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LTC6258 LED Driver with Self-Oscillation

Figure 7a. LED Driver with Self-Oscillation

The circuit above 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.

This 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 capability 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 dependence on the LED voltage in how much current drives the LED. And generation of the on-off, or blinking, comes with the addition of only a handful of low cost components.

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.

Figure 7b. LED Blinker Circuit

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.

The LTC®6258/LTC6259/LTC6260 are single/dual/quad operational amplifiers with low noise, low power, low supply voltage, and rail-to-rail inputs and outputs. They are unity gain stable with or without capacitive loads. They feature 1.3MHz gain-bandwidth product, 0.24V/μs slew rate while consuming only 20μA of supply current per amplifier operating on supply voltages ranging from 1.8V to 5.25V. The combination of low supply current, low supply voltage, high gain bandwidth product and low noise makes the LTC6258 family unique among rail-to-rail input/output op amps with similar supply current. These operational amplifiers are ideal for power efficient applications.




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