Datasheet
LTC6906
8
6906fc
APPLICATIONS INFORMATION
Selecting R
SET
and the Divider Ratio
The LTC6906 contains a master oscillator followed by a
digital divider (see the Block Diagram). R
SET
determines
the master oscillator frequency and the DIV pin sets the
divider ratio, N. The range of frequencies accessible in
each divider ratio overlap, as shown in Figure7. This figure
is derived from the equations in Table 1. For any given
frequency, power can be minimized by minimizing the
master oscillator frequency. This implies maximizing
R
SET
and using the lowest possible divider ratio, N. The
relationship between R
SET
, N and the unloaded power
consumption is shown in Figure 8, where we can clearly
see that supply current decreases for large values of R
SET
.
For a discussion of jitter and divide ratio, refer to page 11.
Minimizing Power Consumption
The supply current of the LTC6906 has four current
components:
• Constant (Independent V
+
, ƒ
OUT
and C
LOAD
)
• Proportional to I
SET
(which is the current in R
SET
)
• Proportional to V
+
, ƒ
OUT
and C
LOAD
• Proportional to V
+
and R
LOAD
An approximate expression for the total supply current is:
I
+
≅ 5μA + 6•I
SET
+ V
+
•ƒ
OUT
•C
LOAD
+ 5pF
()
+
V
+
2•R
LOAD
≅ 5μA + 6•
V
SET
R
SET
+ V
+
•ƒ
OUT
•C
LOAD
+ 5pF
()
+
V
+
2•R
LOAD
V
SET
is approximately 650mV at 25°C, but varies with
temperature. This behavior is shown in the Typical Per-
formance Characteristics section.
Power can be minimized by maximizing R
SET
, minimizing
the load on the OUT pin and operating at lower frequen-
cies. Figure 9 shows total supply current vs frequency
under typical conditions. Below 100kHz the load current
is negligible for the 5pF load shown.
Figure 7. R
SET
vs Desired Output Frequency (LTC6906)
Figure 8. Unloaded Supply Current vs R
SET
OUTPUT FREQUENCY (kHz)
R
SET
(kΩ)
1000
10000
1 100 1000 10000
6906 F07
10
10
100
10
3
1
R
SET
(kΩ)
50
I
SUPPLY
(A)
70
100 1000
6906 F08
30
40
60
80
20
10
0
C
LOAD
= 0
V
+
= 3V
T
A
= 25°C
Figure 9. Supply Current vs Frequency
MASTER OSCILLATOR FREQUENCY (kHz)
0
0
POWER SUPPLY CURRENT (A)
10
30
40
50
800
6906 F09
20
400
200
1000
600 1200
60
1
3
10
70
80
V
+
= 2.7V