LT2178 datasheet(18/28 Pages) LINER

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LTC6258/LTC6259/LTC6260

6258960fa

For more information www.linear.com/LTC6258

APPLICATIONS INFORMATION

Figure 6a. Lower Power LED Driver with Voltage Command

Figure 6b. LED Current

–

2N7002

100k

237

LTC6258

2.2k

97.6k

VIN

100nF

6258 F06a

10nF

Indeed, the circuit works nicely. Once the input voltage is

near 0, the LED current output is 0 and the total supply

current is 20µA. Gain from the input voltage to LED cur-

rent is 0.022A/V, as can be taken from the R2/R3 voltage

divider and the sense resistor value.

LED Current =

VIN

•

R2+R3

Self-Oscillating LED Driver

Taking the circuit of the above application a step further,

the circuit of Figure 7a combines edge detection with use

of the shutdown pin of the LTC6259. R2 and R3 bring in

a divided down copy of the supply voltage as a reference

into the positive terminal. The op amp forces this voltage

on the sense resistor RSENSE in “LED ON” operation. In

that sense this circuit is similar to the one above.

20 µA SUPPLY WHEN LED OFF

MEASURED

CALCULATED

INPUT VOLTAGE (V)

100

6258 F06b

However, whereas the previous circuit assumes an

always-on operation mode, this new circuit hijacks the

shutdown pin. C2 can AC couple fast action signals into

the signal VC. Hence when the gate voltage VG increases

when “LED ON” begins, VC will suddenly rise. VC connects

to the shutdown pin; a rising edge on the shutdown pin

enables the LTC6259, which is already active, to stay on.

However, M3 is also on while M1 is on, and as a result

will work with R9 to charge C2 slowly until VC falls below

the shutdown threshold. At that moment, the active low

shutdown kicks in, and the LTC6259 turns off. A negative

falling VG voltage again feeds through C2, and a falling

VC and hence a falling shutdown pin voltage keeps the

circuit in an “LED OFF” state for some time. M3 turns off,

and C2 discharges until VC is high enough to reactivate

the LTC6259.

It may seem a bit odd to develop such a circuit when a

microprocessor or a LTC6992 can provide on-off capabil-

ity in combination with a single MOSFET and resistor. The

problem with those circuits, however, is the lack of control

over the LED current. In the circuit of this application, a

voltage is controlled across a sense resistor. There is no

dependenceontheLEDvoltageinhowmuchcurrentdrives

the LED. And generation of the on-off, or blinking, comes

withtheadditionofonlyahandfuloflowcostcomponents.

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