LTC6908-1/LTC6908-2 PIN FUNCTIONS (DCB Package/S6 Package)SET (Pin 1/Pin 3): Frequency-Setting Resistor Input. The MOD (Pin 6/Pin 4): Modulation-Setting Input. This three- value of the resistor connected between this pin and V+ state input selects among four modulation rate settings. determines the oscillator frequency. The voltage on this The MOD pin should be tied to ground for the fOUT/16 pin is held by the LTC6908 to approximately 1.1V below modulation rate. Floating the MOD pin selects the fOUT/32 the V+ voltage. For best performance, use a precision modulation rate. The MOD pin should be tied to V+ for the metal film resistor with a value between 20k and 400k fOUT/64 modulation rate. Tying one of the outputs to the and limit the capacitance on this pin to less than 10pF. MOD pin turns the modulation off. To detect a floating V+ (Pin 2/Pin 1): Voltage Supply (2.7V ≤ V+ ≤ 5.5V). This MOD pin, the LTC6908 attempts to pull the pin toward supply must be kept free from noise and ripple. It should midsupply. This is realized with two internal current be bypassed directly to a ground plane with a 0.1µF sources, one tied to V+ and MOD and the other one tied capacitor. to ground and MOD. Therefore, driving the MOD pin high requires sourcing approximately 2µA. Likewise, driving GND (Pin 3/Pin 2): Ground. Should be tied to a ground the MOD pin low requires sinking 2µA. When the MOD plane for best performance. pin is floated, it must be bypassed by a 1nF capacitor to OUT1 (Pin 4/Pin 6), OUT2 (Pin 5/Pin 5): Oscillator ground. Any AC signal coupling to the MOD pin could Outputs. These pins can drive 5k and/or 10pF loads. potentially be detected and stop the frequency modulation. Larger loads may cause inaccuracies due to supply Exposed Pad (Pin 7/NA): Ground. The Exposed Pad must bounce at high frequencies. be soldered to PCB. BLOCK DIAGRAM (S6 Package Pin Numbers) I f MASTER MASTER = 20MHz • 10k • = 20MHz • 10k/RSET V+ – V(SET) fOUT = fMASTER/2 V+ 1 + MASTER OSCILLATOR 0 V+ – V(SET) 1.13V R GAIN = 1 V COMPLEMENTARY SET 6 OUT1 OR 3 – OUT QUADRATURE SET OUTPUTS 90/180 V+ – V(SET) 1-POLE 5 OUT2 V I BIAS SET = R LPF IMASTER SET MUTE OUTPUT UNTIL STABLE IREF MDAC 2 GND POR V+ + 2μA – PSEUDO RANDOM DIVIDE BY CODE GENERATOR CLK 16/32/64 MOD 4 3-STATE DIVIDER SELECT INPUT DECODER + 2μA – DETECT WHEN A CLOCK SIGNAL IS PRESENT AT THE GND CLOCK INPUT MOD INPUT, DISABLE THE MODULATION. 690812 BD Rev B 6 For more information www.analog.com Document Outline Features Applications Description Absolute Maximum Ratings Electrical Characteristics Pin Functions Operation Applications Information Typical Applications Package Description Typical Application
