Datasheet

LTC1041
6
1041fa
APPLICATIO S I FOR ATIO
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If the best possible performance is needed, the inputs to
the LTC1041 must completely settle within 4µs of the start
of the comparison cycle (V
P-P
high impedance to V
+
transition). Also, it is critical that the input voltages do not
change during the 80µs active time. When driving resistive
input networks with V
P-P
, capacitive loading should be
minimized to meet the 4µs settling time requirement.
Further, care should be exercised in layout when driving
networks with source impedances, as seen by the LTC1041,
of greater than 10kΩ (see For R
S
> 10kΩ).
Figure 5. Driving Reference with V
P-P
Output
V
IN
I
L
LTC1041 • AI05
LT1009-2.5
8
7
6
5
1
2
3
4
LTC1041
SET POINT
DELTA
R2
R3
R4
R1
V
+
Figure 3. V
P-P
Output Switch
LTC1041 • AI03
80µs
COMPARATOR ON TIME
8
V
+
47
GND
Q1 P1
V
P-P
Figure 4. Ratiometric Network Driven by V
P-P
R3
R1
SET POINT
GND DELTA
V
IN
V
+
V
P-P
R2
R4
R5
R6
LTC1041 • AI04
8
7
6
5
1
2
3
4
LTC1041
In applications where an absolute reference is required,
the V
P-P
output can be used to drive a fast settling
reference. The LTC1009 2.5V reference settles in ≈ 2µs
and is ideal for this application (Figure 5). The current
through R1 must be large enough to supply the LT1009
minimum bias current (≈ 1mA) and the load current, I
L
.
Internal Oscillator
An internal oscillator allows the LTC1041 to strobe itself.
The frequency of the oscillation, and hence the sampling
rate, is set with an external RC network (see typical curve,
Sampling Rate R
EXT
, C
EXT
). R
EXT
and C
EXT
are connected
as shown in Figure 1. To assure oscillation, R
EXT
must be
between 100kΩ and 10MΩ. There is no limit to the size of
C
EXT
.
At low sampling rates, R
EXT
is very important in
determining the power consumption. R
EXT
consumes
power continuously. The average voltage at the OSC pin
is approximately V
+
/2, giving a power dissipation of
P
REXT
= (V
+
/ 2)
2
/R
EXT
.
Example: assume R
EXT
= 1MΩ, V
+
= 5V, P
REXT
=
(2.5)
2
/10
6
= 6.25/µW. This is approximately four times the
power consumed by the LTC1041 at V
+
= 5V and
f
S
= 1 sample/second. Where power is a premium,
R
EXT
should be made as large as possible. Note that the
power dissipated by R
EXT
is not a function of f
S
or C
EXT
.
If high sampling rates are needed and power consumption
is of secondary importance, a convenient way to get the
maximum possible sampling rate is to make R
EXT
= 100kΩ
and C
EXT
= 0. The sampling rate, set by the controller’s
active time, will nominally be ≈ 10kHz.
To synchronize the Sampling of the LTC1041 to an
external frequency source, the OSC pin can be driven by a
CMOS gate. A CMOS gate is necessary because the input
trip points of the oscillator are close to the supply rails and
TTL does not have enough output swing. Externally driven,
there will be a delay from the rising edge of the OSC input
and the start of the sampling cycle of approximately 5µs.