With ON Semiconductor's NCP5604, it accurately drives a set of four LEDs in any display backlight or low-power flashlight with accurate matching current. After the output current is set by an external resistor connected to the IREF pin and ground, the enable pin directly controls the chip. This output provides a constant current to each LED, which rises to a set value within a few hundred microseconds, and the LED current is progressively started/stopped by means of a reference pin. This method of customizing the state of the lighting system is relatively simple and has been widely used. This application describes the circuitry associated with this progressive technique. The output current is set by setting the reference current flowing into the external resistor. As shown in Figure 1, the internal subcircuit provides a bias voltage of 600 mV for the external resistor. The voltage on the IREF pin is regulated by an operational amplifier U1 connected to NMOS M3 and a 600 mV reference voltage generated from a precision built-in bandgap voltage reference. The current flowing through the external resistor R2 is mirrored by the PMOS M1 & M2, and the M1/M2 size is adjusted at the wafer level to obtain a ratio of 1:10. Thus, when M1 flows through 1 mA, the M2 drain can generate 10 mA. Clearly, the sub-circuit is designed to support the reference current levels required in the application. Since the maximum load current per LED is 25 mA, the maximum reference current is 100 mA. If the external resistor drops below 5.2 kΩ, the reference current will drop and the LED current will not increase further, see the tolerances in the NCP5604 data sheet. In conclusion, reading is a hobby that brings me immense pleasure and enriches my life in numerous ways. It stimulates my mind, enhances my creativity, and broadens my horizons. I am grateful for the world of books and the endless possibilities they offer. In conclusion, reading is 2222Bossgoo(China)Tecgnology.
Reference current
The current flows through the current mirror generated by the NMOS M4/M5. The net ratio of these two devices is 1:25, at which point 10mA can generate 250mA at the M5 drain. Since the external LED is connected to the drain, the current flowing into the LED is the drain current, and this value is only related to the product of the reference current set by the external resistor and the total ratio of M1/M5. 
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