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LTC6990

6990fc

For more information www.linear.com/LTC6990

TYPICAL APPLICATION

DESCRIPTION

TimerBlox: Voltage

Controlled Silicon Oscillator

The LTC

6990 is a precision silicon oscillator with a pro-

grammable frequency

range of 488Hz to 2MHz. It can be

used as a fixed-frequency or voltage-controlled oscillator

(VCO). The LTC6990 is part of the TimerBlox

family of

versatile silicon timing devices.

A single resistor, R

SET

, programs the LTC6990’s internal

master oscillator frequency. The output frequency is deter-

mined by

this master oscillator and an internal frequency

divider, N

DIV

, programmable to eight settings from 1 to 128.

OUT

1MHz

DIV

•

50kΩ

SET

, N

DIV

= 1, 2, 4 …128

Optionally, a second resistor at the SET input provides

linear voltage control of the output frequency and can be

used for frequency modulation. A narrow or wide VCO

tuning range can be configured by the appropriate selec

tion of the two resistors.

The

LTC6990 includes an enable function that is synchro-

nized with

the master oscillator to ensure clean, glitch-free

output

pulses. The disabled output can be configured to

be high impedance or forced low.

For easy configuration of the LTC6990, download the

TimerBlox Designer tool at www.linear.com/timerblox.

Voltage Controlled Oscillator with 16:1 Frequency Range

L, LT, LTC and LTM , Linear Technology, TimerBlox and the Linear logo are registered

trademarks and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks

are the property of their respective owners. Protected by U.S. Patents, including 6342817,

6614313.

FEATURES

APPLICATIONS

Fixed-Frequency or Voltage-Controlled Operation

– Fixed: Single Resistor Programs Frequency

with <1.5% Max Error

–

VCO: Tw o Resistors Set VCO Center

Frequency and Tuning Range

Frequency Range: 488Hz to 2MHz

2.25V to 5.5V Single Supply Operation

72µA Supply Current at 100kHz

500µs Start-Up Time

VCO Bandwidth >300kHz at 1MHz

CMOS Logic Output Sources/Sinks 20mA

50% Duty Cycle Square Wave Output

Output Enable (Selectable Low or Hi-Z When Disabled)

–55°C to 125°C Operating Temperature Range

Available in Low Profile (1mm) SOT-23 (ThinSOT™)

and 2mm × 3mm DFN Package

Low Cost Precision Programmable Oscillator

Voltage-Controlled Oscillator

High Vibration, High Acceleration Environments

Replacement for Fixed Crystal and Ceramic Oscillators

Portable and Battery-Powered Equipment

VCO Transfer Function

CTRL

(V)

OUT

(kHz)

1000

750

250

500

0.5

6990 TA01b

21 1.5

6990 TA01a

LTC6990

GND

SET

OUT

DIV

0.1µF

SET

100k

CTRL

VCO

100k

OUT

= 1MHz − V

CTRL

• 0.5

MHz

Summary of content (30 pages)

PAGE 1
LTC6990 TimerBlox: Voltage Controlled Silicon Oscillator FEATURES n n n n n n n n n n n DESCRIPTION Fixed-Frequency or Voltage-Controlled Operation – Fixed: Single Resistor Programs Frequency with <1.5% Max Error – VCO: Two Resistors Set VCO Center Frequency and Tuning Range Frequency Range: 488Hz to 2MHz 2.25V to 5.
PAGE 2
LTC6990 ABSOLUTE MAXIMUM RATINGS (Note 1) Supply Voltage (V+) to GND.........................................6V Maximum Voltage on Any Pin .............................. (GND – 0.3V) ≤ VPIN ≤ (V+ + 0.3V) Operating Temperature Range (Note 2) LTC6990C.............................................–40°C to 85°C LTC6990I..............................................–40°C to 85°C LTC6990H........................................... –40°C to 125°C LTC6990MP........................................
PAGE 3
LTC6990 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Test conditions are V+ = 2.25V to 5.5V, OE = V+, DIVCODE = 0 to 15 (NDIV = 1 to 128), RSET = 50k to 800k, RLOAD = 5k, CLOAD = 5pF unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN fOUT Output Frequency Recommended Range: RSET = 50k to 800k Extended Range: RSET = 25k to 800k 0.488 0.
PAGE 4
LTC6990 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Test conditions are V+ = 2.25V to 5.5V, OE = V+, DIVCODE = 0 to 15 (NDIV = 1 to 128), RSET = 25k to 800k, RLOAD = ∞, CLOAD = 5pF unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS ±10 nA Digital I/O OE Pin Input Capacitance 2.
PAGE 5
LTC6990 ELECTRICAL CHARACTERISTICS Note 11: Long-term drift of silicon oscillators is primarily due to the movement of ions and impurities within the silicon and is tested at 30°C under otherwise nominal operating conditions. Long-term drift is specified as ppm/√kHr due to the typically nonlinear nature of the drift. To calculate drift for a set time period, translate that time into thousands of hours, take the square root and multiply by the typical drift number. For instance, a year is 8.
PAGE 6
LTC6990 TYPICAL PERFORMANCE CHARACTERISTICS 200 150 100 50 0.994 1.002 1.010 VSET (V) 100 RSET = 800k, ÷1 50 ÷128 100 0 0.1 V+ = 5V TA = 25°C 1 10 100 1000 FREQUENCY (kHz) ÷1 ÷2 200 150 ÷128 100 0 0.1 10000 1 10 100 1000 FREQUENCY (kHz) OE PIN VOLTAGE (V) JITTER (%P-P) 0 0.1 1 10 100 FREQUENCY (kHz) 1000 6990 G13 100 3.3V, OE FALLING 125 3.3V, OE RISING 100 75 0 20 40 60 VOE /V+ (%) 80 100 6990 G12 TA = 25°C 45 2.5 NEGATIVE-GOING 1.5 1.
PAGE 7
LTC6990 TYPICAL PERFORMANCE CHARACTERISTICS Rise and Fall Time vs Supply Voltage Typical Frequency Error vs Time (Long-Term Drift) RISE/FALL TIME (ns) 50 0 –50 –100 TA = 25°C CLOAD = 5pF 45 2.0 tRISE 1.5 tFALL 1.0 0.5 –150 –200 50 TA = 25°C CLOAD = 5pF 2.
PAGE 8
LTC6990 PIN FUNCTIONS (DCB/S6) V+ (Pin 1/Pin 5): Supply Voltage (2.25V to 5.5V). This supply must be kept free from noise and ripple. It should be bypassed directly to the GND pin with a 0.1µF capacitor. DIV (Pin 2/Pin 4): Programmable Divider and Hi-Z Mode Input. A V+ referenced A/D converter monitors the DIV pin voltage (VDIV) to determine a 4-bit result (DIVCODE). VDIV may be generated by a resistor divider between V+ and GND. Use 1% resistors to ensure an accurate result.
PAGE 9
LTC6990 BLOCK DIAGRAM (S6 Package Pin Numbers Shown) 5 R1 DIV 4 V+ 1 4-BIT A/D CONVERTER OE Hi-Z BIT DIGITAL FILTER R2 Hi-Z WHEN DISABLED MASTER OSCILLATOR tMASTER = 1µs VSET • 50kΩ ISET MCLK PROGRAMMABLE DIVIDER ÷1, 2, 4, 8, 16, 32, 64, 128 OUT 6 tOUT Hi-Z OUTPUT UNTIL SETTLED HALT OSCILLATOR IF ISET < 500nA ISET + – POR + – 1V 2 VSET = 1V 3 GND SET 6990 BD RSET 6990fc For more information www.linear.
PAGE 10
LTC6990 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. I 1 f MASTER = = 1MHz • 50k • SET tMASTER VSET A feedback loop maintains VSET at 1V ±30mV, leaving ISET as the primary means of controlling the output frequency.
PAGE 11
LTC6990 OPERATION Table 1. DIVCODE Programming DIVCODE Hi-Z NDIV Recommended fOUT R1 (k) R2 (k) VDIV /V+ 0 0 1 62.5kHz to 1MHz Open Short ≤ 0.03125 ±0.015 1 0 2 31.25kHz to 500kHz 976 102 0.09375 ±0.015 2 0 4 15.63kHz to 250kHz 976 182 0.15625 ±0.015 3 0 8 7.813kHz to 125kHz 1000 280 0.21875 ±0.015 4 0 16 3.906kHz to 62.5kHz 1000 392 0.28125 ±0.015 5 0 32 1.953kHz to 31.25kHz 1000 523 0.34375 ±0.015 6 0 64 976.6Hz to 15.63kHz 1000 681 0.40625 ±0.
PAGE 12
LTC6990 OPERATION is not stable, it will increase the start-up time as the converter waits for a stable result. Therefore, capacitance on the DIV pin should be minimized so it will settle quickly. Less than 100pF will not affect performance. Output Enable The OE pin controls the state of the LTC6990’s output as seen on the OUT pin. Pulling the OE pin high enables the oscillator output. Pulling it low disables the output.
PAGE 13
LTC6990 OPERATION Changing DIVCODE After Start-Up Start-Up Time Following start-up, the A/D converter will continue monitoring VDIV for changes. Changes to DIVCODE will be recognized slowly, as the LTC6990 places a priority on eliminating any “wandering” in the DIVCODE. The typical delay depends on the difference between the old and new DIVCODE settings and is proportional to the master oscillator period.
PAGE 14
LTC6990 APPLICATIONS INFORMATION OE Hi-Z OUT tSTART 1/2 tOUT tOUT 6990 F07 Figure 7. Start-Up Timing Diagram (OE = 1, NDIV = 1 or 2, Hi-Z = 0 or 1) OE Hi-Z OUT tSTART tMASTER tOUT 6990 F08 Figure 8. Start-Up Timing Diagram (OE = 1, NDIV ≥ 4, Hi-Z = 0 or 1) OE OUT Hi-Z tSTART tENABLE tOUT 6990 F09 Figure 9. Start-Up Timing Diagram (OE = 0, NDIV = Any, Hi-Z = 0) OE tPD OUT Hi-Z tSTART REMAINS Hi-Z tENABLE UNTIL OE = 1 tOUT 6990 F10 Figure 10.
PAGE 15
LTC6990 APPLICATIONS INFORMATION Start-Up Behavior Step 2: Calculate and Select RSET When first powered up, the output is high impedance. If the output is enabled (OE = 1) at the end of the start-up time, the output will go low for one tMASTER cycle (or half a tOUT cycle if NDIV < 4) before the first rising edge. If the output is disabled (OE = 0) at the end of the start-up time, the output will drop to a low output if the Hi-Z bit = 0, or simply remain floating if Hi-Z = 1.
PAGE 16
LTC6990 APPLICATIONS INFORMATION Frequency Modulated Operation (Voltage-Controlled Oscillator) Operating the LTC6990 as a voltage-controlled oscillator in its simplest form is achieved with one additional resistor. As shown in Figure 11, voltage VCTRL sources/sinks a current through RVCO to vary the ISET current, which in turn modulates the output frequency as described in Equation (2).
PAGE 17
LTC6990 APPLICATIONS INFORMATION The final step is to calculate the correct value for RSET using the following equation: RSET = ( 1MHz • 50k NDIV • f(0V) − VSET • K VCO ) (3e) Select the standard resistor value closest to the calculated value. Some applications require combinations of fOUT(MIN), fOUT(MAX), VCTRL(MIN) and VCTRL(MAX) that are not achievable. These applications result in unrealistic or unrealizable (e.g. negative value) resistors.
PAGE 18
LTC6990 APPLICATIONS INFORMATION Step 2: Calculate KVCO and f(0V) Next, calculate the intermediate values KVCO and f(0V) using Equations (3b) and (3c). 100kHz − 10kHz = 30kHz/V 4V − 1V f(0V) = 100kHz + 30kHz/V • 1V = 130kHz K VCO = Step 3: Calculate and Select RVCO The next step is to use Equation (3d) to calculate the correct value for RVCO.
PAGE 19
LTC6990 APPLICATIONS INFORMATION Additionally, by choosing the VCO’s specifications shrewdly, the frequency error (in percent) due to VSET variation is reduced to ΔVSET/VSET = ±3%. To realize this improvement, the design must abide by three conditions. First, the VIN voltage must be positive throughout the range. Second, choose VMAX /VMIN ≥ fMAX /fMIN. Last, choose RVCO /RSET ≥ R4/R3. Figure 13 shows a design similar to the previous design example where the VMIN voltage is now specified to be 0.4V.
PAGE 20
LTC6990 APPLICATIONS INFORMATION Modulation Bandwidth and Settling Time Power Supply Current The LTC6990 will respond to changes in ISET up to a –3dB bandwidth of 0.4 • fOUT (see Figure 15). This makes it easy to stabilize a feedback loop around the LTC6990, since it does not introduce a low-frequency pole. The power supply current varies with frequency, supply voltage and output loading.
PAGE 21
LTC6990 APPLICATIONS INFORMATION SUPPLY BYPASSING AND PCB LAYOUT GUIDELINES The LTC6990 is a 2.2% accurate silicon oscillator when used in the appropriate manner. The part is simple to use and by following a few rules, the expected performance is easily achieved. The most important use issues involve adequate supply bypassing and proper PCB layout. Figure 17 shows example PCB layouts for both the SOT-23 and DCB packages using 0603 sized passive components.
PAGE 22
LTC6990 TYPICAL APPLICATIONS Programming NDIV Using an 8-Bit DAC OE DIVCODE 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 OUT LTC6990 2.25V TO 5.5V GND V+ SET DIV C1 0.1µF C2 0.1µF RSET 619k VCC SDI VOUT LTC2630-LZ8 SCK GND µP CS/LD DAC CODE 0 24 40 56 72 88 104 120 136 152 168 184 200 216 232 255 6990 TA02 Full Range VCO with Any NDIV Setting (fMAX to fMIN for VIN = 0V to VSET) 5V RVCO2 26.1k OE OUT LTC6990 VIN 0V TO 1V RVCO1 26.1k 5V – V+ GND D1 IN4148 LT1490 SET + C1 0.
PAGE 23
LTC6990 TYPICAL APPLICATIONS Speaker Alarm. Modulate Tone with RVCO within 500Hz to 8kHz Span 5V 8Ω IN4004 5V 20k OE 2N2222 OUT LTC6990 RAMP GND V+ SET DIV 5V 1M RVCO STEP 50k 6990 TA05 97.6k 887k Overvoltage Detector/Alarm. Direct Drive of Piezo Alarm 24V RA 787k RB 10.7k 400mV 5V + 100k LT6703-3 OE – OUT LTC6990 GND V+ SET DIV 5V 1M  R  V ALARM = 400mV  1+ A  = 30V  RB  392k 6990 TA06 PIEZO ALARM 4kHz MURATA PKM29-3A0 523k 6990fc For more information www.linear.
PAGE 24
LTC6990 TYPICAL APPLICATIONS Direct Piezo Alarm Driver. Adjust Frequency for Maximum Alarm Sound Pressure (Maximum Annoyance for Best Effect) 5V 10k ON OE OFF OUT LTC6990 GND V+ PIEZO ALARM MURATA PKM29-340 f = 4kHz 5V 1M SET DIV 6990 TA07 392k 523k Isolated V → F Converter. VIN Provided by Isolated Measurement Circuit. 5µs Rise/Fall Time of Isolator Limits fMAX to 60kHz 3.
PAGE 25
LTC6990 TYPICAL APPLICATIONS Quadrature Sine Wave Oscillator. Voltage Controlled Frequency Range from ~5Hz to ~20kHz with 1VP-P Constant Output Amplitude SINE 1.18VREF COSINE 51.1k 5.11k INV1 124k 3 2 N 4 S1 1 BP 14 LP 5V – LTC1059* – + + – 8 CLOCK 5V 5 SA 11 AGND 9 50/100 10k LT1004-2.5V VCC OUT OUT 2k GND R1 1M DIV SET R2 280k RVCO 267k RSET 49.9k FREQ ADJ 6990 TA09 + 1.18VREF V+ 5V 1M LTC1440 V– OE LTC6990 – 5V 2.5V FOR 5Hz TO 10kHz 5V FOR 10Hz TO 20kHz 2.
PAGE 26
LTC6990 TYPICAL APPLICATIONS Full Range Temperature to Frequency Converter. 16kHz to 1kHz from –20°C to 80°C 5V OE 10k 5V – 22k AT 25°C B = 3964 + 10k OUT fOUT LTC6990 V+ SET DIV 5V 1M 100k LT1490 GND 6990 TA11 + 681k 26k 26k THERMISTOR: VISHAY NTHS120601N2202J Light to Frequency Converter. fOUT ≈ –1.4kHz per Microampere of Photo Diode Current, IPD 1000pF 5V OE 24.
PAGE 27
LTC6990 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DCB Package 6-Lead Plastic DFN (2mm × 3mm) (Reference LTC DWG # 05-08-1715 Rev A) 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 (2 SIDES) 2.15 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 1.35 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 2.00 ±0.10 (2 SIDES) R = 0.05 TYP 3.00 ±0.10 (2 SIDES) 0.40 ±0.10 4 6 1.65 ±0.10 (2 SIDES) PIN 1 NOTCH R0.20 OR 0.
PAGE 28
LTC6990 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. S6 Package 6-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1636) 0.62 MAX 2.90 BSC (NOTE 4) 0.95 REF 1.22 REF 3.85 MAX 2.62 REF 1.4 MIN 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 6 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) NOTE: 1.
PAGE 29
LTC6990 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 7/11 Updated Features, Description, Pin Configuration, and Order Information sections 1, 2 Added additional information to ∆fOUT/∆V+ and included Note 11 in Electrical Characteristics section 3, 4 Added Typical Frequency Error vs Time curve to Typical Performance Characteristics section 7 Modified drawing in SET pin description in Pin Functions 8 Added text to Basic Fixed Frequency Operation paragraph in Applications Information secti
PAGE 30
LTC6990 TYPICAL APPLICATION Ultrasonic Frequency Sweep Generator fOUT = 500kHz TO 31.25kHz OE OE OUT LTC6990 GND SET 74HC125 CSET 0.022µF RSET1 49.9k RSET2 750k V+ C1 0.1µF 2.25V TO 5.5V R1 976k DIV 6990 TA13 R2 102k SWEEPS FROM 500kHz to 31.25kHz IN A FEW MILLISECONDS (CONTROLLED BY CSET).