LTC6908-1/LTC6908-2

7

Rev C

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IRES ( A)

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.1

10

100

1000

690812 F01

1

V+ = 5V

V+ = 3V

TA = 25C

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 pseudoran-

dom 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 approxi-

mately 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 volt-

age. The master oscillation frequency is:

fMASTER = 20MHz • 10k/RSET

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

Figure 2. RSET vs Desired Output Frequency

Figure 1. V+ – VSET Variation with IRES

OUT1

OUT2

OUT1

LTC6908-1 (COMPLEMENTARY)

LTC6908-2 (QUADRATURE)

OUT2

690812 F03

The master oscillator signal is divided by 2 before driving

the output pins, resulting in the simple formula for the

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

DESIRED OUTPUT FREQUENCY (Hz)

10k

10k

100k

1M

10M

100k

1M

10M

690812 F02

OPERATION