With no additional cost, peripheral components and printed circuit board space, the new white LED driver topology delivers industry-leading efficiency and simple architecture charge pumps.
System designers are currently faced with the daunting challenge of using color portable displays to maximize system functionality and efficiency while minimizing cost and size. It is now time to provide a new LED driver topology for system designers.
White LEDs require a supply voltage of approximately 3.6 volts to achieve proper brightness control. However, most handheld devices use a Li-Ion battery as the power source. They are about 4.2 volts after being fully charged, and about 2.8 volts after being safely discharged. Obviously, white LEDs cannot be directly driven by the battery. An alternative solution is to use a boost circuit that boosts the drive voltage when needed to provide uninterrupted power supply to the LED throughout the battery life cycle.
There are two requirements for the LED driver used in the LCD display. First of all, they must be able to accurately control and match the brightness of each LED, which will maximize the consistency of the display backlight; secondly, the LED driver should be able to increase the input battery voltage, which will ensure the entire battery life cycle. It can provide enough driving voltage for the LED to extend the use time of the device.
Inductor-based LED drivers are typically used to drive string-connected LEDs, and the structure itself provides consistent matching. They also offer variable and optimized voltage rise ratios and therefore have very high power conversion efficiencies. However, inductor-based LED driver solutions have significant drawbacks due to the size and cost of external components and the nuisance of electromagnetic interference (EMI). The bulky energy storage inductors limit the use of this solution in small, low-profile handheld devices.
On the other hand, the charge pump type LED driver provides a very good solution, and the external circuit only needs to use a very small capacitance. This makes it an ideal choice for smaller, thinner portable devices that further drive consumption growth. Each current channel on the charge pump independently drives each of the parallel connected LEDs using a matched current, however, the boost ratio is discrete and depends on the mode of operation (multiplication factor). The number of operating modes available and the current battery voltage determine the power efficiency of the entire charge pump.
A common charge pump solution uses two external flying capacitors to provide three modes of operation (1x, 1.5x, 2x) for boosting. These devices sequentially increase the boost parameters as the battery is consumed. In each boost mode, the maximum output voltage is equal to the input battery voltage multiplier factor. The energy that exceeds the portion of the voltage necessary to drive the LED will be dissipated in the charge pump or current regulator, which reduces the conversion efficiency of the entire circuit.
Embedding more modes of operation helps to limit the excessive voltage gain over the life of the lithium battery, thereby increasing efficiency. Some charge pumps currently offer a fourth mode of operation (1.33 times), which increases the output voltage by 1x, 1.33x, 1.5x, and 2x. The conventional method of achieving 1.33 times boost requires an increase in the number of device pins and external components. Accordingly, more pin packages and a larger area of ​​printed circuit board space are required, which makes the cost of the entire solution much higher than that of the entire solution. Three operating modes of the device.
Figure 1. By adding a 1.33x operating mode, the efficiency of the charge pump solution is equivalent to an inductor-based solution.
The charge pump that boosts the voltage in 1x, 1.33x, 1.5x, and 2x order achieves the efficiency of traditional inductor-based boost converters (Figure 1), while also having the low cost and small cost associated with charge pump solutions. The full benefits of size. In addition, by using the 1.33x operating mode, the voltage that is too high is limited as much as possible, thereby reducing power waste and heat loss (Figure 2).
Figure 2 Comparison of power waste in three-mode and four-mode
There is now an innovative, patent-pending, adaptive fractional-charge pump device that will achieve the fourth while maintaining the simplicity of low-cost and three-mode (1x, 1.5x, and 2x) devices. Charge pump operation mode (1.33 times). The Quad-ModeTM charge pump provides greater efficiency without the need to add external components and associated cost and printed circuit board space. In addition, the 1.33x fractional mode of operation also reduces visible current ripple at the battery terminals. This helps minimize the overall power supply noise, which is an important indicator in portable devices such as mobile phones.
Figure 3. The conventional 1.33x operating mode requires three external flying capacitors.
The conventional 1.33x operating mode (Figure 3) requires three flying capacitors to achieve a 1.33x boost by using two-phase conversion (charging and boosting). Catalyst Semiconductor's new 1.33x conversion architecture (Figure 4) completes the 1.33x boost by adding an additional third conversion phase, which eliminates the need for a third external capacitor.
Figure 4 The new Catalyst 1.33x operating mode architecture eliminates the third flying capacitor
In this new 1.33x boost architecture (Figure 4), the first phase action is to connect the fly capacitors C1 and C2 in series and charge them through the input supply. The second phase action is to connect the capacitor C1 connected to the input supply. C2 is disconnected and transferred to the output for boosting. At the same time, capacitor C2 remains floating due to disconnection from C1. The third phase action is to cascade C1 and C2 and connect the input and output in series to achieve the second boost. Capacitor C1 is reversely connected in the process. Therefore, the anode of capacitor C1 is connected to the input power. The anode of capacitor C2 is connected to the output. Through this three-phase operation, C1 will be charged to one-third of the input voltage, and C2 will be charged to two-thirds of the input voltage, which will raise the output voltage to four-quarters of the input voltage (4) /3 times.
The steady-state output voltage can be obtained by solving the equation for each phase of the voltage determined by Kirchhoff's voltage theorem:
First phase: VIN = VC1 + VC2 (1)
Second phase: VOUT = VIN + VC1 (2)
Third phase: VOUT = VIN - VC1 + VC2 (3)
Replace (2) with (3):
VIN + VC1 = VIN - VC1 + VC2 (4)
VC2 = 2 VC1 (5)
Replace (5) with (1):
VC1 = 1/3 VIN (6)
Replace (6) with (2):
VOUT = 4/3 VIN (7)
Catalyst Semiconductor's newest product, the CAT3636 (Figure 5), already includes this new Quad-Mode TM adaptive fractional charge pump switching architecture. The CAT3636 consists of three groups of six LED drive channels, each consisting of two strictly steady current and matched channels. Complete functionality and dimming control are achieved through a single-wire interface (including address and data) logic, which allows individual and precise settings for individual LED groups. In portable color products with primary and secondary display color LCD backlight systems or RGB LED groups or flash functions, this interface also helps reduce pin and interface connections.
Figure 5 CAT3636 LED Driver Block Diagram: The new four-mode switching architecture eliminates the need for a third external flying capacitor for conventional solutions
System designers can now enjoy efficiency and aesthetics and inductor-based solutions with a simple charge pump solution without the need for additional cost, external components and printed circuit board area. The introduction of the Catalyst CAT3636 four-mode adaptive fractional charge pump is a leap forward for LED drivers in the latest portable products due to the RoHS-compliant miniature 3x3mm low-profile TQFN package.
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