White LED energy-saving lamps are now widely used in handheld devices (such as mobile phones) as backlights for color LCD screens and keyboard lights. The color LED energy-saving lamp is a good indicator to inform users of incoming calls and battery charging. The brightness of LED energy-saving lamps is proportional to the passing current, and the voltage must be high enough to turn them on. The most popular battery today is the lithium battery, whose voltage is usually around 3.6V, but as the battery discharges, the voltage will drop. For these reasons, LED energy-saving lamps cannot be directly powered by this battery, and an adjustable boost converter is necessary. Adjustable charge pump management solutions are very popular, especially in simple and low-cost applications where the total output current is less than 100mA.

1 Differential current drive difference LED energy-saving lamp

The output adjustable charge pump, such as the CAT3200-5 from Catalyst, is one of the aspiring LED energy-saving lamp drivers, which can provide up to 100mA load current while achieving a 5V regulated output. The current of the white LED energy-saving lamp used for LCD backlight is usually 20mA, and the forward voltage VF is about 3.4V at this time. The current on each LED energy-saving lamp is set by a series resistor, which is also called a ballast resistor (see Figure 1). This structure allows different currents to be passed in parallel on the LED energy-saving lamp. A larger resistance value means a lower current on the LED energy-saving lamp. Formula 1 can be used to approximate the current on LED energy-saving lamps (ignoring the interconnection consumption).

Formula 1: VOUT = VF + (LED energy-saving lamp current & TImes; RS)

Here, VF is the forward voltage of the LED energy-saving lamp, and RS is the series resistance.

The output voltage VOUT is stable at 5V. When the forward voltage VF is 3.4V, the current on the LED energy-saving lamp is 20mA, then RS = (5V-3.4V) / 20mA, which is 80 ohms. In this example, three white LED energy-saving lamps are used as the backlight of the screen, and two color LED energy-saving lamps (red and blue, respectively) are used as indicators. Each color LED energy-saving lamp has different forward voltage characteristics. The forward voltage of the red LED energy-saving lamp is very low, and the voltage is usually 2.8V when passing a current of 20mA. Since the indicator LED energy-saving lamp does not eliminate lighting, its current is smaller than the backlight LED energy-saving lamp.

The CAT3200-5 charge pump is a voltage doubler. Only a large capacitor is needed as a storage container to transfer charge from the input to the output. Here we recommend the use of 1μF ceramic capacitors. All the external components required are the other two 1μF capacitors, which are placed on the input and output pins, respectively.

There are other LED energy-saving lamp price driver implementation solutions on the market, such as inductive boost or current-controlled charge pumps. The inductive boost driver uses an inductor to increase the input voltage to drive several LED energy-saving lamps in series. Its advantage is that it can provide the same current on these LED energy-saving lamps, but this structure should not be used as the above application example. Another type of LED energy-saving lamp driver is a current-controlled charge pump. By using an on-chip current regulator, a separate LED energy-saving lamp channel can drive an LED energy-saving lamp. In this case, the size of the current on the LED energy-saving lamp is set by an external resistor. In larger drives, it can be programmed into the chip through a digital interface. The benefit of this structure is that ballast resistance is no longer necessary. Since the current on all LED energy-saving lamps is the same, this management scheme should not be used as the application we discuss.

2 Inrush current

The most important titles often appear when the system is powered on. When the signal from the "enable" input pin transitions from logic low to logic high, the device turns on and starts charging the large capacitor. As a result, the input current increases sharply within a short period of time, producing a so-called "surge" current. One indicator used to profile the advantages of power management chips is how they are reflected in monitoring input current. The risk of high surge current is that the internal rail voltage (VRAIL) drops momentarily and affects the system operation. The drop in rail voltage is a function of the output impedance RS of the power supply and the interconnection impedance RINTER.

The calculation formula of the system rail voltage is: VRAIL = VBAT-IIN & TImes; (RSINTER,

Here, VBAT is the battery voltage and IIN is the input current. For example, if the total series resistance is 1.5 (and the inrush current is 0.5A, the rail voltage is: VRAIL = 3.6V- (0.5A & TImes; 1.5 ohms) = 2.85V.

Low voltage can cause the system to collapse. One way to eliminate the battery output surge current is to increase the input capacitance CIN. As a result, most of the current for charging large capacitors is provided by the capacitor CIN, which can eliminate the inrush current.

The input current waveform reflects that the input current gradually increases when the output voltage increases linearly to the rated 5V. The initial current glitch has only a short 2μs continuous time, so there is no harm, so it has little effect on the input voltage.

The adjustable charge pump provides a good management solution for driving LED energy-saving lamps, and is easy to implement on the PCB board. When selecting this device, some key parameters that must be considered include the output current size, input voltage range, power-up sequence, switching frequency, and low noise event performance.


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