LT2178 datasheet(19/28 Pages) LINER

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LTC6258/LTC6259/LTC6260

6258960fa

For more information www.linear.com/LTC6258

APPLICATIONS INFORMATION

It is interesting to note that the LED current depends on

the supply in this implementation in as much as the supply

feeds through R2 and R3 to create a reference. The supply

also figures into the time of the on and off cycle since the

supply powers the edge detection and relaxation part of

the circuitry. When the supply falls, the LED current drops

and the cycle time increases. This change of behavior

can help in battery powered LED blinking applications to

predict end of life.

The figure shows the sense resistor voltage (red) and the

shutdown pin voltage (blue). The shutdown voltage is

tied to VC; the gate drive couples through C2 as already

described.

Components RF and CF may apparently slow edges down

greatly. Adding this much delay is not essential, but it can

help to smooth out any hiccups that occur when the part

goes through a startup sequence after the shutdown pin

goes inactive. 47µs as a time constant is insignificant in

the time scale of the blinking (10’s or 100’s of ms). The

47µs is much smaller than any time constant associated

with C2 and its resistors.

2N7002

LED

RSENSE

275k

15k

1nF

1µF

RGATE

10k

2N7002

6258 F07a

470k

140k

LTC6258

100

470nF

–

Figure 7a. LED Driver with Self-Oscillation

Figure 7b. LED Blinker Circuit

50ms/Div

–60.0

60.0

120.0

180.0

240.0

300.0

–2.0

–1.0

1.0

2.0

3.0

4.0

6258 F07b

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