An LED is commonly used as a "power on" indicator for many electronic devices. For the LED to produce discernible visible light in daylight, the forward-bias current needs to be in the moderate range (10 to 20 mA). This amount of current may be too large for many low-power designs. Also, it results in wasted power. All that is needed in most cases is a once-in-a-while or on-demand indication that there is power to an electronic device. In Figure 1, National Semiconductor's LM3710, a supervisory circuit, is configured to reduce the power consumption of a traditional LED indicator by 99%.
When VIN is above 4.4 V, green LED1 blinks on for 200 ms and off for 25.6 s, repeatedly. IC1's reset timeout pulses the LED1 on for 200 ms. This 200-ms on period is customizable from 1.4 to 1600 ms at the factory. The LED1 off period is controlled by IC1's watchdog timeout period, which is also customizable from 6.2 ms to 25.6 s.
The watchdog input monitors transitions at WDI. If there are no changes at WDI, then the reset engages. The WDI in Figure 1 is grounded to prevent a change. Thus, the reset timeout and watchdog timeout form two one-shots that produce a repetitive pulse train. The 25.6-s pause between the LED1 flashes can be short-cycled by pressing the momentary switch, SW1.
LED1's current is limited by IC1's RESET pin, which is approximately 13 mA. The 28-µA quiescent current of IC1 can be ignored. The approximate VIN set-point equation to initiate blinking is:
VIN(BLINK) = 3.08 V + R1
(IRESET)
where
IRESET = 13 mA
(for R1 < 200 Ω)
The average current in LED1 is reduced by the ratio of the two time intervals. A 13-mA continuous bias current would be reduced to 0.1 mA in average LED1 current—a 130× power savings.
If VIN goes above 5.5 V, red LED2 illuminates as an overvoltage indicator. The resistor divider of R2 and R3 and the PFI input of IC1 set this power-fault level. IC1's PFI threshold voltage is 1.225 V. This overvoltage condition causes the PFO output to drive LED2 continuously. The VIN set-point equation to initiate an overvoltage condition is:
VIN (OVERVOLTAGE) =
(1.225 V )(R2 + R3)/R3.
In the brief time before VIN reaches 4.4 V, the LED1 is on continuously. Figure 2 shows the current-voltage waveform of the circuit.
Here are a couple of other examples. For a 5-V, ±5% monitor, set R1 = 121 Ω and R3 = 301 kΩ. For a 3.3-V, ±10% monitor, set R1 = 0 Ω and R3 = 511 kΩ. Keep the absolute maximum ratings of IC1 in mind during the design.
An optional capacitor, C1, can be added to provide an energy reservoir that will increase the brightness of LED1's flash and its average current. Values in the microfarad range are needed to create a noticeable change.
