LTC1041CS8 datasheet(6/8 Pages) LINER

6 / 8 page

LTC1041

1041fa

APPLICATIO S I FOR ATIO

If the best possible performance is needed, the inputs to

the LTC1041 must completely settle within 4

µs of the start

of the comparison cycle (VP-P high impedance to V+

transition). Also, it is critical that the input voltages do not

change during the 80

µsactivetime.Whendrivingresistive

input networks with VP-P, capacitive loading should be

minimized to meet the 4

µs settling time requirement.

Further, care should be exercised in layout when driving

networks with source impedances, as seen by the LTC1041,

of greater than 10k

Ω (see For RS > 10kΩ).

Figure 5. Driving Reference with VP-P Output

VIN

LTC1041 • AI05

LT1009-2.5

LTC1041

SET POINT

DELTA

V+

Figure 3. VP-P Output Switch

LTC1041 • AI03

µs

COMPARATOR ON TIME

V+

GND

VP-P

Figure 4. Ratiometric Network Driven by VP-P

SET POINT

GND

DELTA

VIN

V+

VP-P

LTC1041 • AI04

LTC1041

In applications where an absolute reference is required,

the VP-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, IL.

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 REXT, CEXT). REXT and CEXT are connected

as shown in Figure 1. To assure oscillation, REXT must be

between 100k

Ω and 10MΩ. There is no limit to the size of

CEXT.

At low sampling rates, REXT is very important in

determining the power consumption. REXT consumes

power continuously. The average voltage at the OSC pin

is approximately V+/2, giving a power dissipation of

PREXT = (V+/ 2)2/REXT.

Example:

assume REXT = 1MΩ, V+ = 5V, PREXT =

(2.5)2/106 = 6.25/

µW. This is approximately four times the

power consumed by the LTC1041 at V+ = 5V and

fS = 1 sample/second. Where power is a premium,

REXT should be made as large as possible. Note that the

power dissipated by REXT is not a function of fS or CEXT.

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 REXT = 100kΩ

and CEXT = 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 5

µs.

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