Application of White LED Driver Cat3636 in Portable Equipment

At present, various consumer electronic products are constantly given new functional features, and with the improvement of complexity, the corresponding power consumption also increases. How to avoid the excessive growth of power consumption by improving the design has become an important topic in front of hardware design engineers. For example, for products such as mobile phones, PMP and GPS, the power consumed by the LCD backlight accounts for a very large proportion of the power consumption of the whole machine, and the LCD screen has a further increasing trend, which will further increase the corresponding power. Therefore, how to reduce the power consumption of LCD backlight has become one of the primary problems to reduce the system power consumption.

At present, the mainstream architectures of LCD screens on the market are CSTN and TFT, both of which need a certain brightness backlight to achieve visible images. The brightness demand of the backlight is basically proportional to the size of the LCD screen, but also related to the surrounding environment and the subjective feeling of users. The more popular backlight scheme in China is that the LCD screen below 2.8 inches adopts 2 4 white LEDs, and the LCD screen of 2.8 4.3 inches adopts more than 5 white LEDs. Considering the service life, index and price of LED, the current of single LED is generally controlled at 15 25mA.

Most portable devices are powered by lithium batteries. The discharge range of lithium battery is about 3.2 4.2V, while the forward voltage required for white LED in normal operation is between 3.2 3.8V (if = 20mA @ VF = 3.5V). When the load of the system increases suddenly, it will cause battery voltage fluctuation. Therefore, driving white led directly with lithium battery will cause abnormal phenomena such as dark screen or flashing screen.

The solution to this problem is to insert a primary boost driving circuit between the lithium battery and the white LED. At present, there are two commonly used architectures: one is inductor boost type, and the other is charge pump boost type.

For the driving scheme of 5 6 white LEDs in portable devices, although the inductive boost architecture has more high efficiency advantages than the traditional charge pump boost architecture, noise and the size of peripheral devices are still difficult to overcome in this scheme. In order to drive 5 6 white LEDs arranged in series, the inductive boost chip needs to output at least 16 (3.2V / LED & tips; 5) 21V (3.5V / LED & tips; 6) driving voltage, and its inherent switching circuit ripple is a large noise source for other small signal circuits. If the PWM signal is used to control the enable end of the boost chip for dimming operation, not only the PWM signal itself, but also the fluctuating driving voltage will have an adverse impact on other circuits.

Cat3636 is a charging pump capacitor boost white LED driver, which is especially suitable for LCD screen with 5 6 white LEDs as backlight. Similar to the popular capacitor boost architecture, it does not need inductors, only a few small capacity ceramic capacitors are needed in the periphery, and the small-size 3mm & tides are also used in itself; 3mm square tqfn-16 package. Different from the popular capacitor boost architecture, it adopts the patented Quad modetm charge pump technology of catalyst semiconductor company, which can effectively improve the conversion efficiency of backlight LED driver and reduce the power consumption of backlight circuit.

At present, most popular charge pump LED drivers provide only three working modes according to the ratio of output voltage to input voltage: 1 times, 1.5 times and 2 times. Quad modetm architecture adds a fourth working mode - 1.33 times. The 1.33x working mode makes the output boost voltage as small as possible, which greatly reduces the useless power consumption and subsequent heat loss of the device. At the same time, the 1.33 times working mode also effectively reduces the input switching current at the battery end, which not only effectively prolongs the working time of the battery, but also minimizes the input noise of the whole system. In particular, in order to realize the 1.33 times working mode, the cat3636 still adopts the peripheral configuration of the popular charge pump LED driver, and only two flying capacitors are used to realize voltage conversion, which makes the chip do not have to increase the number of pins due to the increase of working mode, so that the device can be packaged with small but still cheap TQFN, which is conducive to actual production and procurement.

Quad mode charge pump principle

Cat3636 adopts quad-modetm charge pump boost architecture. Its principle is different from inductive boost circuit. The output voltage and input voltage have a discrete multiple relationship. The charge pump has four working modes: 1 times, 1.33 times, 1.5 times and 2 times.

The double mode working principle of quad-modetm charge pump is shown in Figure 1. In the first phase, input the power supply VIN to charge the two external capacitors C1 and C2. At this time, the two external capacitors are connected in parallel, and the a terminal of the capacitor is connected to VIN and the B terminal is grounded. The voltage between the two poles of the capacitor is the input voltage, that is, Vc1 = vc2 = Vin. In phase 2, terminal B of external capacitor is connected to VIN and terminal A is connected to Vout, which is Vout = VIN VC = 2vin. Due to the repeated conversion of the first phase and the second phase, the charge is continuously "pumped" to the output.

Fig. 1 principle of 2x Mode Boost

Similar to 2x mode, the working principle of 1.5x mode is shown in Figure 2. In the first phase, C1 and C2 are connected in series between VIN and ground, and the input voltage Vin charges capacitors C1 and C2, Vc1 = vc2 = 1 / 2vin. In the second phase, the two external capacitors are disconnected from the ground and connected to Vout. At this time, Vout = VIN VC = VIN 1 / 2vin = 3 / 2vin. Similarly, this process is repeatedly converted to achieve 1.5 times the boost.

Figure 2 1.5x Mode Boost principle

Different from the traditional 1.33x mode, quad-modetm charge pump can realize 1.33x mode by using only two external capacitors. In the first phase, VIN charges the external capacitors C1 and C2, and C1 and C2 are connected in series. In the second phase, the capacitors C1 and C2 are disconnected from the input power supply VIN, and C1 is inversely connected to VIN and Vout. At this time, C2 remains floating. In the third phase, C1 and C2 are connected to VIN in series, and the positive pole of C2 is connected to Vout. The steady-state output voltage can be solved according to Kirchhoff's voltage law:

Phase 1: VIN = Vc1 vc2 (1)

Phase 2: Vout = VIN Vc1 (2)

Phase 3: Vout = vin-vc1 vc2 (3)

Substitute equation (2) into equation (3) to obtain:

VIN VC1=VIN-VC1VC2 (4)

VC2=2VC1 (5)

Replace equation (5) with equation (1):

VC1=1/3VIN (6)

Then substitute equation (6) into equation (2) to obtain:

VOUT=4/3VIN。

Figure 3 catalyst's innovative 1.33x mode architecture

If the input voltage Vin is greater than the forward voltage drop VF of the LED, the LED does not need to be boosted, and the quad-modetm charge pump operates in the double mode.

According to the principle of energy conservation, the input power Pi of cat3636 is equal to the power pl consumed by the external led plus the power PE consumed by itself, that is, pi = PL PE. The power consumed by cat3636 itself mainly includes charge pump voltage conversion power consumption PC, passive power consumption PS of internal constant current source, power pf consumed by internal logic function module, and heat loss Pt, i.e. PE = PC PS pf Pt. As shown in Figure 4.

Figure 4 power consumption distribution

Conversion efficiency of cat3636 η= PL/PI=PL/(PCPLPSPFPT)。 Since PF and Pt values are relatively small, η ≈PL/(PCPLPS)。 Under the condition of constant current operation, the power consumption pl of LED is approximately constant. Therefore, in the same boost mode, with the decrease of input voltage, the output voltage decreases, and the voltage acting on the internal constant current source also decreases. Therefore, the power consumption PS of constant current source also decreases, and the conversion efficiency of cat3636 η rise; Under the same input voltage, the higher the mode and the higher the output voltage, the greater the power consumed by the internal constant current source and the lower the conversion efficiency. This is why the LED Driver with 1.33x mode has higher comprehensive conversion efficiency than the driver with only 1.5x or 2x mode. Fig. 5 is a diagram of the conversion efficiency of the cat3636 operating within the lithium discharge range.

Figure 5 cat3636 conversion efficiency

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