LTC6908CDCB-1 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC6908CDCB-1

Resistor Set SOT-23 Oscillator with Spread Spectrum Modulation

Linear Technology 

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OPERATIO

As shown in the Block Diagram, the LTC6908’s master

oscillator is controlled by the ratio of the voltage between

the V+ and SET pins and the current entering the SET pin

(IMASTER). When the spread spectrum frequency modula-

tion (SSFM) is disabled, IMASTER is strictly determined by

the (V+ – VSET) voltage and the RSET resistor. When SSFM is

enabled, IMASTER is modulated by a filtered pseudorandom

noise (PRN) signal. Here the IMASTER current is a random

value uniformly distributed between (ISET – 10%) and (ISET

+ 10%). In this way the frequency of the master oscillator

is modulated to produce an approximately flat frequency

spectrum that is centered at the frequency set by the ISET

current, with a bandwidth equal to approximately 20% of

the center frequency.

The voltage on the SET pin is forced to approximately 1.1V

below V+ by the PMOS transistor and its gate bias volt-

age. This voltage is accurate to ±5% at a particular input

current and supply voltage (see Figure 1). The LTC6908

is optimized for use with resistors between 20k and 400k,

corresponding to output frequencies between 250kHz and

5MHz. Accurate frequencies up to 10MHz (RSET = 10k)

are attainable if the supply voltage is greater than 4V. The

RSET resistor, connected between the V+ and SET pins,

locks together the (V+ – VSET) voltage and the current ISET.

This allows the parts to attain excellent frequency accuracy

regardless of the precision of the SET pin voltage. The

master oscillation frequency is:

fMASTER = 20MHz • 10k/RSET

The master oscillator signal is divided by 2 before driving

the output pins, resulting in the simple formula for the

1.4

TA = 25°C

1.3

1.2

V+ = 5V

1.1

V+ = 3V

1.0

0.9

0.8

0.1

1

10

100

1000

IRES (µA)

690812 F01

Figure 1. V+ – VSET Variation with IRES

LTC6908-1/LTC6908-2

output frequency, fOUT, below (see Figure 2):

fOUT = 10MHz • 10k/RSET

When the spread spectrum frequency modulation (SSFM)

is disabled, the frequency fOUT is the final output fre-

quency. When SSFM is enabled, 0.9 • fOUT is the minimum

output frequency and 1.1 • fOUT is the maximum output

frequency.

Both outputs are nominally 50% duty cycle. There are 2

possible output configurations for the LTC6908, shown

in Figure 3.

Output Configurations

The only difference between the two versions of the

LTC6908 is the phase relationship between the two outputs.

The LTC6908-1 outputs are 180 degrees out of phase and

the LTC6908-2 outputs are 90 degrees out of phase. These

convenient output options are useful in synchronizing the

clocking of multiple phase switching regulator designs. In

very high current applications, a significant improvement

10M

1M

100k

10k

10k

100k

1M

10M

DESIRED OUTPUT FREQUENCY (Hz)

690812 F02

Figure 2. RSET vs Desired Output Frequency

LTC6908-1 (COMPLEMENTARY)

OUT1

OUT2

LTC6908-2 (QUADRATURE)

OUT1

OUT2

690812 F03

Figure 3. Output Waveforms for LTC6908-1, LTC6908-2

690812fa

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