ANALOG DEVICES LTC 6900 CS5 Datasheet

LTC6900

Low Power, 1kHz to 20MHz

Resistor Set SOT-23 Oscillator

FEATURES

n

One External Resistor Sets the Frequency

n

1kHz to 20MHz Frequency Range

n

500μA Typical Supply Current, VS = 3V, 3MHz

n

Frequency Error ≤1.5% Max, 5kHz to 10MHz

= 25°C)

(T

A

n

Frequency Error ≤2% Max, 5kHz to 10MHz

= 0°C to 70°C)

(T

A

n

±40ppm/°C Temperature Stability

n

0.04%/V Supply Stability

n

50% ±1% Duty Cycle 1kHz to 2MHz

n

50% ±5% Duty Cycle 2MHz to 10MHz

n

Fast Start-Up Time: 50µs to 1.5ms

n

100 CMOS Output Driver

n

Operates from a Single 2.7V to 5.5V Supply

n

Low Profi le (1mm) ThinSOT™ Package

APPLICATIONS

n

Portable and Battery-Powered Equipment

n

PDAs

n

Cell Phones

n

Low Cost Precision Oscillator

n

Charge Pump Driver

n

Switching Power Supply Clock Reference

n

Clocking Switched Capacitor Filters

n

Fixed Crystal Oscillator Replacement

n

Ceramic Oscillator Replacement

DESCRIPTION

The LTC®6900 is a precision, low power oscillator that is
easy to use and occupies very little PC board space. The
oscillator frequency is programmed by a single external
resistor (R
accuracy operation (≤1.5% frequency error) without the
need for external trim components.

The LTC6900 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. The frequency-setting
resistor can vary from 10k to 2M to select a master
oscillator frequency between 100kHz and 20MHz (5V
supply). The three-state DIV input determines whether the
master clock is divided by 1, 10 or 100 before driving the
output, providing three frequency ranges spanning 1kHz
to 20MHz (5V supply). The LTC6900 features a proprietary
feedback loop that linearizes the relationship between R
and frequency, eliminating the need for tables to calculate
frequency. The oscillator can be easily programmed using
the simple formula outlined below:

f

=10MHz •

OSC

L, LT, LTC, LTM, Linear Technology and the Linear logo 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.

). The LTC6900 has been designed for high

SET

100,

DIV Pin = V

⎛
⎜

N•R

⎝

20k

SET

⎞
⎟

⎠

,N=

⎧
⎪

DIV Pin =Open

10,

⎨
⎪

DIV Pin =GND

1,

⎩

+

SET

TYPICAL APPLICATION

Clock Generator

10k ≤ R

SET

≤ 2M

5V

f

OSC

0.1μF

= 10MHz •

1

+

V

OUT

LTC6900

2

GND

3

SET

DIV

6900 TA01a

20k

()

N • R

SET

1kHz ≤ f

5

4

OSC

5V, N = 100

N = 1

≤ 20MHz

OPEN, N = 10

R

vs Desired Output Frequency

SET

10000

1000

(kΩ)

100

SET

R

10

÷100 ÷10 ÷1

1

1k 100k 1M 10M

10k

DESIRED OUTPUT FREQUENCY (Hz)

6900 TA01b

100M

6900fa

1

LTC6900

PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS

(Note 1)

Supply Voltage (V+) to GND .........................–0.3V to 6V

+

DIV to GND ....................................–0.3V to (V

SET to GND ....................................– 0.3V to (V

+ 0.3V)

+

+ 0.3V)
Operating Temperature Range (Note 8)

LTC6900C ............................................–40°C to 85°C

LTC6900I .............................................–40°C to 85°C

GND

SET

TOP VIEW

+

V

1
2
3

S5 PACKAGE

5-LEAD PLASTIC TSOT-23

T

= 150°C, θJA = 256°C/W

JMAX

54OUT

DIV

Storage Temperature Range ..................–65°C to 150°C

Lead Temperature (Soldering, 10 sec)...................300°C

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LTC6900CS5#PBF LTC6900CS5#TRPBF LTZM 5-Lead Plastic TSOT-23 –40°C to 85°C
LTC6900IS5#PBF LTC6900IS5#TRPBF LTZM 5-Lead Plastic TSOT-23 –40°C to 85°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container.

Consult LTC Marketing for information on non-standard lead based fi nish parts.
For more information on lead free part marking, go to:

For more information on tape and reel specifi cations, go to:

http://www.linear.com/leadfree/

http://www.linear.com/tapeandreel/

ELECTRICAL CHARACTERISTICS

The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. V+ = 2.7V to 5.5V, RL= 5k, CL = 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, 3) V

R

SET

∆f/∆T Frequency Drift Overtemperature

∆f/∆V Frequency Drift Over Supply (Note 3) V

Frequency-Setting Resistor Range |∆f| < 1.5% V+ = 5V

(Note 3)

Timing Jitter (Note 4) Pin 4 = V

Long-Term Stability of Output
Frequency

Duty Cycle (Note 7) Pin 4 = V

+

= 5V 5kHz ≤ f ≤ 10MHz
5kHz ≤ f ≤ 10MHz, LTC6900C
5kHz ≤ f ≤ 10MHz, LTC6900I
1kHz ≤ f < 5kHz
10MHz < f ≤ 20MHz

+

V

= 3V 5kHz ≤ f ≤ 10MHz
5kHz ≤ f ≤ 10MHz, LTC6900C
5kHz ≤ f ≤ 10MHz, LTC6900I
1kHz ≤ f < 5kHz

SET

+

= 3V

= 63.2k

≤ 400k

SET

≤ 400k

SET

≤ 400k

SET

SET

= 20k to 400k

V

= 63.2k

R

SET

+

= 3V to 5V, R

+

Pin 4 = Open, 20k ≤ R
Pin 4 = 0V, 20k ≤ R

Pin 4 = 0V (DIV by 1), R

, 20k ≤ R

+

or Open (DIV Either by 100 or 10)

±0.5

●

●

±0.5±2±1.5

●

●

20
20

●

●

●

●

±0.004 %/°C

0.04 0.1 %/V

300 ppm/√kHr

49
45

±2
±2

0.1

0.2

0.6

50
50

±1.5
±2.0
±2.5

±2.0
±2.5

400
400

51
55

%
%
%
%
%

%
%
%
%

k
k

%
%
%

%
%

2

6900fa

LTC6900

ELECTRICAL CHARACTERISTICS

The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T
All voltages are with respect to GND.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V
I

V
V
I

V

V

t

t

+

S

IH

IL

DIV

OH

OL

r

f

Operating Supply Range
Power Supply Current R

High Level DIV Input Voltage
Low Level DIV Input Voltage
DIV Input Current (Note 5) Pin 4 = V

High Level Output Voltage (Note 5) V+ = 5V IOH = –1mA

Low Level Output Voltage (Note 5) V+ = 5V IOL = 1mA

OUT Rise Time
(Note 6)

OUT Fall Time
(Note 6)

= 25°C. V+ = 2.7V to 5.5V, RL= 5k, CL = 5pF, Pin 4 = V+ unless otherwise noted.

A

2.7 5.5 V

●

= 400k, Pin 4 = V+, RL = ∞ V+ = 5V

SET

f

= 5kHz V+ = 3V

OSC

= 20k, Pin 4 = 0V, RL = ∞ V+ = 5V

R

SET

f

= 10MHz V+ = 3V

OSC

+

Pin 4 = 0V V

I

+

= 3V IOH = –1mA

V
I

I

+

= 3V IOL = 1mA

V
I

= –4mA

OH

= –4mA

OH

= 4mA

OL

= 4mA

OL

V+ = 5V

+

= 5V

V+ = 5V Pin 4 = V+ or Floating, RL = ∞
Pin 4 = 0V, R

+

= 3V Pin 4 = V+ or Floating, RL = ∞

V
Pin 4 = 0V, R

= ∞

L

= ∞

L

V+ = 5V Pin 4 = V+ or Floating, RL = ∞
Pin 4 = 0V, R

+

= 3V Pin 4 = V+ or Floating, RL = ∞

V
Pin 4 = 0V, R

= ∞

L

= ∞

L

●

●

●

●

V+ – 0.4 V

●

●

●

–4

●

4.8

●

4.5

●

2.7

●

2.2

●

●

●

●

●

0.32

0.29

0.92

0.68

2

–2

4.95

4.8

2.9

2.6

0.05

0.2

0.1

0.4
14

7

19
11

13

6

19
10

0.42

0.38

1.20

0.86

mA
mA

mA
mA

0.5 V
4µA

0.15

0.4

0.3

0.7

µA

V
V

V
V

V
V

V
V

ns
ns

ns
ns

ns
ns

ns
ns

Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.

Note 2: Frequencies near 100kHz and 1MHz may be generated using two
different values of R

(see the Selecting the Divider Setting Resistor

SET

paragraph in the Applications Information section). For these frequencies,
the error is specifi ed under the following assumption: 20k < R

SET

≤ 200k.

Note 3: Frequency accuracy is defi ned as the deviation from the
f

equation.

OSC

Note 4: Jitter is the ratio of the peak-to-peak distribution of the period to
the mean of the period. This specifi cation is based on characterization and
is not 100% tested. Also, see the Peak-to-Peak Jitter vs Output Frequency
curve in the Typical Performance Characteristics section.

Note 5: To conform with the Logic IC Standard convention, current out of
a pin is arbitrarily given as a negative value.

Note 6: Output rise and fall times are measured between the 10% and 90%
power supply levels. These specifi cations are based on characterization.

Note 7: Guaranteed by 5V test.
Note 8: The LTC6900C is guaranteed to meet specifi ed performance from

0°C to 70°C. The LTC6900C is designed, characterized and expected to
meet specifi ed performance from –40°C to 85°C but is not tested or
QA sampled at these temperatures. The LTC6900I is guaranteed to meet
specifi ed performance from –40°C to 85°C.

6900fa

3

LTC6900

TYPICAL PERFORMANCE CHARACTERISTICS

Frequency Variation vs R

4

TA = 25°C

3

GUARANTEED LIMITS APPLY OVER
20kΩ ≤ R

2

1

0

–1

VARIATION (%)

–2

–3

–4

1k

≤ 400kΩ

SET

10k 100k 1M

2.0

1.5

1.0

SET

TYPICAL HIGH

TYPICAL LOW

R

(Ω)

SET

6900 G01

Supply Current
vs Output Frequency

TA = 25°C

= 5pF

C

L

÷100, 5V

1.00

0.75

0.50

0.25

–0.25

VARIATION (%)

–0.50

–0.75

–1.00

÷1, 5V

÷10, 5V

Frequency Variation Over
Temperature

R

= 63.4k

SET

÷1 OR ÷10 OR ÷100

TYPICAL

HIGH

0

TYPICAL

LOW

–20 0 20 40 60 80

–40

TEMPERATURE (°C)

6900 G02

Output Resistance
vs Supply Voltage

140

120

OUTPUT SOURCING CURRENT

100

80

Peak-to-Peak Jitter
vs Output Frequency

1.0

0.9

0.8

0.7

)

0.6

P-P

0.5

0.4

JITTER (%

0.3

0.2
÷100

0.1

0

1k

OUTPUT FREQUENCY (Hz)

T

= 25°C

A

÷1, VA = 5V

÷1, VA = 3V

÷10

100k 1M10k 10M

6900 G03

SUPPLY CURRENT (mA)

0.5

÷100, 3V ÷10, 3V ÷1, 3V

0

0

1k 10k 100k 1M 10M

OUTPUT FREQUENCY (Hz)

LTC6900 Output Operating at
20MHz, VS = 5V

V+ = 5V, R

1V/DIV

0V

= 10k, CL = 10pF

SET

12.5ns/DIV

6900 G04

6900 G06

OUTPUT RESISTANCE (Ω)

60

OUTPUT SINKING CURRENT

40

2.5 3.0

4.0

3.5
SUPPLY VOLTAGE (V)

LTC6900 Output Operating at
10MHz, VS = 3V

V+ = 3V, R

1V/DIV

0V

= 20k, CL = 10pF

SET

25ns/DIV

4.5

5.0

5.5

6.0

6900 G05

6900 G07

6900fa

4

PIN FUNCTIONS

LTC6900

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 a ground plane with a 0.1µF capacitor.

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 LTC6900 to approximately 1.1V below the V
voltage. For best performance, use a precision metal fi lm
resistor with a value between 10k and 2M 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 GND

BLOCK DIAGRAM

for the ÷1 setting, the highest frequency range. Floating
Pin 4 divides the master oscillator by 10. Pin 4 should be

+

tied to V

for the ÷100 setting, the lowest frequency range.
To detect a fl oating DIV pin, the LTC6900 attempts to pull
the pin toward midsupply. Therefore, driving the DIV pin
high requires sourcing approximately 2µA. Likewise, driv­ing DIV low requires sinking 2µA. When Pin 4 is fl oated,
it should preferably be bypassed by a 1nF capacitor to
ground or it should be surrounded by a ground shield to
prevent excessive coupling from other PCB traces.

OUT (Pin 5): Oscillator Output. This pin can drive 5k and/
or 10pF loads. Heavier loads may cause inaccuracies due
to supply bounce at high frequencies. Voltage transients,
coupled into Pin 5, above or below the LTC6900 power
supplies will not cause latchup if the current into/out of
the OUT pin is limited to 50mA.

= (V+ – V

V

RES

+

V

1

R

SET

I

RES

SET

3

V

BIAS

GND

2

PATENT PENDING

+

–

I

RES

GAIN = 1

) = 1.1V TYPICALLY

SET

ƒ

MO

MASTER OSCILLATOR

= 10MHz • 20kΩ •

I

RES

(V+ – V

SET

PROGRAMMABLE

DIVIDER (N)

(÷1, 10 OR 100)

DIVIDER

)

SELECT

THREE-STATE

INPUT DETECT

V

+
–

+
–

GND

OUT

2µA

DIV

2µA

5

+

4

6900 BD

6900fa

5

LTC6900

OPERATION

As shown in the Block Diagram, the LTC6900’s master os­cillator is controlled by the ratio of the voltage between the

+

and SET pins and the current (I

V

) is entering the SET

RES

pin. The voltage on the SET pin is forced to approximately

+

1.1V below V

by the PMOS transistor and its gate bias
voltage. This voltage is accurate to ±8% at a particular
input current and supply voltage (see Figure 1).

A resistor R
“locks together” the voltage (V

, connected between the V+ and SET pins,

SET

+

– V

) and current, I

SET

RES

,
variation. This provides the LTC6900’s high precision. The
master oscillation frequency reduces to:

ƒMO=10MHz •

⎛
⎜

⎝

20kΩ

R

SET

⎞
⎟

⎠

The LTC6900 is optimized for use with resistors between
10k and 2M, corresponding to master oscillator frequen­cies between 100kHz and 20MHz.

To extend the output frequency range, the master oscillator
signal may be divided by 1, 10 or 100 before driving OUT

(Pin 5). The divide-by value is determined by the state of
the DIV input (Pin 4). Tie DIV to GND or drive it below 0.5V
to select ÷1. This is the highest frequency range, with the
master output frequency passed directly to OUT. The DIV
pin may be fl oated or driven to midsupply to select ÷10,
the intermediate frequency range. The lowest frequency

+

range, ÷100, is selected by tying DIV to V

+

within 0.4V of V

, divider setting and output frequency, including the

R

SET

. Figure 2 shows the relationship between

or driving it to

overlapping frequency ranges near 100kHz and 1MHz.
The CMOS output driver has an on resistance that is typi-

cally less than 100. In the ÷1 (high frequency) mode,
the rise and fall times are typically 7ns with a 5V supply
and 11ns with a 3V supply. These times maintain a clean
square wave at 10MHz (20MHz at 5V supply). In the ÷10
and ÷100 modes, where the output frequency is much lower,
slew rate control circuitry in the output driver increases
the rise/fall times to typically 14ns for a 5V supply and
19ns for a 3V supply. The reduced slew rate lowers EMI
(electromagnetic interference) and supply bounce.

1.4

1.3

1.2

SET

– V

+

1.1

= V

RES

V

1.0

0.9

0.8

Figure 1. V+ – V

V+ = 3V

10.1

V+ = 5V

10 100 1000

I

(µA)

RES

Variation with I

SET

6900 F01

RES

10000

1000

(kΩ)

100

SET

R

Figure 2. R

÷100 ÷10 ÷1

10

1

10k

1k 100k 1M 10M

DESIRED OUTPUT FREQUENCY (Hz)

vs Desired Output Frequency

SET

100M

6900 F02

6

6900fa

APPLICATIONS INFORMATION

LTC6900

SELECTING THE DIVIDER SETTING AND RESISTOR

The LTC6900’s master oscillator has a frequency range
spanning 0.1MHz to 20MHz. However, accuracy may suffer
if the master oscillator is operated at greater than 10MHz
with a supply voltage lower than 4V. A programmable
divider extends the frequency range to greater than three
decades. 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 desired frequency.

In general, any given oscillator frequency (f

) should

OSC

be obtained using the lowest master oscillator frequency.
Lower master oscillator frequencies use less power and
are more accurate. For instance, f
obtained by either R
10MHz or R
The R

SET

= 200k, N = 10, master oscillator = 1MHz.

SET

= 200k approach is preferred for lower power

= 20k, N = 100, master oscillator =

SET

= 100kHz can be

OSC

and better accuracy.

Table 1. Frequency Range vs Divider Setting

DIVIDER SETTING FREQUENCY RANGE

< 100kHz

< 20k),

SET

*

÷1 ⇒ DIV (Pin 4) = GND
÷10 ⇒ DIV (Pin 4) = Floating
÷100 ⇒ DIV (Pin 4) = V

*

At master oscillator frequencies greater than 10MHz (R
the LTC6900 may experience reduced accuracy with a supply voltage
less than 4V.

+

>500kHz

50kHz to 1MHz

After choosing the proper divider setting, determine the
correct frequency-setting resistor. Because of the linear
correspondence between oscillation period and resistance,
a simple equation relates resistance with frequency.

ALTERNATIVE METHODS OF SETTING THE OUTPUT
FREQUENCY OF THE LTC6900

The oscillator may be programmed by any method that
sources a current into the SET pin (Pin 3). The circuit in
Figure 3 sets the oscillator frequency using a programmable
current source and in the expression for f

is replaced by the ratio of 1.1V/I

R

SET

CONTROL

, the resistor

OSC

. As already

explained in the Operation section, the voltage difference

+

between V

and SET is approximately 1.1V, therefore, the
Figure 3 circuit is less accurate than if a resistor controls
the oscillator frequency.

Figure 4 shows the LTC6900 confi gured as a VCO. A voltage
source is connected in series with an external 20k resis­tor. The output frequency, f

, will vary with V

OSC

that is the voltage source connected between V

CONTROL

+

and the

,

SET pin. Again, this circuit decouples the relationship

+

between the input current and the voltage between V
and SET; the frequency accuracy will be degraded. The
oscillator frequency, however, will monotonically increase

OSC

CONTROL

1

2

3

10MHz

.

182kHz TO 18MHz (TYPICALLY ±8%)

+

V

LTC6900

GND

SET

N

EXPRESSED IN (A)

5

OUT

4

N = 1

DIV

6900 F03

20kΩ

••I

1.1V

CONTROL

with decreasing V

+

V

I

CONTROL

1µA TO 100µA

0.1µF

Figure 3. Current Controlled Oscillator

CONTROL

ƒ

I

R

= 20k •

SET

(R

SETMIN

= 10k, R

Any resistor, R
oscillator, f

OSC

100

⎛

10MHz

⎜

N•f

⎝

, tolerance adds to the inaccuracy of the

SET

⎞
⎟

⎠

OSC

SETMAX

, N =

= 2M)

⎧

⎪

10

⎨

1

⎪

⎩

.

+

V

CONTROL

0V TO 1.1V

V

0.1µF

+
–

R

SET

20k

ƒ

OSC

TYPICAL f
±0.5%, V
±8%, V

CONTROL

1

2

3

10MHz

N

OSC

CONTROL

+

OUT

V

LTC6900

GND

SET

DIV

6900 F04

20k

• • 1 –
R

SET

ACCURACY

= 0V

= 0.5V

()

Figure 4. Voltage Controlled Oscillator

5

4

N = 1

V

CONTROL

1.1V

6900fa

7

LTC6900

APPLICATIONS INFORMATION

POWER SUPPLY REJECTION

Low Frequency Supply Rejection (Voltage Coeffi cient)

Figure 5 shows the output frequency sensitivity to power
supply voltage at several different temperatures. The
LTC6900 has a guaranteed voltage coeffi cient of 0.1%/V
but, as Figure 5 shows, the typical supply sensitivity is
twice as low.

High Frequency Power Supply Rejection

The accuracy of the LTC6900 may be affected when its
power supply generates signifi cant noise with a frequency
content in the vicinity of the programmed value of f

OSC

. If a
switching power supply is used to power the LTC6900, and
if the ripple of the power supply is more than 20mV, make
sure the switching frequency and its harmonics are not
related to the output frequency of the LTC6900. Otherwise,
the oscillator may show additional frequency error.

If the LTC6900 is powered by a switching regulator and
the switching frequency or its harmonics coincide with
the output frequency of the LTC6900, the jitter of the
oscillator output may be affected. This phenomenon will
become noticeable if the switching regulator exhibits
ripples beyond 30mV.

START-UP TIME

The start-up time and settling time to within 1% of the
fi nal value can be estimated by t

START

≅ R

+ 10µs. Note the start-up time depends on R

(3.7µs/k)

SET

and it is

SET

independent from the setting of the divider pin. For in­stance with R

= 100k, the LTC6900 will settle with 1%

SET

of its 200kHz fi nal value (N = 10) in approximately 380µs.
Figure 6 shows start-up times for various R

resistors.

SET

Figure 7 shows an application where a second set resistor

is connected in parallel with set resistor R

R

SET2

SET1

via
switch S1. When switch S1 is open, the output frequency
of the LTC6900 depends on the value of the resistor R

SET1

.
When switch S1 is closed, the output frequency of the
LTC6900 depends on the value of the parallel combination
of R

SET1

and R

SET2

.

The start-up time and settling time of the LTC6900 with
switch S1 open (or closed) is described by t

START

shown
above. Once the LTC6900 starts and settles, and switch
S1 closes (or opens), the LTC6900 will settle to its new
output frequency within approximately 70µs.

Jitter

The Peak-to-Peak Jitter vs Output Frequency graph, in the
Typical Performance Characteristics section, shows the
typical clock jitter as a function of oscillator frequency and
power supply voltage. The capacitance from the SET pin,
(Pin 3), to ground must be less than 10pF. If this require­ment is not met, the jitter will increase.

8

0.15
R

= 63.2k

SET

PIN 4 = FLOATING (÷10)

0.10

0.05

0

FREQUENCY DEVIATION (%)

–0.05

2.5

3.0 3.5 4.0 4.5
SUPPLY VOLTAGE (V)

Figure 5. Supply Sensitivity Figure 6. Start-Up Time

25°C

–40°C

85°C

5.0 5.5

6900 F05

70

60

50

40

30

20

FREQUENCY ERROR (%)

10

–10

63.2k

0

20k

200 400 800

0

TIME AFTER POWER APPLIED (µs)

400k

600

TA = 25°C

+

= 5V

V

6900 F06

1000

6900fa

APPLICATIONS INFORMATION

LTC6900

S1

R

SET1

R

SET2

3V OR 5V

1

2

3

+

V

LTC6900

GND

SET

OUT

DIV

6900 F07

5

÷100

4

÷1

Figure 7

f

= 10MHz •

OSC

OR

+

V

f

= 10MHz •

OSC

÷10

20k

()

N • R

SET1

20k

()

//R

N • R

SET1

SET2

A Ground Referenced Voltage Controlled Oscillator

The LTC6900 output frequency can also be programmed by
steering current in or out of the SET pin, as conceptually
shown in Figure 8. This technique can degrade accuracy

+

as the ratio of (V

– V

dependent of the value of R

SET

) / I

is no longer uniquely

RES

, as shown in the LTC6900

SET

Block Diagram. This loss of accuracy will become noticeable
when the magnitude of I

is comparable to I

PROG

RES

. The

frequency variation of the LTC6900 is still monotonic.
Figure 9 shows how to implement the concept shown in

Figure 8 by connecting a second resistor, R
SET pin and a ground referenced voltage source, V

, between the

IN

IN

.

For a given power supply voltage in Figure 9, the output
frequency of the LTC6900 is a function of V

+

and (V

f

OSC

⎡
⎢
⎢

1+

⎢
⎢

⎣

– V

=

()

SET

10MHz

N

− V

V

IN

V

RES

) = V

•

+

:

RES

20k

RINR

⎛
⎜

•

⎜
⎜

1+

⎜

⎝

SET

1

R

R

IN

SET

•

⎤

⎞

⎥

⎟

⎥

⎟

⎥

⎟
⎟

⎥

⎠

⎦

(1)

, RIN, R

IN

SET

When V

= V+, the output frequency of the LTC6900 as-

IN

sumes the highest value and it is set by the parallel com­bination of R

, is independent of the value of V

f

OSC

the accuracy of f
When V

and R

IN

OSC

is less than V+, and expecially when VIN ap-

IN

. Also note, the output frequency,

SET

= (V+ – V

RES

SET

is within the data sheet limits.

) so

proaches the ground potential, the oscillator frequency,

, assumes its lowest value and its accuracy is affected

f

OSC

by the change of V

RES

= (V+ – V
by ±8%, assuming the variation of V
perature coeffi cient of V

is 0.02%/°C.

RES

By manipulating the algebraic relation for f

). At 25°C V

SET

+

is ±5%. The tem-

RES

above, a

OSC

varies

simple algorithm can be derived to set the values of external
resistors R

and RIN, as shown in Figure 9.

SET

1. Choose the desired value of the maximum oscillator

frequency, f
voltage V

IN(MAX)

OSC(MAX)

, occurring at maximum input

≤ V+.

2. Set the desired value of the minimum oscillator fre-

quency, f
V

IN(MIN)

3. Choose V

OSC(MIN)

≥ 0.

RES

, occurring at minimum input voltage

= 1.1 and calculate the ratio of RIN/R

SET

from the following:

R

IN

=

R

SET

⎛

f

V

()

IN(MAX)

− V

V

+

RES

OSC(MAX)

−

⎜

f

⎝

OSC(MIN)

⎡

f

()

OSC(MAX)

⎢

f

⎢

OSC(MIN)

⎣

⎞

V

()

⎟

IN(MIN)

⎠

− V

+

−1

⎤
⎥

−1
⎥

⎦

(2)

+

V

0.1µF

R

SET

I

PR

Figure 8. Concept for Programming via Current Steering

1

+

V

LTC6900

2

GND

3

SET

I

RES

OUT

DIV

6900 F08

5

5V

÷100

4

÷10

OPEN

÷1

+

V

+

0.1µF

V

R

SET

RES

–

R

IN

+

V

IN

–

Figure 9. Implementation of Concept Shown in Figure 8

1

2

3

+

V

LTC6900

GND

SET

OUT

DIV

6900 F09

5

f

OSC

5V

÷100

4

÷10

OPEN

÷1

6900fa

9

LTC6900

APPLICATIONS INFORMATION

Once RIN/R

R

SET

⎡

V

⎢

()

IN(MAX)

⎢
⎢
⎢
⎢

⎣

is known, calculate R

SET

10MHz

=

N

− V

V

•

+

RES

20k

f

OSC(MAX)

+ V

⎛

R

⎜

R

⎝

SET

RES

IN

from:

SET

•

R

IN

SET

⎤

⎞

⎥

⎟

⎠

⎥
⎥
⎥
⎥

⎦

(3)

⎛

1+

⎜

R

⎝

⎞
⎟

⎠

Example 1:
In this example, the oscillator output frequency has small

excursions. This is useful where the frequency of a system
should be tuned around some nominal value.

+

= 3V, f

Let V
f

OSC(MIN)

OSC(MAX)

= 1.5MHz for VIN = 0V. Solve for RIN/R
Equation (2), yielding R
Equation (4). R

IN

tor values, use R
Figure 10 shows the measured f

= 2MHz for V

IN/RSET

= 9.9R

SET

= 110k (1%) and RIN = 1.1M (1%).

SET

= 9.9/1. R

= 1.089M. For standard resis-

IN(MAX)

vs VIN. The 1.5MHz

OSC

= 3V and

= 110.1k by

SET

SET

by

to 2MHz frequency excursion is quite limited, so the curve

vs V

of f

OSC

is linear.

IN

Example 2:
Vary the oscillator frequency by one octave per volt. As-

sume f

OSC(MIN)

= 1MHz and f

OSC(MAX)

= 2MHz, when the
input voltage varies by 1V. The minimum input voltage is
half supply, that is V

+

= 3V.

V
Equation (2) yields R

= 142.8k. RIN = 1.273R

R

SET

IN/RSET

resistor values, use R

= 1.5V, V

IN(MIN)

IN(MAX)

= 1.273 and Equation (3) yields

= 181.8k. For standard

SET

= 143k (1%) and RIN = 182k

SET

(1%). Figure 11 shows the measured f

= 2.5V and

vs VIN. For VIN

OSC

higher than 1.5V, the VCO is quite linear; nonlinearities
occur when V

becomes smaller than 1V, although the

IN

VCO remains monotonic.

Maximum VCO Modulation Bandwidth

The maximum VCO modulation bandwidth is 25kHz; that
is, the LTC6900 will respond to changes in V

at a rate

IN

up to 25kHz. In lower frequency applications however, the
modulation frequency may need to be limited to a lower
rate to prevent an increase in output jitter. This lower limit
is the master oscillator frequency divided by 20, (f

OSC

/20).
In general, for minimum output jitter the modulation fre­quency should be limited to f

/20 or 25kHz, whichever

OSC

is less. For best performance at all frequencies, the value
for f
when V

should be the master oscillator frequency (N = 1)

OSC

is at the lowest level.

IN

10

2.00
RIN = 1.1M

1.95

R

= 110k

SET

+

V

= 3V

1.90
N = 1

1.85

1.80

1.75

(MHz)

OSC

f

1.70

1.65

1.60

1.55

1.50

0

0.5 1 1.5 2 2.5 3

Figure 10. Output Frequency vs Input Voltage

VIN (V)

6900 F10

3000

RIN = 182k
R

= 143k

SET

+

2500

V

= 3V

N = 1

2000

(kHz)

1500

OSC

f

1000

500

0

0

0.5 1 1.5 2 2.5 3

Figure 11. Output Frequency vs Input Voltage

VIN (V)

6900 F11

6900fa

APPLICATIONS INFORMATION

LTC6900

Example 3:

+

= 3V, f

V
V

IN(MAX)

R

IN/RSET

OSC(MAX)

= 2.5V, V

= 8.5, R

= 5MHz, f

IN(MIN)

= 43.2k, RIN = 365k

SET

OSC(MIN)

= 0.5V

= 4MHz, N = 1

Maximum modulation bandwidth is the lesser of 25kHz
or f

OSC(MIN)

Maximum V

/20 (4MHz/20 = 200kHz)

modulation frequency = 25kHz

IN

Example 4:

+

= 3V, f

V
V

IN(MAX)

R

IN/RSET

OSC(MAX)

= 2.5V, V

= 3.1, R

= 400kHz, f

IN(MIN)

= 59k, RIN = 182k

SET

OSC(MIN)

= 0.5V

= 200kHz, N = 10

PACKAGE DESCRIPTION

S5 Package

5-Lead Plastic TSOT-23

(Reference LTC DWG # 05-08-1635)

0.62

MAX

0.95
REF

Maximum modulation bandwidth is the lesser of 25kHz or
f

OSC(MIN)

Maximum V

Table 2. Variation of V

RIN || R

20k 0.98V 1.03V
40k 1.03V 1.08V
80k 1.07V 1.12V
160k 1.1V 1.15V
320k 1.12V 1.17V
V

RES

Note: All of the calculations above assume V
≈ 1.1V. For completeness, Table 2 shows the variation of VRES against
various parallel combinations of R
with V
then recalculate the resistor values using the new value for V

/20 calculated at N =1 (2MHz/20 = 100kHz)

modulation frequency = 25kHz

IN

for Various Values of RIN || R

RES

(VIN = V+)V

SET

= Voltage across R

≈ 1.1V, then use Table 2 to get a better approximation of V

RES

SET

2.90 BSC
(NOTE 4)

, V+ = 3V V

RES

RES

and R

IN

(VIN = V+). Calulate fi rst

SET

RES

= 1.1V, although V

SET

, V+ = 5V

.

RES

RES

RES

,

1.22 REF

3.85 MAX

2.62 REF

RECOMMENDED SOLDER PAD LAYOUT

PER IPC CALCULATOR

0.20 BSC

DATUM ‘A’

0.30 – 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

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 representa­tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

1.4 MIN

0.09 – 0.20
(NOTE 3)

2.80 BSC

1.50 – 1.75
(NOTE 4)

1.00 MAX

PIN ONE

0.95 BSC

0.80 – 0.90

1.90 BSC

0.30 – 0.45 TYP
5 PLCS (NOTE 3)

0.01 – 0.10

S5 TSOT-23 0302 REV B

6900fa

11

LTC6900

TYPICAL APPLICATION

THERMISTOR

Temperature-to-Frequency Converter

5V

R

100k

C1

T

0.1µF

R

: YSI 44011 800 765-4974

T

1

2

3

+

V

LTC6900

GND

SET

OUT

DIV

6900 TA02

5

4

Output Frequency vs Temperature

1400

1200

1000

800

600

FREQUENCY (kHz)

400

MAX
TYP
MIN

f

OSC

=

10MHz

10

20k

•
R

T

200

0

–20–100 102030405060708090

TEMPERATURE (°C)

6900 TA03

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LTC1799 1kHz to 30MHz ThinSOT Oscillator Identical Pinout, Higher Frequency Operation

12

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear.com

6900fa

LT 0709 REV A • PRINTED IN USA

© LINEAR TECHNOLOGY CORPORATION 2002