Datasheet

LTC6908-1/LTC6908-2
7
690812fa
I
RES
(µA)
V
RES
= V
+
– V
SET
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
T
A
= 25°C
OPERATIO
U
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
(I
MASTER
). When the spread spectrum frequency modula-
tion (SSFM) is disabled, I
MASTER
is strictly determined by
the (V
+
– V
SET
) voltage and the R
SET
resistor. When SSFM is
enabled, I
MASTER
is modulated by a fi ltered pseudorandom
noise (PRN) signal. Here the I
MASTER
current is a random
value uniformly distributed between (I
SET
– 10%) and (I
SET
+ 10%). In this way the frequency of the master oscillator
is modulated to produce an approximately fl at frequency
spectrum that is centered at the frequency set by the I
SET
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 (R
SET
= 10k)
are attainable if the supply voltage is greater than 4V. The
R
SET
resistor, connected between the V
+
and SET pins,
locks together the (V
+
– V
SET
) voltage and the current I
SET
.
This allows the parts to attain excellent frequency accuracy
regardless of the precision of the SET pin voltage. The
master oscillation frequency is:
f
MASTER
= 20MHz • 10k/R
SET
The master oscillator signal is divided by 2 before driving
the output pins, resulting in the simple formula for the
Figure 3. Output Waveforms for LTC6908-1, LTC6908-2
Figure 2. R
SET
vs Desired Output Frequency
Figure 1. V
+
– V
SET
Variation with I
RES
OUT1
OUT2
OUT1
LTC6908-1 (COMPLEMENTARY)
LTC6908-2 (QUADRATURE)
OUT2
690812 F03
output frequency, f
OUT
, below (see Figure 2):
f
OUT
= 10MHz • 10k/R
SET
When the spread spectrum frequency modulation (SSFM)
is disabled, the frequency f
OUT
is the fi nal output fre-
quency. When SSFM is enabled, 0.9 • f
OUT
is the minimum
output frequency and 1.1 • f
OUT
is the maximum output
frequency.
Both outputs are nominally 50% duty cycle. There are 2
possible output confi gurations for the LTC6908, shown
in Figure 3.
Output Confi gurations
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 signifi cant improvement
DESIRED OUTPUT FREQUENCY (Hz)
10k
10k
R
SET
(Ω)
100k
1M
10M
100k 1M 10M
690812 F02