Low current flash lamp driving solution based on CP2130

1. Driving requirements for low current flash lamps

For various reasons, the flashlights in most mobile phones, MP4s, PDAs and other products on the market are implemented with low-brightness, low-cost LEDs. Such flashlights are usually composed of several white light LED cores, some of which are directly connected in parallel or in series internally, with only 2 connection pins; some provide 6 pins for users to freely configure in series or parallel form externally. Since the series-connected flashlights need to be driven by a Boost chip based on an inductor, the inductor is too large and the EMI interference is serious, making it difficult to use in handheld devices. Therefore, most handheld device applications currently use parallel-connected flashlights.

Figure 1 Several typical flash tube die connection methods

Limited by the current technological level and price of LEDs, flashes in handheld device applications are usually only allowed to have a continuous current of about 30mA per light and a peak current of no more than 100mA (manufacturers have strict requirements on the duration and frequency of the peak current). Due to the small current, the brightness of the flash is low (about 5000mcd), which obviously cannot meet the requirements of professional camera flashes (mostly xenon lamps). However, because LED flashes are low in cost and simple to implement, they are still the main choice for handheld devices.

To realize the flash function, you need to add a flash driver chip. It is not only required to be able to light the light, but also to be able to flexibly set the working current, and to be able to easily switch between Torch mode and Flash mode. When working in Torch mode, the flash continues to emit light, which can be used as a flashlight or background lighting when shooting short videos, and can also be used as a pre-flash before taking pictures in dark environments. When working in Flash mode, it is used as a short-term flash when taking pictures to increase the exposure. Whether the flash driver chip can flexibly set the driving current and conveniently switch modes determines the ease of use of the chip.

2. Traditional solutions are large in area and low in efficiency

When a larger current is passing through, the forward voltage drop VF (4-4.5V) of the flash tube is usually higher than the voltage of the lithium battery, so most parallel flash driver chips use Charge Pump boost technology to increase the battery voltage to a higher value to drive the LED. In traditional solutions, a Charge Pump chip with a constant voltage output is commonly used to drive a small current flash. Figure 2 shows the application of a typical product. The chip outputs a constant voltage (4.5V or 5V) and sets the current in Torch mode through the RB current limiting resistor; in Flash mode, the Flash Gate (N-channel MOSFET) is closed, and the two current limiting resistors RB and RF are connected in parallel, and the equivalent resistance value is reduced, resulting in a significant increase in the current flowing through the LED lamp, an increase in brightness, and a "flash".

Figure 2 Traditional low current flash lamp driving solution

The solution in Figure 2 has two obvious disadvantages:

A chip works in a fixed 2x boost mode, with constant voltage output and very low efficiency.

B. Because the current through the LED lamp is relatively large in Torch and Flash modes, the current limiting resistor needs to use 0603 or 0805 packaging, and the Flash Gate generally also needs to use SOT23 packaging. This not only increases the circuit area, but also increases the solution cost.

Some improved products use fractional charge pump working mode, which slightly improves the conversion efficiency, but still uses constant voltage output and requires external current limiting resistors and MOS switches, and still cannot overcome the above shortcomings.

3 CP2130's new flash driver solution

Some dedicated flash driver chips on the market can solve the above problems very well, but these products are usually designed for flash applications with currents up to 500mA or even ampere levels, and the chip prices are very high, which forces many customers to continue using traditional solutions.

CP2130 is a charge pump LED driver chip with 1x and 1.5x adaptive mode switching. It can support applications with up to 5 LED lamps and PWM dimming with a wide frequency range. The drive current is set by the RSET resistor of the ISET pin. It uses a small (3mm x 3mm) QFN16 package. Based on the patented Smart Mirror technology, it can provide ultra-precise current matching for each LED channel. The typical application scheme of driving a 2.8-inch LCD backlight is shown in Figure 3.

Figure 3 CP2130 backlight drive solution

The specially designed current drive structure of CP2130 can also meet the driving requirements of small current flashlights. It can ensure the driving capability of 150mA continuous output and 300mA peak output, which meets the driving requirements of small current flashlights of most mobile phones, MP4, PDA and other products. See Figure 4 for a typical application diagram.

Figure 4 CP2130 low current flash driver solution

In the scheme of Figure 4, the EN signal is used as chip enable. When EN is low, the chip enters shutdown mode with extremely low leakage current. When EN is high, the chip is in working state. GPIO-FLASH is used to select and switch between Torch and Flash modes.

When GPIO_FLASH is set to high impedance (or set as an input port), the resistance of ISET is only RSET. At this time, it is set to a smaller drive current and the chip works in Torch mode. When GPIO_FLASH outputs a low level, the equivalent resistance of ISET is the parallel connection of RADJ and RSET. The equivalent resistance value becomes smaller. At this time, it is set to a larger drive current and the chip enters Flash mode. It can be seen that the IO signal of GPIO_FLASH can be used to easily switch between Torch mode and Flash mode.

Taking the typical application of 80mA @ Torch, 250mA @ Flash as an example, the two resistors can be selected according to the following rules: when the output is 80mA, 5 outputs are connected in parallel, which is equivalent to 16mA output per channel. According to Formula 1, RSET is easily obtained to be 10KΩ; when the output is 250mA, it is equivalent to 50mA output per channel. According to Formula 1, the equivalent RSET is easily obtained to be 3.1KΩ (the parallel value of the actual RSET and RADJ); at this time, it can be calculated that RADJ is about 4.5KΩ, and the adjacent nominal resistance can be used.

You can also directly output GPIO_FLASH high level to enter Torch mode, and output low level to enter Flash mode. In this case, IO control is relatively simple, but it is relatively complicated to calculate the resistance values ​​of RSET and RADJ through formula 2.

In formula 1 and formula 2, RSET is the equivalent resistance value of the ISET pin to ground, and ILED is the set output current value of each channel of CP2130. The unit of each resistor in formula 2 is KΩ, and VIO is the IO voltage of GPIO_FLASH in V, such as low level is 0V and high level is 2.8V.

It should be noted that the GPIO port used to control mode switching should not have a pull-up or pull-down resistor inside, otherwise the actual current will deviate from the current calculated by the above formula. Users can choose a port without a pull-up or pull-down resistor, or a port that can turn off the pull-up or pull-down resistor to switch the mode.

Comparing the CP2130 driver solution in Figure 4 with the traditional solution in Figure 2, it is easy to see that:

ACP2130 is an adaptive mode switching charge pump, a current output device with high average efficiency.

B CP2130 only needs a few ordinary resistors and capacitors in 0402 package to set and adjust the flash current. The solution has a small board area and low implementation cost.

4 Problems with practical applications

In actual applications, the placement of the flash unit and its condenser has a certain impact on the brightness and effect. It should be designed reasonably based on the manufacturer's recommendations and the product structure and appearance. At the same time, attention should be paid to the actual heat dissipation of the application. Poor heat dissipation will cause the temperature of the flash to be too high, affecting its life. Usually, an aging test should be conducted on the whole product to test whether the flash is safe when the flash lasts for a long time (such as 200-300ms) and works continuously (such as flashing 5,000 times or 10,000 times).

In addition, I learned that in some products, the flash is directly connected to the battery through a MOSFET load switch, and the flash is driven directly from the lithium battery. This method is very undesirable. Since the voltage of the lithium battery varies greatly, this will cause serious differences in the brightness of the flash. When the battery voltage is low, the flash brightness deteriorates sharply. When the battery voltage is high, the efficiency is low, and power is wasted. Therefore, a flash driver chip must be used to manage the flash.

5 Conclusion

As a parallel LED driver chip, CP2130 is not only capable of backlight driving, but also can meet the application requirements of small current flash. The circuit is simple to implement, the board area is small, and the cost is low. Compared with traditional constant voltage output drive solutions, it has great advantages and is particularly suitable for small current flash drive of handheld devices.