Datasheet LTC6906 (Analog Devices) - 7

LTC6906
THEORY OF OPERATION
The LTC6906 is a precision, resistor programmable oscilla- The period and frequency are determined exclusively by tor (see the Block Diagram). It generates a square wave at RSET and the precision internal capacitor. Importantly, the the OUT pin with a period directly proportional to the value value of VSET is immaterial, and the LTC6906 maintains of an external resistor, RSET. A feedback circuit measures its accuracy even though VSET is not a precision reference and controls the oscillator frequency to achieve the high- voltage. est possible accuracy. In equilibrium, this circuit ensures The digital dividers shown in the Block Diagram further that the current in the SET pin, ISET, is balanced by IFB. divide the master oscillator frequency by 1, 3 or 10 pro- IFB is proportional to the master oscillator frequency, so ducing: we have the relationship: ISET = IFB = VSET • ƒOSC • COSC (1) ƒ = ƒOSC OUT N (5) where COSC is a precision internal capacitor: and COSC = 10pF for the LTC6906 t Solving for the oscillator period: OUT = N • tOSC (6) Table 1 gives specific frequency and period equations for t = 1 = VSET OSC • COSC the LTC6906. The Applications Information section gives ƒ I OSC SET (2) further detail and discusses alternative ways of setting the LTC6906 output frequency. This is the fundamental equation for the LTC6906. It holds regardless of how the SET pin is driven. When a resistor, RSET, is connected from the SET pin to ground, we have the relationship: VSET =RSET I SET (3) so t = 1 = OSC RSET • COSC ƒ OSC (4)
Table 1. Output Frequency Equations PART NUMBER FREQUENCY PERIOD DIVIDER RATIOS
LTC6906 ⎧ ⎛ 10, DIV Pin = V + 100k⎞ ⎛R ⎞ ⎪ ƒ = 1MHz ⎜ ⎟ = SET OUT • ⎜ ⎟ t N • 1μs • ⎜ ⎟ N = ⎨ 3, DIV Pin = Open N R ⎝ ⎠ OUT ⎝100k⎠ SET ⎪ 1, ⎩ DIV Pin = GND 6906fc 7