Datasheet LTC6907 (Analog Devices) - 7

LTC6907
U THEORY OF OPERATIO
The LTC6907 is a precision, resistor programmable oscil- so lator (see Block Diagram). It generates a square wave at 1 the OUT pin with a period directly proportional to the value t = = R • C OSC SET OSC ƒ (4) of an external resistor, RSET. A feedback circuit measures OSC and controls the oscillator frequency to achieve the high- The period and frequency are determined exclusively by est possible accuracy. In equilibrum, this circuit ensures R that the current in the SET pin, I SET and the precision internal capacitor. Importantly, the SET, is balanced by IFB. IFB value of V is proportional to the master oscillator frequency, so we SET is immaterial, and the LTC6907 maintains its accuracy even though V have the relationship: SET is not a precision reference voltage. ISET = IFB = VSET • ƒOSC • COSC (1) The digital dividers shown in the Block Diagram further Where COSC is a precision internal capacitor: divide the master oscillator frequency by 1, 3 or 10 producing: COSC = 5pF for the LTC6907 Solving for the oscillator period: ƒ ƒ OSC = OUT (5) N 1 V t SET = = • C OSC ƒ (2) and I OSC OSC SET tOUT = N • tOSC (6) This is the fundamental equation for the LTC6907. It holds regardless of how the SET pin is driven. When a resistor, Table 1 gives specific frequency and period equations for R the LTC6907. The Applications Information section gives SET, is connected from the SET pin to ground, we have the relationship: further detail and discusses alternative ways of setting the LTC6907 output frequency. VSET = RSET (3) ISET
Table 1. Output Frequency Equations PART NUMBER FREQUENCY PERIOD DIVIDER RATIOS
LTC6907 ⎧ ,DIV Pin = + 10 V 4 ⎛ 50 ⎞ R ⎞ ƒ MHz k = SET = ⎛ • 250 • ⎪ OUT • t N ns OUT N = 3, DIV Pin = Open ⎨ N ⎝⎜R ⎝⎜ k 50 ⎠⎟ SET ⎠ ⎟ ⎪ ⎩1, DIV Pin = GND 6907fa 7