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

Jinguang Jiang; Gaojian Tan; Zeyu Zhang; Xifeng Zhou

doi:10.1088/1674-4926/36/2/025004

J. Semicond. > 2015, Volume 36 > Issue 2 > 025004, doi: 10.1088/1674-4926/36/2/025004

SEMICONDUCTOR INTEGRATED CIRCUITS

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

Jinguang Jiang^{1, 2, 3,}, Gaojian Tan², Zeyu Zhang⁴ and Xifeng Zhou²

+ Author Affiliations

Corresponding author: Jinguang Jiang, E-mail: jgjiang09@aliyun.com

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.

Key words: LED driver, DC—DC converter, UVLO, soft-start, PWM dimming

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Fig. 1. Architecture of a traditional LED driver.

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Fig. 2. The block digram of the proposed LED driver.

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Fig. 3. The proposed soft-start circuit.

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Fig. 4. Circuit of the proposed step voltage generator.

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Fig. 5. Ideal soft-start waveform of $I_{\rm L}$ and $I_{\rm O}$.

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Fig. 6. The dimming circuit and ON/OFF controller.

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Fig. 7. Ideal dimming waveforms of $I_{\rm L}$ and $I_{\rm O}$.

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Fig. 8. The UVLO circuit.

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Fig. 9. Chip microphotograph of the proposed LED diver (chip area is 1.2 $\times $ 0.765 mm$^{2})$.

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Fig. 10. Experimental circuit board of the co-operation between this LED driver IC and a series of four LED chips.

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Fig. 11. The simulation result of the dimming function.

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Fig. 12. The simulation characteristic of ON/OFF control.

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Fig. 13. The simulation result of UVLO circuit.

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Fig. 14. The simulation results of soft-start function.

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Fig. 15. (a) Measured dimming waveform of EN and $I_{\rm O}$ when $D_{\rm EN}$ $=$ 0.6. (b) Measured dimming waveform of EN and $I_{\rm O}$ when $D_{\rm EN}$ $=$ 0.8.

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Fig. 16. ON/OFF control waveform of EN and $I_{\rm O}$.

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Fig. 17. UVLO waveform of $V_{\rm IN}$ and $V_{\rm RF}$ ($I_{\rm O})$.

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Fig. 18. Soft-start waveform of $V_{\rm IN}$, $I_{\rm L}$ ($V_{\rm RL})$ and $I_{\rm O}$ ($V_{\rm RF})$.

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Table 1. Accuracy value of $I_{\rm LP}$ and $V_{\rm ST}$.

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Table 2. Comparison between this work and related LED drivers.

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Received: 02 July 2014 Revised: Online: Published: 01 February 2015

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.

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

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