Design and Research of Oscillator Circuit with Soft Starting in Power Management Chip

In this paper, an oscillator circuit with a soft start function is designed for DC-DC converters, and its working principle is analyzed. Based on the NUVOTON 0.35 μm BCD technology, the circuit is fully simulated and optimized via Cadence software. Under the typical environment of 5 V and 27°C, the typical oscillation frequency of the oscillator is 800 kHz and the duty ratio is 20%. The expected index is reached and the circuit function is realized. At the same time, a temperature-independent current bias is designed to charge and discharge a capacitor in the oscillator so that the frequency of the oscillator is very little affected by temperature. Under the simulation condition of -40°C–140°C, the operation of the oscillator is stable and its frequency variation is 2.95% over the above temperature range.


Introduction
At present, whether in the field of national defense military equipment, or civil equipment, the power supply system is inseparable.DC-DC chip is widely used in all kinds of electronic equipment due to their characteristics of small size and high efficiency [1].It is an indispensable type of power supply for the rapid development of today's electronic industry.
The oscillator is an important part of the switching power supply chip [2].As the core module of the DC-DC switching power converter chip, the performance of the oscillator has a direct impact on the power supply chip.This requires that the oscillator can produce a clock signal that does not vary substantially with temperature.When the circuit is just started, the steady state of the circuit has not been completely established, so the logic control circuit may output a high level for a long time to make the high-voltage power tube turn on at the full duty ratio, and the chip will output a large pulse current, which exceeds the current that the load can bear, easily causing load damage.To avoid the generation of starting surge current, this paper designs an oscillator circuit with a soft start based on the structure of a constant current source charge-discharge stretching oscillator with a double comparator [3].

Soft start circuit design
The soft start circuit designed in this paper is shown in Figure 1.The soft start circuit is composed of current-limiting MOS tube Mb1~Mbn, soft start capacitor C, comparator, transmission gate, inverter, and Rise_pul.VREF is connected to the upper voltage of 2.1 V, TG is the transmission gate, Rise pul is the circuit that outputs short pulses along the rising edge of the input, and M1-M10 is the comparator.IBIAS1 is the soft start current bias.The current is scaled down by Ma and Mb1~Mbn current mirrors to generate a soft starting current I of nA level [4].A large capacitor is charged by a small current, and the capacitor voltage rises slowly.The upper voltage of the oscillator is replaced by the capacitor voltage to realize the soft start function of the system [5].Soft startup time t can be expressed as: The specific working principle is as follows: the capacitor C starts charging from 0 V, and the voltage rises slowly, but it is lower than VREF.The comparator outputs a high level, TG2 is switched on, and VH outputs capacitor voltage.When the capacitor voltage is higher than VREF, the comparator output is low level, TG1 on, and VH output VREF at the same time when the comparator is reversed.Rise pul outputs a short pulse, and then through a phase inverter, the M14 tube is on for a very short time, and the capacitor is charged rapidly, which avoids flipping the comparator output.

Temperature-independent bias current design
In the application of integrated circuits, the operating temperature is often in the range of -40 ႏ-140 ႏ.Therefore, to provide a stable and ideal current source for the circuit or system, the influence of temperature on the reference current source circuit should be minimized [6].The current circuit designed in this paper is shown in Figure 2. The operational amplifier AMP, NMOS tube M1, and resistor R1 form a voltage-current converter.Because ܹ ଶ ‫ܮ‬ ଶ Τ : ܹ ଷ ‫ܮ‬ ଷ Τ = 1: 1 and ‫ܫ‬ ଶ = ‫ܫ‬ ଵ , VREF is 1.2 V voltage generated by band-gap reference.R1 and R2 are poly resistances with negative temperature characteristics, so I1 is a positive temperature coefficient current.

Oscillator circuit design
In integrated circuits, the relaxation oscillator is a nonlinear electronic oscillator circuit that produces a non-sinusoidal repetitive output signal [7]. Figure 3 shows the oscillator circuit designed in this paper, which is composed of an energy storage capacitor, comparator, inverter, RS flip-flop, and D flip-flop.Ibias connects temperatures-independent current bias, COMP1's reverse voltage VH is the soft starting voltage, COMP2's forward voltage VL is 0.4 V, pin 20 out outputs a 20% duty cycle square wave signal, and half out pin outputs square wave signal with 50% duty cycle.
In the initial state, the upper plate voltage of capacitor C1 is 0, and capacitor C1 is charged by current I1.When the voltage on the capacitor exceeds the lower limit voltage VL of COMP2, COMP2 outputs a low level, COMP1 outputs a low level.Both inv1 and inv2 output a high level, and the RS trigger outputs an unchanged low level.When the upper limit voltage VH of the comparator COMP1 is exceeded by the voltage on the capacitor C1, COMP1 inverts the output high level, COMP2 continues to output a low level, inv1 outputs a low level, inv2 outputs a high level, and RS flip-flop sets "1" to output high level.when the RS flip-flop outputs a high level, the NMOS tube M4 is on, and the PMOS tube M3 is off, so the capacitor C1 starts to discharge.When the upper voltage of the capacitor C1 is lower than the upper voltage of COMP1 VH, COMP1 reverses to the low level, COMP2 continues to output a low level, inv1, and inv2 output a high level, and the output of the RS trigger is unchanged.When capacitor C1's voltage continues to drop to COMP2's lower limit voltage VL, COMP2 turns over to a high level, COMP1 continues to output a low level, inv1 outputs a high level, inv2 outputs a low level, and RS trigger sets "0" to output low level.NMOS tube M4 is closed, and PMOS tube M3 is switched on.We charge the capacitor, move on to the next cycle, and repeat it.
The oscillator cycle is determined by the capacitor charge and discharge time, assuming a capacitor charging time of ‫ݐ߂‬ ଵ and the discharge time of ‫ݐ߂‬ ଶ .The capacitor charge current is I1, and the discharge current is I2, which can be obtained from the following formula [8]: Capacitor charging time is: Capacitor discharge time is: The period of the oscillator is: where ߂ܸ = ‫ܪܸ‬ െ ‫,ܮܸ‬ as can be seen from Formula (14), the frequency of the oscillator is: In conclusion, the expected square wave signal can be obtained by adjusting the width-length ratio of capacitors C1 and M1 to M5 during the whole working cycle.

Soft start circuit simulation
Simulation conditions: power supply voltage is 5 V, temperature is 27 ႏ, and TT processes angle.The designed soft start time is t=4 ms.Through the calculation of Formula (1), the soft start capacitor C is designed to be 4.1 pF, the soft start current is designed to be 2 nA, the upper voltage of the oscillator VH is 2.1 V, and the initial voltage of the capacitor is 0 V. Figure 4 shows the simulation results of the soft start circuit.As can be seen from Figure 4, the initial voltage of the upper plate of the soft-start capacitor C is 0 V.After the circuit is started, the voltage of the upper plate of the capacitor C slowly rises and reaches the upper voltage of the oscillator 2.1 V after 4 ms, and then is replaced by the reference voltage of 2.1 V.The upper voltage of the oscillator no longer changes, and the soft start function is completed.

Simulation of reference current independent of temperature
Simulation conditions: the power supply voltage is 5 V, TT processes angle, and the reference current is scanned from -40 ႏ to 140 ႏ.Specific simulation results are as follows:

Oscillator circuit simulation
The transient simulation was carried out on the oscillator under the conditions of the supply voltage of 5 V, process angle of TT, and temperature of 27 ႏ.The transient simulation results were shown in Figure 6.The oscillator output amplitude is 5 V, the oscillation period is 1.25 ȝS, the duty ratio is 20%, and the oscillation frequency is 800 KHz.Table 1 shows the oscillation frequency of the oscillator in the temperature range of -40 ႏ-140 ႏ, with stable performance and a frequency offset of 2.95%.It is better than the frequency offset in [9].

Conclusion
An oscillator circuit suitable for a DC-DC switching power converter chip is designed by using a constant current source to charge and discharge the oscillation capacitor in the oscillator with double comparators included.At the same time, a soft start circuit is applied to avoid the generation of surge current [10].The simulation results show that the output frequency of the oscillator is little affected by the temperature, and the variation is only 2.95% over a temperature range of -40 ႏ to +140 ႏ.The performance of the circuit fully meets the requirements of use in the design of a DC-DC switching power supply.

Figure 4 .
Figure 4.The soft start output signal

Figure 5 .
Figure 5. Reference current as a function of temperature

Figure 6 .
Figure 6.Transient simulation of the oscillator

Table 1 .
Results of oscillator frequency variation with temperature