New efficient and compact white LED driver solution

New white LED driver topologies offer industry-leading efficiency and simple charge pump architecture without adding cost, external components, or PCB space.

System designers are currently faced with a daunting challenge to maximize system functionality and efficiency while minimizing cost and size with color portable displays. The time has come for a new LED driver topology to be available to system designers. White

LEDs require a supply voltage of approximately 3.6 volts to achieve proper brightness control. However, most handheld devices are powered by lithium-ion batteries, which are approximately 4.2 volts when fully charged and approximately 2.8 volts when safely discharged. Obviously, white LEDs cannot be driven directly from batteries. An alternative solution is to use a boost circuit to increase the drive voltage when needed, thereby providing a constant supply voltage to the LEDs throughout the battery life cycle.

LED drivers used in LCD displays have two requirements. First, they must be able to accurately control and match the brightness of each LED, which will maximize the consistency of the display backlight; second, the LED driver must be able to step up the input battery voltage, which will ensure that sufficient drive voltage is provided to the LEDs throughout the battery life cycle, thereby extending the device's operating time.

Inductor-based LED drivers are commonly used to drive LEDs connected in series, which inherently provides consistent matching. They also provide variable and optimized voltage step-up ratios, resulting in very high power conversion efficiency. However, inductor-based LED driver solutions have significant drawbacks due to the size and cost of external components, as well as annoying electromagnetic interference (EMI). The bulky energy storage inductor limits the application of this solution in small handheld devices with slender and low appearance.

On the other hand, charge pump-based LED drivers provide a very good solution, and their external circuits require only very small capacitors. This makes them ideal for smaller and thinner portable devices that are further driving consumer growth. Each current channel on the charge pump independently drives each parallel-connected LED with a matched current, but the step-up ratio is discrete and determined by different operating modes (multiplication factors). The number of available operating modes and the current battery voltage determine the power efficiency of the entire charge pump.

Common charge pump solutions use two external flying capacitors to provide three operating modes (1x, 1.5x, 2x) for step-up. As the battery is depleted, these devices gradually increase the step-up parameters. In each boost mode, the maximum output voltage is equal to the input battery voltage multiplied by the multiplication factor. The energy of the voltage that exceeds the part necessary to drive the LED will be consumed in the charge pump or current regulator, which reduces the conversion efficiency of the entire circuit.

Embedding more operating modes helps to limit excessive voltage gain throughout the life cycle of the lithium battery, thereby improving efficiency. Some charge pumps currently provide a fourth operating mode (1.33 times), which increases the output voltage by 1 times, 1.33 times, 1.5 times, and 2 times in sequence. The conventional method of achieving a 1.33 times boost requires an increase in the number of device pins and external components, which in turn requires a package with more pins and a larger area of ​​printed circuit board space, making the cost of the entire solution much higher than that of a device with only three operating modes.

Figure 1 By adding a 1.33x operating mode, the efficiency of the charge pump solution is equivalent to that of the inductor-based solution

. The charge pump that boosts the voltage by 1x, 1.33x, 1.5x, and 2x achieves the efficiency of the traditional inductor-based boost converter (Figure 1), while also having all the benefits of low cost and small size associated with the charge pump solution. In addition, by using the 1.33x operating mode, the over-boosted voltage is minimized, thereby reducing power waste and the resulting heat loss (Figure 2).

Figure 2 Comparison of power waste in three-mode and four-mode

An innovative, US patent-pending adaptive fractional charge pump device is now available that enables a fourth charge pump operating mode (1.33x) while maintaining the low cost and simplicity of three-mode (1x, 1.5x, and 2x) devices. The Quad-Mode TM charge pump provides higher efficiency without the need to add external components and the associated cost and printed circuit board space. In addition, the 1.33x fractional operating mode reduces the visible current ripple at the battery terminal. This helps minimize the overall power supply noise, which is a very important indicator in portable devices such as mobile phones.

Figure 3 Conventional 1.33x operation mode requires three external flying capacitors

Conventional 1.33x operation mode (Figure 3) requires three flying capacitors to achieve 1.33x boost by using two-phase conversion (charging and boosting). Catalyst Semiconductor's new 1.33x conversion architecture (Figure 4) achieves 1.33x boost by adding an additional third conversion phase, which eliminates the third external flying capacitor that is usually required.

Figure 4 New Catalyst 1.33x operation mode architecture eliminates the third flying capacitor

In this new 1.33x boost architecture (Figure 4), the first phase action is to connect the flying capacitors C1 and C2 in series and charge them through the input power supply. The second phase action is to disconnect the capacitors C1 and C2 connected to the input power supply and transfer them to the output end to achieve boost. At the same time, capacitor C2 remains floating because it is disconnected from C1. The third phase action is to connect C1 and C2 in series and connect them in series between the input and output to achieve a second boost. Capacitor C1 is reversed in this process. Therefore, the positive terminal of capacitor C1 is connected to the input power supply, and the positive terminal of capacitor C2 is connected to the output end. 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 can increase the output voltage to four-thirds (4/3) times the input voltage.

The steady-state output voltage can be obtained by solving the voltage equations for each phase determined by Kirchhoff's voltage theorem:
Phase 1: VIN = VC1 + VC2 (1)
Phase 2: VOUT = VIN + VC1 (2)
Phase 3: 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 latest product, the CAT3636 (Figure 5), already incorporates this new Quad-Mode TM adaptive fractional charge pump switching architecture. The CAT3636 contains three groups of six LED driver channels, each group containing two strictly regulated and matched channels. Through a single-wire interface (including address and data) logic, complete functional and dimming control can be achieved, which can set each LED group individually and accurately. In portable products with color LCD backlight systems or RGB LED groups or flash functions containing main and sub-displays, this interface also helps to reduce the number of pins and interface connections.

Figure 5 CAT3636 LED driver block diagram: The new four-mode switching architecture eliminates the third external flying capacitor required by conventional solutions

System designers can now use a simple charge pump solution while enjoying efficiency comparable to inductor-based solutions without the added cost, external components and printed circuit board area. With a tiny 3x3mm low-profile RoHS-compliant TQFN package, the introduction of the Catalyst CAT3636 four-mode adaptive fractional charge pump is a leap forward for LED drivers in today's latest portable products.