^{1}

^{*}

^{2}

^{2}

^{3}

^{3}

^{2}

^{4}

^{5}

^{5}

^{5}

^{2}

A high power buck-boost switch-mode LED driver delivering a constant 350 mA with a power efficient current sensing scheme is presented in this paper. The LED current is extracted by differentiating the output capacitor voltage and maintained by a feedback. The circuit has been fabricated in a standard 0.35 μm AMS CMOS process. Measurement results demonstrated a power-conversion efficiency over 90% with a line regulation of 8%/V for input voltage of 3.3 V and current output between 200 mA and 350 mA.

High-power LED has been widely used as flashlight for camera phones and electric torch for night vision. The most commonly used high-power LED is driven at 350 mA, and LED manufacturers are constantly working on driving LED at higher output current, so that it can provide sufficient light output for broader lighting applications [

As Li-ion batteries with a voltage range from 2.7 V to 4.2 V, it’s commonly used as power source for handheld devices. A high-power LED driver has to step up or step down this supply voltage to drive a 350 mA high-power LED of forward voltage ranging from 3.4 V to 3.7 V. This makes buck-boost converter one of the most suitable candidates. In this paper, an LED-current sensing circuit that is suitable for buck-boost and boost converter is presented. In the following sections, the operation principle and implementation of the proposed LED-current sensing circuit will be presented together with the measurement results.

The most common approach [2-7] for sensing the LED current is illustrated in

Typical reference voltage used is between 110 mV to 1.23 V [1-7] so that Rsense ranges 314 Ohm to 3.5 Ohm. The power consumption can then be estimated to be in the range of 38.5 mW to 430 mW, which is relatively high. An the same time, the power consumption of the sensing circuit increase with the square of output current as given in Equation (1), which is undesirable for high current circuits.

The output current of a buck-boost converter, however, can be directly extracted from the capacitor voltage at the output [_{in} to inductor L where the energy is stored. Meanwhile, the high power LED is disconnected from the inductor and output-current is solely provided by output capacitor C_{O}. Therefore, current flowing out of output capacitor is equal to the LED-current in this time period, and this LED-current is equal to the slope of decreasing V_{O} based on equation (2).

By differentiating the output voltage, the current information can be obtained without using a sensing resistor network and thus achieve significant power saving.

The overall system of a Buck-boost LED driver with proposed LED-current sensing circuit is shown in

modulation in PWM control and is voltage-programmed. Power transistors S_{P}_{1}, S_{N}_{1} S_{P}_{2} and, S_{N}_{2}, inductor L and output capacitor C_{O} form the power stage of the Buck-boost LED driver. Resistor R_{fb}_{1} and R_{fb}_{2} form the resistive feedback network to feedback a portion of the output voltage to the system control. There is a dead time control block to produce a delay between the switching of S_{P}_{1}, S_{N}_{1} and S_{P}_{2}, S_{N}_{2} during each switching cycle to prevent shoot-through current in the power stage. There are also drivers to turn on and off the power transistors as they are very large in size. An oscillator OSC is used to generate a ramp and clock signal for both PWM control blocks. A dimming control block is implemented to allow PWM dimming of LED. There is also a start-up current limiting block to limit the power transistor and inductor current during system start up, and a shut-down current limiting block to perform similar function when the system is under dimming operation.

System diagram of the proposed LED-current sensing circuit is shown in _{diff} and C_{diff} form the differentiator to generate V_{diff}. Sampling

switch and capacitor form the sample and hold circuit that sample the output of the differentiator during the ‘set’ period and hold it for the remaining time. The second error amplifier compares the sampled voltage V_{samp} and compare with reference voltage V_{ref} to record the LED-current.

To simplify the analysis, the sampling switch and voltage buffer are assumed to be ideal. The gain of error amplifier is assumed to be G_{ma}R_{oa} and the current drawn by feedback resistor is negligible. The sample voltage of the proposed LED-current sensing circuit during (1 − D)T < t < T is given by:

From Equation (3), the relationship that the sampled DC voltage is proportional to the slope of the capacitor voltage only holds at relatively high frequency such that the “1” in the denominator of (3) become negligible.

Also from the pole, we can see that the bandwidth of the proposed sensing circuit is limited by the differentiating capacitor, resistor and the gain of the error amplifier. The Bode plot of the proposed LED-current sensing circuit is shown in _{LED} and V_{samp} implies that the DC value and slow changes in LED-current is completely filtered out from the system control loop by the differentiator, and thus is not regulated. Intuitively it seems that I_{LED} is not regulated. However, the information of LED-current actually appears in two frequency ranges: 1) the DC value of I_{LED}R_{O} that is filtered out by the proposed LED-current sensing circuit and 2) the differentiated value of capacitor voltage over time during each switching cycle. It is the second frequency range that is of interested because we are not regulating V_{O} but from the slope of V_{O} that gives the LED-current. The output capacitor has to be chosen carefully in this case such that the cut-off frequency of the LED-current sensing circuit cannot be too low; otherwise, the sampled voltage V_{samp} may become output voltage dependent.

The behavior of the differentiator can be studied by performing Fourier analysis on the output voltage during (1 − D)T < t ≤ T which is assumed to be triangular with the form

The result from the Fourier analysis (on the full triangular wave) is given by

Including all harmonics of the fundamental frequency of the output voltage will require a differentiator with unlimited bandwidth and is not practical. In general, including up to the second harmonics of the signal will be enough [_{diff} = 1.25 MΩ and C_{diff} = 4.7 pF have been chosen in our circuit implementation. Transient simulation has been performed and the result shown in _{P}_{2} during each switching cycle, which is also the current passed to the LED and output capacitor. It is shown that the differentiator can give reasonably accurate sensing result and V_{diff} is about 1.33 V for an LED-current of 350 mA.

As V_{diff} is sampled during (1 − D)T < t ≤ T in every switching cycle, the transfer function of the LED-current sensing circuit should not be directly multiplied to the transfer function of the whole converter. The sampling circuit acts as a continuous to discrete converter and at the same time a reconstruction filter that convert the continuous signal V_{diff}(t) into a discrete signal and reconstruct it by a zero-order hold interpolation in between each sample [_{samp}[n] can be represented by

As the sampling frequency is equal to the frequency of the differentiator output, V_{samp} extracts the output value of the differentiator when (1 − D)T < t < T [_{LED} and V_{samp}. The resulted transfer function is given by [

(7)

The proposed LED driver has the same configuration as a

typical Buck-boost converter with leading-edge-modulation except for the feedback network. From the transfer function given by Equation (7), a right-half-plane (RHP) zero ω_{d} exists and can be moved to left-half-plane (LHP) when the condition below is fulfilled.

For the proposed design, it is very hard to fulfill such criterion and thus the RHP zero is placed after UnityGain-Frequency (UGF). RHP zero ω_{d} occurs at around 12.4 kHz, and the natural frequency ω_{o} of the LED driver occurs at around 13.3 kHz with Q estimated to be about 1. As the RHP zero locates quite close to the natural frequency, extending bandwidth with a zero is not very effective, hence dominant pole compensation is used to ensure stability.

To achieve the compensation, the transfer function A(s) of the compensation network used should place the UGF of the system at around 1 kHz. A simple differential amplifier and an off-chip compensation capacitor are used to implement the compensation network. The Bode plot of the overall system and that of the power stage alone is shown in

The proposed LED-current sensing scheme is applied to a voltage-mode PWM Buck-boost LED driver and implemented in AMS 0.35 μm process. The chip micrograph is shown in

In steady state measurement, a supply voltage is 3.6 V and preset output-current is 350 mA. Switching frequency is 1 MHz.

_{diff} and the buffered sampled voltage V_{samp} with a supply voltage is 3.6 V. Because of the switching noise coupled from switching nodes of the driver, there is distortion in the waveform of V_{diff}. However, as the value of V_{diff} is sampled in instants when the driver is not switching, and the value is held constant thereafter, the switching noise induced distortion that appears in V_{diff} is not passed to V_{samp}, as shown in the figure. The measured V_{samp} is 1.39V for an output current of 350 mA.

A plot of output-current for different reference voltages V_{ref} is shown in

factor is 1.85 mV/mA. When supply voltage V_{in} is changed from 2.7 V to 3.6 V, the output current for the same reference voltage V_{ref} does not change significantly.

The output current of the circuit is also measured in response to line variation and load variation. As the actual resistance of the high power LED depends on the manufacturing company, two resistors of 10 Ω and 4.7Ω are used. The preset output current is 350 mA, and input voltage V_{DD} is swept from 2.7 V to 3.6 V. Measurement result is shown in

lation measured is below 30 mA/V.

350 mA output current, hence efficiency is not very seriously degraded even when input voltage is close to its minimum value.

_{in }= 3.3 V or 350 mA for V_{in }= 3.6 V. When the output current is low, the efficiency is lower for high input voltage due to the increase of switching loss with increasing input voltage.

In this paper, an LED-current sensing circuit that consumes minimal power is proposed. The proposed sensing circuit senses LED-current by differentiating the voltage of the output capacitor and consumes less than 420 μW power. It is implemented in a Buck-boost LED driver and the design is fabricated in AMS 0.35 μm process. Measurement results confirm a reasonable sensing accuracy and line/load regulation. The maximum power efficiency is measured to be 92%. As the power consumption of the proposed LED-current sensing circuit does not

increase proportionally with the LED-current, the circuit is particularly useful when LED-current continues to increase in the future.

This work is supported by Industry Education and Research Foundation of PKU-HKUST Shenzhen-Hongkong Institution (sgxcy-hzjj-201204), by the Guangdong Natural Science Foundation (S2011040001822) and the Fundamental Research Project of Shenzhen Science & Technology Foundation (JCYC20120618163025041). This work is also supported by the National natural Science Funds of China (61204033, 61204043).