The flow chart as depicted in Fig. 16(a) includes the start-up operation, the protection functions, and the MHCC operation. When the power is turned on, the chip begins the start-up procedure. As a result, it is necessary to have the under-voltage lockout (UVLO) circuit as shown in Fig. 16(b) to determine whether the power supply is greater than 2.5V or not. If the supplying voltage is not high enough, the closed-loop is not adequate to be established in order to avoid oscillating. Before the supplying voltage approaches the pre-defined voltage level, the value of UVLO is high and can be expressed as (27).
In this paper, the supplying voltage should be larger than 2.5V to guarantee a stable closed-loop operation. Before the closed-loop operation, the output is connected to the input supplying voltage through the inductor. Once the power supply is greater than 2.5V, the start-up circuit takes over the operation. That is the switching converter will begin to boost the output to a higher output voltage level. During this start-up procedure, the maximum inductor current IL_max_startup is limited below 1.13A.
In the implementation of the start-up circuit, it is easy to divide the resistor RV in Fig. 11 into three small resistors, RV1~RV3 in Fig. 16(b). Good layout matching can guarantee the accuracy the current sensing and the protection function. According to (21), the divided voltage VST at node X can be used to limit the inductor current during the start-up period. Thus, the value of IL_max_startup can be expressed as (28).
(
1)
When the output voltage Vo approaches to 12V, the feedback voltage VFB will be greater
MHCC operation. The maximum inductor current will be raised to a higher level to avoid the overloading condition during the MHCC operation. The voltage VOCP at node Y is used to compare with Vref to detect the over current protection (OCP) condition and thus the protection current of the OCP protection is higher than that of the start-up period. The expression of OCP is shown in (29). When the OCP condition occurs, the ST_OK will be set to 0 and the high voltage (HV) N-MOSFET, HN24G5, will be turned off to let the system return to the start-up operation.
1 when o V2 ref
OCP = I ×R >V (29)
(a)
IO
Fig. 16. (a) The flow chart includes the start-up operation, the protection functions, and the MHCC operation. (b) The start-up and protection circuits.
Chapter 5
Experimental Results
The proposed boost convert with the MHCC technique was fabricated by TSMC 0.25 µm CMOS process. The threshold voltages of nMOSFET and pMOSFET are 0.477 V and -0.596 V, respectively. The off-chip inductor and output capacitor are 6.8 µH and 10 µF, respectively. The output voltage Vo is 12V to drive 3 white LEDs in series. The specification is listed in Table I. The chip micrograph is shown in Fig. 17 and the chip area is about 1480 µm × 2780 µm including the test pads.
C_Multiplier Peak Detect Error Amp.
BIAS
Fig. 17. Chip micrograph.
TABLE I THE DESIGN SPECIFICATION
Characteristics Typ. Unit
Supply Voltage (Vin) 3.5~4.5 V
Output Voltage (Vo) 12 V
Output Current (Iload) 70~270 mA
Input Inductor (L) 6.8 µH Equilibrium series Resistance of
the inductor (DCR) 45 mΩ
Output capacitor (CO) 10 µF
Equilibrium series resistance of
the output capacitor (RESR) 50 mΩ
Operation temperature 0~100 ℃
Experimental results of conventional boost converter are shown in Fig. 18. The input voltage is 4V. The load currentchanges from 70mA to 270mA. The slew rate of load current is 200mA/us. The undershoot voltage and overshoot voltage are 338mV and 308mV, respectively. The recovery time is 141µs when load current changes from light to heavy load or heavy to light. And the load regulation is 0.16mV/mA. It is obvious to see that the bandwidth is limited by the existence of the RHP zero. Under the low-bandwidth design, the transient response can’t be speeded up. Thus, the slow-response output voltage may cause the LEDs in series have a little of luminance variation when the dimming control is applied on the LED brightness control.
(a)
(b)
11.845V 11.877V
VOUT
IL
Recovery time = 141µs
Iload=270mA 70mA within 2µs
Iload
32mV Load regulation =0.16mV/mA Overshoot voltage
=308mV
(c)
Fig. 18. Waveforms in conventional boost converter with hysteresis control when load current changes from 70mA to 270mA within 2µs.
Compared to the slow-response of the conventional design, the experimental results of the proposed boost converter are shown in Fig. 19. The test setting is the same as the conventional design. The undershoot voltage and overshoot voltage are 300mV and 234mV, respectively. The recovery time of light load to heavy load is 19.5µs and the recovery time of
heavy load to light load is 18.8µs. Experimental results show the improvement in transient response is higher than 7.2 times when load current changes from light to heavy or vice versa compared to the conventional boost converter design. The improvement comes from the new HCC and the ACC techniques. The output of the error amplifier can be settled rapidly by the ACC technique to define the low band of the hysteresis window. Besides, the new HCC technique can adaptively adjust the on-time and off-time values to speed up the transient response time. The load regulation, which is 0.11mV/mA, is also better than that of the conventional design due to the ACC technique.
(a)
(b)
Fig. 19. Waveforms in the proposed boost converter with the MHCC technique (a) when load current changes from 70mA to 270mA within 2µs and (b) when load current changes from 270mA to 70mA within 2µs.
The experimental results of the proposed boost converter are shown in Fig. 20 when the load current changes from 70mA to 220mA or vice versa. The fast transient performance can be easily seen since the MHCC technique can speed up the transient response over a wide load current range. Certainly, the fast transient performance has its limitation. The load current step decreases to about 100mA and 50mA as shown in Fig. 21 (a) and (b), respectively. The transient response time has a little improvement compared to that of the large load current step since the ACC technique has only a little effect on the whole system.
Fortunately, the ACC technique consumes a little power since it works only when the output voltage has a large variation. Fig. 22 shows the power conversion efficiency. It demonstrates that the power conversion efficiency is nearly not influenced with or without the ACC technique. The power consumption overhead is merely 1%. The maximum power conversion efficiency is about 90%. The start-up waveforms are also shown in Fig. 23. The comparison
summary between the conventional and the proposed MHCC techniques is listed in TABLE II.
(a)
(b)
Fig. 20. Waveforms in the proposed boost converter with the MHCC technique when load current changes from 70mA to 220mA within 2µs.
V
OUTI
LI
load70mA
170mA
∆I=100mA
Recovery time of light-to-heavy load =13µs Undershoot voltage= 140mV
Recovery time of heavy-to-light load =13µs Overshoot voltage= 142mV
(a)
(b)
Fig. 21. Waveforms in the proposed boost converter with the MHCC technique when load current changes from light to heavy within 2µs.
Power conversion efficiency (η)
Fig. 22. Power conversion efficiency.
Fig. 23. The start-up waveforms when the input voltage is slowly ramped up and the output loading is 70mA.
TABLE II THE SUMMARIES OF THE CONVENTIONAL AND MHCCTECHNIQUES
Characteristics Conditions MHCC Technique Conventional Technique
Supply Voltage -- 4V 4V
Output Voltage -- 12V 12V
Output Current
Variation -- 200mA 200mA
Overshoot Voltage Iload=270mAÆ70mA
within 2µs 234mV 308mV
Undershoot Voltage Iload=70mAÆ270mA
within 2µs 300mV 338mV
Recovery time of heavy-to-light load
Iload=270mAÆ70mA
@ 0.1% rated voltage within 2µs
18.8µs 141µs
Recovery time of light-to-heavy load
Iload=70mAÆ270mA
@ 0.1% rated voltage within 2µs
19.5µs 141µs
Load Regulation -- 0.11mV/mA 0.16mV/mA
Chapter 6
Conclusion
This paper proposes a modulated hysteretic current control (MHCC) technique to improve transient response of DC-DC boost converters, which suffer from low bandwidth due to the existence of right-half-plane (RHP) zero. The new HCC technique can adaptively adjust the on-time and off-time values of the PWM waveform to speed up the transient response.
Besides, the ACC technique can settle the low band value of the hysteresis generated by the error amplifier to further improve the transient response. Thus, the low-bandwidth and large overshoot and undershoot voltages due to the RHP zero can be improved. The proposed boost converter with the MHCC technique fabricated by TSMC CMOS 0.25 um process can demonstrate the fast transient performance to verify its advantage in the driving capability of the LEDs in series. A good driving capability and accuracy can be guaranteed by the proposed MHCC technique according the fast transient performance in the experimental results.