LTC6907 datasheet(16/24 Pages) AD

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LTC6991

Rev. D

For more information www.analog.com

Self-Resetting Circuits

The RST pin has hysteresis to accommodate slow-

changing input voltages. Furthermore, the trip points

are proportional to the supply voltage (see Note 6 and

the RST Threshold Voltage vs Supply Voltage curve in

Typical Performance Characteristics). This allows an RC

time constant at the RST input to generate a delay that is

nearly independent of the supply voltage.

A simple application of this technique allows the LTC6991

output to reset itself, producing a well-controlled pulse

once each cycle. Figure 13a and Figure 13b show circuits

that produce approximately 1µs pulses once a minute.

The only difference is in the POL bit setting, which con-

trols whether the pulse is positive or negative.

APPLICATIONS INFORMATION

LTC6991

2.25V TO 5.5V

0.1µF

6991 F13b

OUT

392k

60 SECONDS

0.9µs PULSE WIDTH

VRST(FALLING)

V+

tPULSE = –RPW • CPW • In

tPULSE ≈ –2.26k • 470pF • In(0.43)

tPULSE ≈ 0.9µs

RSET

715k

RST

GND

SET

OUT

V+

DIV

CPW

470pF

RPW

2.26k

(

)

6991 F14

LTC6991

RST

GND

SET

OUT

V+

DIV

0.1µF

RSET

RVCO

VCTRL

V+

(a) Self-Resetting Circuit (DIVCODE = 4)

Figure 13b. Self-Resetting Circuit (DIVCODE = 11)

Figure 13.

Figure 14. Voltage-Controlled Oscillator

LTC6991

2.25V TO 5.5V

0.1µF

OUT

392k

RSET

715k

RST

GND

SET

OUT

V+

DIV

CPW

470pF

RPW

2.26k

6991 F13a

60 SECONDS

1µs PULSE WIDTH

VRST(RISING)

V+

tPULSE = –RPW • CPW • In

tPULSE ≈ –2.26k • 470pF • In(1 – 0.61)

tPULSE ≈ 1µs

1–

(

)

Voltage Controlled Frequency

With one additional resistor, the LTC6991 output frequency

can be manipulated by an external voltage. As shown in

Figure 14, voltage VCTRL sources/sinks a current through

RVCO to vary the ISET current, which in turn modulates the

output frequency as described in Equation (3).

fOUT =

1MHz • 50kΩ

1024 • NDIV • RVCO

• 1+

RVCO

RSET

−

VCTRL

VSET

⎛

⎝

⎜

⎞

⎠

⎟ (3)

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