs frequency frequency (mhz) 0 supply current (ma) 10 12 14 200 6905 g01 8 4 4 1 1 6 0 2 50 100 150 4 18 16 5.5v 2.7v 2 2 r set (k ? ) 10 frequency error (%) 0 0.20 0.40 22 6905 g02 C0.20 C0.40 12 14 16 18 20 24 C0.60 C0.80 0.60 supply voltage (v) 2.5 C0.40 frequency error (%) C0.20 0.20 0.40 0.60 4.5 1.40 6905 g03 0 3.5 3 5 4 5.5 0.80 1.00 1.20 frequency error vs r set frequency error vs supply voltage note 6: a heat sink may be required to keep the junction temperature below the absolute maximum when the output is shorted indefinitely. note 7: the ltc6905c is guaranteed functional over the operating temperature range. note 8: the ltc6905 is guaranteed to meet specified performance from 0 c to 70 c. the ltc6905c-xxx is designed, characterized and expected to meet specified performance from C40 c to 85 c but is not tested or qa sampled at these temperatures. the ltc6905i-xxx is guaranteed to meet specified performance from C40 c to 85 c. note 9: the ltc6905 is optimized for the performance with a 3v power supply voltage. please consult ltc marketing for parts optimized for 5v operation.
ltc6905 4 6905fb ltc6905 output operating at 17.5mhz, v s = 3v ltc6905 output operating at 170mhz, v s = 3v r out vs v + jitter vs frequency supply voltage (v) 2.5 0 output resistance ( ? ) 5 15 20 25 4.5 45 6905 g04 10 3.5 3 5 4 5.5 30 35 40 frequency (mhz) 0 0 peak-to-peak jitter (%) 0.20 0.60 4 2 1 0.80 1.00 40 80 100 180 6905 g05 0.40 20 60 120 140 160 1.20 12.5ns/div 6905 g06 1ns/div 6905 g07 typical perfor a ce characteristics uw temperature ( c) C40 C1.0 percentage error (%) C0.8 C0.4 C0.2 0 1.0 0.4 0 40 60 6905 g08 C0.6 0.6 0.8 0.2 C20 20 80 100 120 frequency vs temperature uu u pi fu ctio s v + (pin 1): voltage supply (2.7v v + 5.5v). this supply must be kept free from noise and ripple. it should be bypassed directly to the gnd (pin 2) with a 0.1 f capacitor or higher. gnd (pin 2): ground. should be tied to a ground plane for best performance. set (pin 3): frequency-setting resistor input. the value of the resistor connected between this pin and v + deter- mines the oscillator frequency. the voltage on this pin is held by the ltc6905 to approximately 1v below the v + voltage. for best performance, use a precision metal film resistor with a value between 10k and 25k and limit the capacitance on this pin to less than 10pf. div (pin 4): divider-setting input. this three-state input selects among three divider settings, determining the value of n in the frequency equation. pin 4 should be tied to v + for the 1 setting, the highest frequency range. floating pin 4 divides the master oscillator by 2. pin 4 should be tied to gnd for the 4 setting, the lowest frequency range. to detect a floating div pin, the ltc6905 attempts to pull the pin toward midsupply. this is realized with two internal current sources, one tied to v + and pin 4 and the other one tied to ground and pin 4. therefore, driving the div pin high re- quires sourcing approximately 15 a. likewise, driving div low requires sinking 11 a. when pin 4 is floated, it should be bypassed by a 1nf capacitor to ground or it should be surrounded by a ground shield to prevent excessive cou- pling from other pcb traces. out (pin 5): oscillator output. this pin can drive 5k ? and/or 5pf loads. for larger loads, refer to the applications information section. 1v/div 1v/div
ltc6905 5 6905fb block diagra w theory of operatio u as shown in the block diagram, the ltc6905s master oscillator is controlled by the ratio of the voltage between the v + and set pins and the current entering the set pin (i res ). the voltage on the set pin is forced to approxi- mately 1v below v + by the pmos transistor and its gate bias voltage. a resistor r set , connected between the v + and set pins, locks together the voltage (v + C v set ) and current, i res , variation. this provides the ltc6905s high precision. the master oscillation frequency reduces to: f mhz k r mhz mo set = ? + 168 5 10 15 .? . to extend the output frequency range, the master oscilla- tor signal is divided by 1, 2 or 4 before driving out (pin 5). the ltc6905 is optimized for use with resistors between 10k and 25k, corresponding to oscillator fre- quencies between 17.225mhz and 170mhz. the divide- by value is determined by the state of the div input (pin 4). tie div to v + or drive it to within 0.4v of v + to select 1. this is the highest frequency range, with the figure 1. r set vs output frequency master output frequency passed directly to out. the div pin may be floated or driven to midsupply to select 2, the intermediate frequency range. the lowest frequency range, 4, is selected by tying div to gnd or driving it below 0.5v. figure 1 shows the relationship between r set , divider setting and output frequency, including the over- lapping frequencies. output frequency (mhz) 10 r set ( ? ) 15 20 4 2 1 6905 f01 10 5 60 110 160 30 25 C + C + + C 1 3 gain = 1 v + v bias i res i res r set set gnd master oscillator programmable divider ( 1, 2 or 4) v res = 1v 5% (v + C v set ) three-state input detect gnd v + 15 a 6905 bd 15 a out divider select 5 div 4 2 f osc = f mo n
ltc6905 6 6905fb applicatio s i for atio wu uu selecting the divider setting and resistor the ltc6905s master oscillator has a frequency range spanning 68.9mhz to 170mhz. a programmable divider extends the frequency range from 17.225mhz to 170mhz. table 1 describes the recommended frequencies for each divider setting. note that the ranges overlap; at some frequencies there are two divider/resistor combinations that result in the same frequency. choosing a higher divider setting will result in less jitter at the expense of slightly higher supply current. table 1. frequency range vs divider setting divider setting frequency range 1 ? div (pin 4) = v + 68.9mhz to 170mhz 2 ? div (pin 4) = floating 34.45mhz to 85mhz 4 ? div (pin 4) = gnd 17.225mhz to 43mhz after choosing the proper divider setting, determine the correct frequency-setting resistor. because of the linear correspondence between oscillation period and resis- tance, a simple equation relates resistance with frequency. rk mhz f n mhz n set osc = ? ? ? ? ? ? ? ? ? ? ? 10 168 5 15 1 2 4 ? . ?C. , = (r setmin = 10k, r setmax = 25k) any resistor, r set , tolerance adds to the inaccuracy of the oscillator, f osc . start-up time the start-up time and settling time to within 1% of the final frequency is typically 100 s. maximum output load the ltc6905 output (pin 5) can drive a capacitive load (c load ) of 5pf or more. driving a c load greater than 5pf depends on the oscillators frequency (f osc ) and output resistance (r out ). the output rise time or fall time due to r out and c load is equal to 2.2 ? r out ? c load (from 10% to 90% of the rise or fall transition). if the total output rise time plus fall time is arbitrarily specified to be equal to or less than 20% of the oscillators period (1/f osc ), then the maximum output c load in picofarads (pf) should be equal to or less than [45454/(r out ? f osc )] (r out in ohms and f osc in mhz). example: an ltc6905 is operating with a 3v power supply and is set for a f osc = 50mhz. r out with v + = 3v is 27 ? (using the r out vs v + graph in the typical performance characteristics). the maximum output c load should be equal to or less than [45454/(27 ? 50)] = 33.6pf. the lowest resistive load pin 5 can drive can be calculated using the minimum high level output voltage in the elec- trical characteristics. with a v + equal to 5.5v and 4ma output current, the minimum high level output voltage is 5v and the lowest resistive load pin 5 can drive is 1.25k (5v/4ma). with a v + equal to 2.7v and 4ma output current, the minimum high level output voltage is 1.9v and the lowest resistive load pin 5 can drive is 475 ? (1.9v/4ma). frequency accuracy and power supply noise the frequency accuracy of the ltc6905 may be affected when its power supply generates noise with frequency contents equal to f mo /64 or its multiples (f mo is the internal ltc6905 master oscillator frequency before the divider and f mo /64 is the master oscillator control loop fre- quency). if for example, the master oscillator frequency is set equal to 80mhz and the ltc6905 is powered by a switching regulator, then the oscillator frequency may show an additional error if the switching frequency is 1.4mhz (80mhz/64). jitter and power supply noise if the ltc6905 is powered by a supply that has frequency contents equal to the output frequency then the oscillators jitter may increase. in addition, power supply ripple in excess of 20mv at any frequency may increase jitter. jitter and divide ratio at a given output frequency, a higher master oscillator frequency and a higher divide ratio will result in lower jitter and higher power supply dissipation. indeterminate jitter percentage will decrease by a factor of slightly less than the square root of the divider ratio, while determinate jitter will not be similarly attenuated. please consult the speci- fication tables and jitter vs frequency graph showing jitter at various divider ratios.
ltc6905 7 6905fb applicatio s i for atio wu uu jitter and stray capacitance on the set pin (pin 3) the stray capacitance on the set pin (pin 3) should be limited to 10pf or less to avoid increased jitter or unstable oscillation. ltc6905 suggested critical component layout in order to provide the specified performance, it is re- quired that the frequency setting resistor r set and the supply bypass capacitor be placed as close as possible to the ltc6905. the following additional rules should be followed for best performance: 1) the bypass capacitor must be placed as close as possible to the ltc6905, and no vias should be placed between the capacitor and the ltc6905. the bypass capacitor must be on the same side of the circuit board as the ltc6905. information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 2) the resistor r set should be placed as close as possible to the ltc6905, and the connection of r set to v cc should be closely shared with the bypass capacitor. the resistor r set may be placed on the opposite side of the board from the ltc6905, directly underneath the by- pass capacitor. 3) if a ground plane is used, the connection of the ltc6905 to the ground plane should be as close as possible to the ltc6905 gnd pin and should be composed of multiple, high current capacity vias. package descriptio u s5 package 5-lead plastic tsot-23 (reference ltc dwg # 05-08-1635) 1.50 C 1.75 (note 4) 2.80 bsc 0.30 C 0.45 typ 5 plcs (note 3) datum a 0.09 C 0.20 (note 3) s5 tsot-23 0302 pin one 2.90 bsc (note 4) 0.95 bsc 1.90 bsc 0.80 C 0.90 1.00 max 0.01 C 0.10 0.20 bsc 0.30 C 0.50 ref note: 1. dimensions are in millimeters 2. drawing not to scale 3. dimensions are inclusive of plating 4. dimensions are exclusive of mold flash and metal burr 5. mold flash shall not exceed 0.254mm 6. jedec package reference is mo-193 3.85 max 0.62 max 0.95 ref recommended solder pad layout per ipc calculator 1.4 min 2.62 ref 1.22 ref figure 2. ltc6905 suggested critical component layout ltc6905 6905 f02 c r
ltc6905 8 6905fb linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2005 lt/lt 0905 rev b ? printed in usa related parts part number description comments ltc1799 1khz to 33mhz thinsot oscillator single output, high frequency operation ltc6900 1khz to 20mhz thinsot oscillator single output lower power ltc6902 multiphase oscillator with spread spectrum modulation 2-, 3- or 4-phase outputs ltc6903/ltc6904 1khz to 68mhz serial port programmable oscillator 3-wire or i 2 c tm programmable ltc6905-xxx series fixed frequency ltc6905 high accuracy, no external resistor ltc6906 micropower, 10khz to 1mhz resistor set thinsot oscillator ultralow power, resistor sets frequency i 2 c is a trademark of philips electronics n.v. applicatio s i for atio wu uu figure 3. current controlled oscillator figure 4. voltage controlled oscillator f n mhz k vv r i vv mhz osc set set cntrl set = ? ? ? ? ? ? ? + ? ? ? ? ? ? ? ? ? ? ? ? ? ? + + 1 168 5 10 15 .? ? C C C . i cntrl frequency 100khz example (figure 3): v set = (v + C 1v), r set = 10k, n = 1 f mhz k i mhz osc cntrl = ? () + [] 168 5 1 10 1 5 .?C ? . f n mhz k vv r vv r vv mhz osc set set set cntrl cntrl set = ? ? ? ? ? ? ? + ? ? ? ? ? ? ? ? ? ? ? ? ? ? + + 1 168 5 10 15 .? ? C C C C . v cntrl frequency 100khz example (figure 4): v set = (v + C 1v), r set = 10k, r cntrl = 33.2k, n = 1, v + = 3v f mhz k k vv k mhz osc cntrl = ? ?? ? ? ? ? ? ? + ? ? ? ? ? ? 168 5 10 1 10 2 33 2 15 .? ? C C . . alternative methods of setting the output frequency of the ltc6905 the ltc6905 may be programmed by any method that sources a current into the set pin (pin 3). the accuracy of the programming is best with a simple resistor because the ltc6905 takes into account both the voltage at the set pin and the current into the set pin when generating the output frequency. since the voltage at the set pin can vary by as much as 5%, setting the frequency using a current rather than a resistor will result in as much as 5% addi- tional inaccuracy in the output frequency. figure 3 shows a method to control the frequency of the ltc6905 using a current source. r set , in this case, sets a maximum frequency according to the regular expression for f osc . the current source will subtract current from the set pin to lower the frequency. figure 4 shows a method for controlling the frequency of the ltc6905 using a voltage source. in this case, r set sets a constant current into the set pin, and r cntrl will sub- tract from this current in order to change the frequency. increasing v cntrl will increase the output frequency. v + 1 2 3 5 f osc 69.8mhz to 170mhz v + 0.1 f i cntrl 0 a to 60 a r set 10k 6905 f03 4 gnd v + n = 1 ltc6905 set out div v + 1 2 3 5 v + = 3v 0.1 f r set 10k v cntrl 0v to 2v 6905 f04 4 gnd v + n = 1 ltc6905 set out div + C r cntrl 33.2k f osc 69.8mhz to 170mhz
|
