A novel dimmable LED driver with soft-start and UVLO circuits

    Corresponding author: Jinguang Jiang, jgjiang09@aliyun.com
  • 1. GNSS Research Center, Wuhan University, Wuhan 430072, China
  • 2. School of Physics and Technology, Wuhan University, Wuhan 430072, China
  • 3. Suzhou Institute, Wuhan University, Suzhou 215123, China
  • 4. School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

Key words: LED driverDC—DC converterUVLOsoft-startPWM dimming

Abstract: A fully integrated LED driver based on a current mode PWM boost DC—DC converter with constant output current is proposed. In order to suppress the inrush of current and the overshoot voltage at the start up state, a soft-start circuit is adopted. Additionally, to adjust the LED brightness without color variation over the full dimming range and achieve high efficiency, a PWM dimming circuit is presented. Furthermore, to keep the loop stability of the LED driver, an internal slope compensation network is designed to avoid the sub-harmonic oscillation when the duty cycle exceeds 50%. Finally, a UVLO circuit is adopted to improve the reliability of the LED driver against the input voltage changing. The LED driver has been fabricated with a standard 0.5 μm CMOS process, and only occupies 1.21 × 0.76 mm2. Experimental results show that the brightness of the LED can be adjusted by an off-chip PWM signal with a wide adjusting range. The inductor current and output current increase smoothly over the whole load range. The chip is in the UVLO condition when the input voltage is below 2.18 V and has achieved about 137 μs typical start-up time.

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1.   Introduction
  • Nowadays, high brightness white light emitting diodes (LED) have become an advanced solid-state light source because of their high efficiency, long lifetime, less environmental impact, and smooth dimming[1]. One of the most important applications of LEDs is offline lighting, such as general illumination liquid crystal display (LCD) backlights. The brightness of LEDs is directly proportional to their forward current not to their forward voltage. Namely, LEDs are current-driven devices[2]. To ensure each LED has a similar luminance, an effective way is to connect them in a series[3].

    The block diagram of a traditional LED driver is depicted in Figure 1. This traditional LED driver is a current-mode PWM boost DC-DC converter with constant current output. In case of interior lighting driver solutions or LCD backlight solutions, dimming functionality should be provided additionally, as it has become a prominent and indispensable feature[4]. Since dimming means to decrease the energy consumption of a luminaire, high energy efficiency is also desired at low dimming levels to improve these energy savings. A well-known measure to control LED brightness is to adjust the LED forward current by either amplitude modulation (AM) or pulse-width modulation (PWM), as luminous flux is almost proportional to the LED current flow[5]. As AM dimming can cause color variation, a PWM dimming method is proposed to adjust the LED forward current in this paper.

    The traditional LED driver works in a controlling loop converting the error signal CA into a driving signal CK with a variable duty cycle for the switching element. The error amplifier (EA) is unbalanced at the beginning of the start-up transient, thus causing the converter to work at the largest duty cycle. This situation makes the inductor current, $I_{\rm L}$, rise above its equilibrium value; as a result, an inrush current appears[6]. The $I_{\rm L}$ will remain above its equilibrium value for a little time, as the inductor current cannot change instantaneously. This causes the output voltage, $V_{\rm O}$, to rise abruptly and exceed its regulated value; therefore, an overshoot voltage appears. As a result, the whole LED driver may be damaged by the inrush current and overshoot voltage at the start-up period. Therefore, a soft-start circuit is required to eliminate the inrush current and overshoot voltage[7].

    Due to the weak ability of bearing a short-time overload of MOSFET and IGBT, the LED driver can be easily damaged when it accumulates energy caused by over-voltage and over-current[8]. Protection circuits, including over current protection (OCP) and over voltage protection (OVP), must be adopted to keep the high reliability of the LED driver. OCP and OVP limit the maximum value of the inductor current $I_{\rm L}$ and output voltage $V_{\rm O}$ respectively.

    When the LED driver is working, the input voltage $V_{\rm IN}$ may drop down transiently and fall below the specific threshold. As a result, the LED driver will breakdown or closedown and output wrong current[9]. Moreover, the on-chip MOSFET power switches may be burned out, and then huge transient current may occur, which can easily damage the whole LED driver. To avoid this incident, an under-voltage-lockout (UVLO) circuit must be employed to guarantee that the LED driver operates correctly. The UVLO circuit also ensures a correct saturation level for the MOSFET power switches.

    A novel LED driver based on a current-mode PWM boost DC-DC converter with constant output current is proposed in this paper. The LED driver contains a soft-start circuit, an OCP circuit, an OVP circuit, an UVLO circuit and a dimming circuit. The whole scheme is introduced in Section 2. The circuit implementation is presented in Section 3. Simulation and measurement results are shown in Section 4 and the conclusions are drawn in Section 5.

2.   Architecture of the proposed LED driver
  • The block diagram of the proposed LED driver is presented in Figure 2. Off chip components include the input capacitor $C_{\rm IN}$, output capacitor $C_{\rm O}$, inductor L, freewheeling diode D and feed-back resistor $R_{\rm F}$. The $R_{\rm F}$ senses the output current $I_{\rm O}$ then feeds the $I_{\rm O}$ back to the EA. The stable value of $I_{\rm O}$ is determined by the $R_{\rm F}$ and has no relationship with LEDs. The stable value of $I_{\rm O}$ can be expressed as:
    $I_{\rm O} =\frac{V_{\rm B0} }{R_{\rm F}}, $(1)
    where $V_{\rm B0}$ is a reference voltage produced by the Bandgap {\&} Bias block.

    All functional modules including MOSFET power switches are integrated on a single chip. For the on-chip system, Driver{\_}N drives the two MOSFET power switches NM1 and NM2 simultaneously. The LEDs, $R_{\rm F}$, EA, PWM comp, RS latch{\_}D, Driver{\_}N and NM2 constitute a controlling loop. The controlling loop converts the error signal, CA, into a driving signal, Dim{\_}CK, with a variable duty cycle to control the power switch, NM2. NM1, $R_{1}$ and $R_{2}$ constitute a current sensor, which detects the peak current flowing through $L$, and provides voltage $V_{\rm SENSE}$ for slope compensation. The Ramp generator, Slope compensation and inductor current sensor constitute an internal slope compensation network. When the duty cycle is larger than 50 % under current-mode PWM operation, sub-harmonic oscillation will occur in the controlling loop. Fortunately, it can be suppressed by the internal slope compensation network, and the stability of the controlling loop can be maintained effectively in this way. In addition, OSC supplies a system clock of 1.4 MHz, and Bandgap {\&} Bias offers reference voltages and bias currents. The OCP and OVP provide an over current protection cycle by cycle and over voltage protection respectively. The UVLO ensures the high reliability of the LED driver against the input voltage, $V_{\rm IN}$, changing. Furthermore, the inductor current sensor, OCP, OVP and Step voltage generator constitute a soft-start circuit. The soft-start circuit eliminates the inrush current of $I_{\rm L}$ and the overshoot voltage of $V_{\rm O}$ at the start-up state. The off-chip signal EN can adjust $I_{\rm O}$ through the RS Latch{\_}D block, and can also shut down the LED driver by the ON/OFF controller.

3.   Circuit implementation

    3.1.   Soft-start circuit

  • Traditionally a soft-start circuit sets a steadily rising reference voltage for EA to meet the dynamic voltage scaling performance of the DC-DC converter. Normally the soft-start circuit contains a DAC to generate the steadily increasing voltage. In this way, the inrush current can be suppressed effectively, but DAC requires extra power and costs a large silicon area[10]. Thus, an improved soft-start circuit is proposed and redrawn in Figure 3.

    The soft-start circuit contains four main components i.e., OCP, OVP, the inductor current sensor and step voltage generator. The soft-start circuit abandons the complex DAC block and saves layout. The stair voltage $V_{\rm ST}$ is set for the OCP to control the inductor current increases by steps. That means the inrush current of $I_{\rm L}$ is eliminated. During every step of $I_{\rm L}$, the increasing rate of $I_{\rm O}$ is limited, so the $V_{\rm O}$ will not rise up abruptly and exceed its regulated value. In this way, the overshoot voltage of $V_{\rm O}$ is suppressed. The inductor current sensor senses the inductor current, and transfers it to the OCP. The OCP compares $V_{\rm SENCE}$ with $V_{\rm ST}$ to limit the peak value of $I_{\rm L}$. The peak inductor current, $I_{\rm LP}$, can be given as:
    $I_{\rm LP} =N \frac{V_{\rm ST} }{R_2 }=\frac{V_{\rm ST} }{R_2 }\frac{(W/L)_{\rm NM1} }{(W/L)_{\rm NM2} }, $(2)
    where $(W/L)_{\rm NM1}$ and $(W/L)_{\rm NM2}$ are the W/L of NM1 and NM2 respectively, and $R_{2}$ is the inductor current sense resistor. The proposed step voltage generator is depicted in Figure 4.

    MOSFETs NM$_{\rm dm}$ ($m$ $=$ 1, 2, 3, 4) act as switches. $V_{\rm ST}$ has five steps while Q$_{1}$, Q$_{2}$, Q$_{3}$, Q$_{4}$ turn to 0 from 1 one by one under the control of delay blocks. $V_{\rm ST}$ can be expressed as:
    $V_{\rm ST} =\frac{V_{\rm B1} }{R_2} \frac{\bar {Q}_1 R_{\rm d1} +\bar {Q}_2 R_{\rm d2} +\bar {Q}_3 R_{\rm d3} +\bar {Q}_4 R_{\rm d4}} {R_{\rm d0} +\bar {Q}_1R_{\rm d1} +\bar {Q}_2 R_{\rm d2} +\bar {Q}_3 R_{\rm d3} +\bar {Q}_4 R_{\rm d4} }, $(3)
    where $V_{\rm B1}$ is a reference voltage produced by the Bandgap {\&} Bias block. The five steps of peak inductor current can be expressed as:

    The detail of $I_{\rm LP}$ and $V_{\rm ST}$ is described in Table 1.

    The ideal soft-start waveforms of $I_{\rm L}$ and Io are presented in Figure 5. $I_{\rm O}$ and $I_{\rm L}$ need five steps to achieve stability. Each step of $I_{\rm L}$ consists of two sections.

    (1) $I_{\rm L}$ rises up linearly.

    $T_{1}$ is the time $I_{\rm L}$ takes to rise to $I_{{\rm LP}(n)}$ from $I_{{\rm LP}(n-1)}$ ($n$ $=$ 2, 3, 4, 5) while the LED driver works using the maximum duty cycle. $I_{{\rm LP}(n)}$ is the peak value of $I_{\rm L}$ at the nth step. As the driver is a boost DC-DC converter, $T_{1}$ can be expressed as:

    (2) $I_{\rm O}$ rises up while $I_{\rm L}$ remains $I_{{\rm LP}(n)}$. There are two situations.

    (a) $I_{\rm L}$ remains $I_{{\rm LP}(n)}$ until the next step arrives. $T_{2}$ can be given as:
    $T_2 =T_{\rm D} -T_1 , $(6)
    where $T_{\rm D}$ is the delay time of the delay block.

    (b) $I_{\rm L}$ remains $I_{{\rm LP}(n)}$ until $I_{\rm O}$ achieves stability. To analyse the start-up time, a load resistor R is used to replace the LEDs. The resistor R and the output capacitor $C_{\rm O}$ are both charged by $I_{\rm L}$ at this period. The charge process can be expressed as follows:

    $I_{\rm R}$, the current flowing through $R$, can be given as:

    $I_{\rm R0}$ is the initial value of $I_{\rm R}$. $I_{\rm R(t)}$ is the final value of $I_{\rm R}$ after t time. $I_{{\rm R}(n)}$, the final value of $I_{\rm R}$ after the nth step, can be written as:

    Since TD $\ll$ $RC$/4, $I_{{\rm R}(n)}$ can be simplified as:

    $T_{\rm S}$, the whole time of soft-start, can be given as:

    $I_{\rm O}$ reaches its stable value $I_{\rm OS}$ after the nth step. As mentioned above, $I_{\rm O}$ rises up by five successively increasing rates until it achieves stability. At the same time, the OVP limits the maximum of $V_{\rm O}$. Therefore, the proposed soft start circuit can eliminate the inrush current and the overshoot voltage at the start up state.

  • 3.2.   Dimming scheme and ON/OFF control

  • The dimming circuit and ON/OFF controller are shown in Figure 6. Off-chip digital signal EN has two functions: dimming control and ON/OFF control.

    (1) Dimming control

    For the dimming control, there are four kinds of LED driving methods i.e., amplitude modulation dimming (AM) with linear regulators, AM dimming with switching regulators, pulse width modulation (PWM) dimming with linear regulators, and PWM dimming with switching regulators[11]. To adjust the LED brightness without color variation over the full dimming range and obtain high efficiency, a PWM dimming circuit with switching regulators is proposed, as shown in Figure 6. With this, the LEDs operate with full current when EN is high, and operate with zero while EN is low. The ideal dimming waveforms of $I_{\rm L}$ and $I_{\rm O}$ are described in Figure 7.

    When EN turns to low from high, NM2 is closed, and then $I_{\rm L}$ decreases while $I_{\rm O}$ continues to increase a little bit. Once $I_{\rm L}$ $<$ $I_{\rm O}$, $V_{\rm O}$ and $I_{\rm O}$ decrease as $C_{\rm O}$ discharges. $I_{\rm O}$ and $I_{\rm L}$ will be zero, once $V_{\rm O}$ drops below the threshold voltage of LEDs. When ENB turns to high from low, the driver will redo the soft-start process. The average LED current is proportional to the duty-cycle of the PWM signal EN. The average of the output current $I_{\rm O}$ can be expressed as:
    $I_{\rm O} =\frac{V_{\rm B1} }{R_{\rm F} } D_{\rm PWM} =\frac{V_{\rm B1} }{R_{\rm F} } \frac{T_{\rm H} }{T}, $(12)
    where $D_{\rm PWM}$ is the duty cycle of EN, which ranges from 0 to 1. $T$ and $T_{\rm H}$ are the cycle and high level duration of EN respectively. When EN is low, $I_{\rm O}$ and $I_{\rm L}$ are much less than their full value and NM2 has no power consumption. Therefore, the proposed dimming scheme almost has no effect on the LED driver's efficiency. The typical frequency of the dimming signal is 0.1-1 kHz.

    (2) ON/OFF control

    Digital logic of EN provides an electrical ON/OFF control of the power supply. Connecting EN to GND and sustaining the level more than 4 ms will completely turn off the driver. The LED driver will be locked by the ON/OFF controller when UV turns to low.

  • 3.3.   UVLO circuit

  • The UVLO circuit is presented in Figure 8. During the procedure of the input voltage $V_{\rm IN}$ falling down, hysteresis drift is introduced and different threshold voltage is brought in. Under voltage threshold, $V_{\rm HU}$, and release voltage threshold, $V_{\rm HR}$, are expressed as:
    \begin{split} {}& V_{\rm HU} ={\rm VB}_{\rm U2} +{\rm VTH}_2, \\[2mm]& V_{\rm HR} ={\rm VB}_{\rm U1} +{\rm VTH}_5. \\ \end{split} (13)
    VTH$_{2}$ and VTH$_{5}$ are the threshold voltage of PM$_{\rm U2}$ and PM$_{\rm U5}$ respectively, and VTH$_{2}$ $=$ VTH$_{5}$. VB$_{\rm U2}$ and VB$_{\rm U1}$ are reference voltages produced by the Bandgap {\&} Bias block. $V_{\rm HR}$ $>$ $V_{\rm HU}$, as VB$_{\rm U2}$ $>$ VB$_{\rm U1}$. When $V_{\rm IN}$ $<$ $V_{\rm HU}$, $I_{\rm P1}$ and $I_{\rm P2}$ are zero. This makes $V_{\rm UV1}$ turn to zero as NM$_{\rm U2}$ still remains on. As a result, UV turns to zero, then the LED driver is in the UVLO condition and $I_{\rm O}$ is zero. When $V_{\rm IN}$ $>$ $V_{\rm HR}$, $I_{\rm P1}$ will be built. Then $V_{\rm UV1 }$ turns to high, as the $W/L$ of PM$_{\rm U5}$ and PM$_{\rm U6}$ are much bigger than NM$_{\rm U2}$'s. Therefore, $V_{\rm UV2}$ is zero, and then $I_{\rm P2}$ is built to reinforce the high level of UV. As a result, UVLO protection is released, and the LED driver works reliably.

    A conventional UVLO circuit needs extra comparators and amplifiers to ensure it operates correctly. The proposed UVLO circuit realizes the function without using complex comparators and amplifiers. Therefore, the proposed UVLO reduces noise, increases stability and saves layout.

4.   Simulation and measurement results
  • The proposed LED driver is fabricated in a CMOS CSMC 0.5 $\mu $m process. The chip microphotograph is shown in Figure 9. The experimental circuit board of the co-operation between this LED driver IC and a series of four LED chips is shown in Figure 10. The total die size of the proposed circuit including NM2 is only 1.21 $\times$ 0.76 mm$^{2}$, and the power MOSFET NM2 occupies 0.622 $\times $ 0.76 mm$^{2}$. The LED driver was simulated using Cadence Spectre and measured by a digital oscilloscope, Agilent DS07104A. The power consumption of the LED driver is 823 $\mu $A @ 3.6 V under a typical working environment. Using a testing package of the LED driver, the reference voltage $V_{\rm B}$ produced by the Bandgap {\&} Bias block can be measured. In order to measure $I_{\rm L}$, a nominal resistor, $R_{\rm L}$, is connected between $L$ and $V_{\rm IN}$. $I_{\rm L}$ and $I_{\rm O}$ are represented by $V_{\rm RL}$ (the voltage of $R_{\rm L})$ and $V_{\rm RF}$ (the voltage of $R_{\rm F})$ respectively. The off-chip capacitor $C_{\rm O}$, inductor $L$, input capacitor $C_{\rm IN}$ and resistor $R_{\rm F}$ for the LED driver in Figure 2 are 4.7 $\mu $F, 10 $\mu $H, 4.7 $\mu $F and 2 $\Omega $ respectively.

  • 4.1.   Simulation result

  • Firstly, the simulation result of the dimming function is presented in Figure 10. It can be seen from Figure 10 that the duty cycle of $I_{\rm O}$ is the same with EN's while $D_{\rm EN}$ (the duty cycle of EN) changes. That means the average value of $I_{\rm O}$ is adjusted by the duty cycle of EN.

    Secondly, the simulation characteristics of the ON/OFF control are presented in Figure 11. As is shown in the figure, the chip will be completely turned off when EN is zero and sustains the level for more than 4 ms.

    Then, the simulation results of the UVLO circuit are shown in Figure 12. It can be seen from the figure that the chip is in the UVLO condition when $V_{\rm IN}$ is lower than 2.18 V, and the UVLO protection is released when $V_{\rm IN}$ is larger than 2.28 V.

    Finally, the simulation characteristics of the soft-start function are shown in Figure 13. As is shown in the figure, $I_{\rm L}$ rises up by five steps, and $I_{\rm O}$ increases smoothly at the start up state. That means the inrush current and overshoot voltage are suppressed. The typical start-up time is 137 $\mu $s.

  • 4.2.   Measurement result

  • Figures 14(a) and 14(b) show the measured dimming waveform of EN and $I_{\rm O}$ when $D_{\rm EN}$ is 0.6 and 0.8 respectively. Figure 15 shows the measured ON/OFF control waveform of EN and $V_{\rm B}$. Figure 16 shows the measured UVLO waveform of $V_{\rm IN}$ and $I_{\rm O}$. Figure 17 shows the measured Soft-start waveform of $V_{\rm IN}$, $I_{\rm L}$ and $I_{\rm O}$. The measurement result is well consistent with the simulation result. A performance comparison between the proposed LED driver and other related LED drivers is listed in Table 2. This work saves more layout area and cost, while providing more functions such as soft-start, UVLO and smooth dimming. All these simulation and measurement results show that the proposed LED driver is a good choice for offline lighting with a dimming function.

5.   Conclusion
  • A novel LED driver based on a current-mode PWM boost DC-DC converter with constant output current is presented in the paper. Almost all modules, such as the soft-start circuit, UVLO circuit, dimming circuit and power switch are integrated on a single chip. Only five off-chip components are needed to make the LED driver work normally. Internal slope compensation is used to avoid sub-harmonic oscillation for the sake of improving load capability and keeping loop stability. Additionally, in order to adjust the LED brightness without color variation over the full dimming range and obtain high efficiency, the PWM dimming method with switching regulators is adopted. Moreover, the UVLO circuit guarantees the reliability of the LED driver when the input voltage changes. The soft-start circuit suppresses the inrush current and the overshoot voltage at the start up state. Finally, the chip is fabricated under a standard CSMC CMOS 0.5 $\mu $m process, and only occupies 1.21 $\times$ 0.76 mm$^{2}$. The power consumption of the LED driver is 823 $\mu $A @ 3.6 V under a typical working environment. The input voltage is 2.5-5.5 V, the output current ranges between 1-1000 mA, and the maximum output voltage is 18~V.

Figure (18)  Table (8) Reference (14) Relative (20)

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