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 Figure7. 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
+
A + 6•I
SET
+ V
+
•ƒ
OUT
•C
LOAD
+ 5pF
()
+
V
+
2•R
LOAD
A + 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