LTC6906 Linear Technology, LTC6906 Datasheet - Page 8

• Manufacturer Part Number: LTC6906
• Description: Micropower
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
LTC6906
Selecting R
The LTC6906 contains a master oscillator followed by a
digital divider (see Block Diagram). R
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 minimiz-
ing the master oscillator frequency. This implies maxi-
mizing R
N . The relationship between R
power consumption is shown in Figure 8, where we can
clearly see that supply current decreases for large values
of R
page 11.
Minimizing Power Consumption
The supply current of the LTC6906 has four current
components:
An approximate expression for the total supply current is:
V
temperature. This behavior is shown in the Typical Perfor-
mance Characteristics.
Power can be minimized by maximizing R
the load on the OUT pin and operating at lower frequen-
cies. Figure 9 shows total supply current vs frequency
I
8
+
SET
≅ µ +
• Constant (Independent V
• Proportional to I
• Proportional to V
• Proportional to V
≅ µ +
SET
5
5
is approximately 650mV at 25°C, but varies with
A
A
. For a discussion of jitter and divide ratio, refer to
SET
6
6
SET
•
•
and using the lowest possible divider ratio,
I
R
SET
V
SET
SET
and the Divider Ratio
+
+
U
V
SET
+
V
+
+
+
, ƒ
•
and R
ƒ
•
(which is the current in R
OUT
OUT
ƒ
U
OUT
LOAD
and C
•
+
, ƒ
(
•
C
SET
(
LOAD
C
OUT
LOAD
, N and the unloaded
W
LOAD
and C
+
SET
+
5
pF
5
SET
determines the
pF
)
LOAD
+
)
, minimizing
+
2
U
2
)
•
R
V
•
R
LOAD
SET
V
+
LOAD
+
)
under typical conditions. Below 100kHz the load current
is negligible for the 5pF load shown.
Guarding Against PC Board Leakage
The LTC6906 uses relatively large resistance values for
R
SET pin current is typically only 6.5µA. Thus, only 6.5nA
leaking into the SET pin causes a 0.1% frequency error.
Similarly, 1G of leakage resistance across R
R
SET
SET
Figure 7. R
) causes the same 0.1% error.
to minimize power consumption. For R
10000
1000
Figure 8. Unloaded Supply Current vs R
100
10
80
70
60
50
40
30
20
10
0
100
1
SET
vs Desired Output Frequency (LTC6906)
OUTPUT FREQUENCY (kHz)
÷10
10
÷3
R
SET
100
÷1
(kΩ)
1000
C
V
T
LOAD
+
A
= 3V
= 25°C
6906 F07
6906 F09
10000
= 0
1000
SET
SET
SET
= 1M, the
(1000 •
6906fa