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Operation of an inverting DC-DC converter with charge pumping and LDO in the LM 27762 microcircuit

https://doi.org/10.38013/2542-0542-2019-1-35-43

Abstract

The paper focuses on the study of operation of an inverting DC-DC converter with charge pumping and LDO, which are the part of the combined bipolar secondary power supply LM 27762. The measurements covered two modes amply included in the range of normalized parameters with input voltages equal to 3.5 and 5.5 V, and output voltages 1.8 and 4.9 V, respectively, as well as the mode with an input voltage of 5.0 V and an output voltage 4.7 V, with a difference in the input and output voltage not exceeding the normalized limit of the voltage drop across LDO . Pulsations of voltages were measured at various load currents from 15 to 250 mA at the positive and negative poles of the flying capacitor, at the output of the charge pumping system and, in the latter case, also at the output of the microcircuit. Based on the results obtained, it was shown for the first time how at low load currents up to about 100 mA the charge pumping system operates in the burst mode, and at higher currents, it operates in the charge pumping mode with a constant frequency

For citations:


Bityukov V.K., Petrov V.A., Sotnikova A.A. Operation of an inverting DC-DC converter with charge pumping and LDO in the LM 27762 microcircuit. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2019;(1):35-43. https://doi.org/10.38013/2542-0542-2019-1-35-43

Introduction

Low-noise secondary power supplies find ever wider use in portable mobile devices, such as medical equipment, hand-held instrumentation, wireless communication systems, earphones, and others. The primary power source for such consumers in most cases is lithium-ion batteries or rechargeable storage batteries. Apart from the small size requirement, microcircuits of secondary power supplies face high demands on the output voltage stability, small voltage rip­ples, and high converter efficiency. One of the microcircuit types providing stabilised voltages of positive and negative polarities are microcir­cuits with charge pumping.

The majority of the manufacturers of elec­tronic components offer microcircuits intended for constructing either boost converters or buck converters, or polar-inverting converters. There is just a small number of microcircuits that can be used for obtaining regulated unipolar stabi­lised voltage in the mode of both decrease and increase of the input voltage. Operation of such microcircuits is analysed in [1][2][3][4][5].

The main drawbacks of the microcircuits developed in the early 2000’s and being pro­duced to this day, those that employ a classical technology of charge pumping, are fairly high output voltage ripple, normally amounting to 20...40 mV, and considerable drop of efficiency at deviations of the input/output voltage ratios from the optimal values. Numerous attempts have been made to reduce the output voltage rip­ples and increase efficiency in different modes. In the microcircuit of unipolar DC-DC converter LTC3245 (Linear Technology), according to the information published in 2013 [6], the charge pumping system, unlike the classical scheme, can operate in two modes. The mode with low out­put voltage ripples is implemented by means of constant charge pumping frequency. Therewith, the output voltage stabilisation system employs two comparators. Each comparator has an adju­stable offset, whose value increases (decreases) proportionally to increase (decrease) of the load current. In this way, output voltage stabilisation is achieved through regulation of charge amount transferred within one cycle. The output voltage ripples in this mode do not exceed 20 mV. How­ever, at low load currents in the mode with con­stant charge pumping frequency, the efficiency factor is reduced considerably. To increase it, the microcircuit can be set to the pumping mode using a pulse burst (BURSTMODE). A change­over from one pumping mode to the other can be

effected by supplying a logic signal of high or low level to the output of BURST microcircuit. It must be pointed out that in the burst mode ef­ficiency increase is accompanied by the growth of the output voltage ripples, which, according to [6], typically amount to 50 mV.

Microcircuits of bipolar DC-DC converters employing charge pumping have emerged only very recently. In the search systems and releases of electronic component manufacturers, referen­ces to just two microcircuits could be found. It follows from the search results that the first one of them was combined microcircuit LTC3260 (Linear Technology) [7]. Negative voltage is generated by an inverter with charge pumping, fol­lowed by LDO (linear stabiliser with low voltage drop), and positive output voltage is generated by positive-voltage LDO. In this microcircuit, to ob­tain a higher efficiency factor, the charge pumping system in the negative voltage generation path, similarly to LTC3245, operates in the pulse burst mode under low load currents and in the constant frequency mode under high load currents. In the negative voltage path, LDO reduces voltage rip­ples at the output, which is of special importance for the burst mode. According to the data given in the paper [7], respective ripple amplitude at the microcircuit output is below 10 mV.

A later development of a combined bipolar electric power source employing charge pumping is microcircuit LM27762 (Texas Instruments), considered in this paper.

The paper is a continuance to the cycle of works [1][2][3][4][5] in which DC-DC converters with charge pumping were studied. It reflects the results of the first research stage of bipolar secondary power supplies with charge pumping and focuses on the negative voltage generation only.

Object of study and measurement method

In LM27762 specifications, the indicated pos­sible input voltage range is from 2.7 to 5.5 V [8]; therewith, the converter allows to simulta­neously obtain two output voltages of positive and negative polarity lying within 1.5...5 V, with load currents up to 250 mX. To generate positive voltage of desired magnitude, LDO is used. Negative voltage is obtained by means of inverter followed by LDO. This paper only considers operation of that microcircuit section which serves to obtain output voltage of nega­tive polarity.

The operating principle of a charge pumping system with voltage inversion [8], as used in LM27762, is shown in Fig. 1. The inverter circuit has three external capacitors: input Cin, flying
C1, and capacitor Ccpout, preceding LDO and sometimes designated in [8] as Ccp. Inverted input voltage Vin is drawn from input capacitor Cin. The charge is transferred to capacitor Ccpout. by means of four switches S1...S4 and flying capacitor C1. The transfer cycle consists of two stages. At the first stage, flying capacitor C1 is charged from the input voltage on Cin at closing of switches S1 and S3, and at the second stage the charge is transferred from C1 to Ccpout at closing of switches S2 and S4. Since in this case the positive pole of Ccpout is connected to ground, voltage at its negative pole, close to value –Vin, is supplied to the output and to the regulating system via feedback circuit. The algorithm of regulating system operation is not described in [8]. Determining this algorithm was one of the objectives of this paper.

 

Fig. 1. Voltage inverting principle [8]

According to [8], clock frequency of the charge pumping system of negative output voltage is equal to 2 MHz. Such a high clock frequency makes it possible to decreaseoutput resistance and voltage ripples [8]. Unfortunately, the table of microcircuit electrical characteristics given in [8] is very limited, and the data in it pertain to one case of circuit implementation only, in which input voltage Vin = 5 V, the input capacitor and both output ones Cin = Cout+ = Cout- = 2,2 µF, flying capacitor C1 = 1 µF, and capacitor Ccpout = 4,7 µF. According to the table of [8], the value of voltage drop across negative LDO, given only for one load current of 100 mА and one output voltage of –5 V, is equal to 30 mV. Proceeding from the very limited typical characteristics presented in the graphical form, it just can be concluded that the magnitudes of voltage drop across LDO strongly depend on the load current and at 250 mА may reach 100 mV, with output voltage ripples (voltages at LDO output) amounting to 1...3 mV.

Analysing all the characteristics of LM27762 microcircuit given in [8], it has to be stated that they are plainly insufficient both for determining the algorithms of its operation in
different modes and for calculating voltage ripples in different points of its connection layout. This was actually the main objective of research undertaken in this paper.

It was important for the job to have a possibility to avail of a ready-made test printed-circuit board from the LM27762 manufacturer –evaluation module LM27762EVM [9]. A schematic electrical diagram of this module, with arrangement of check points for connection of instrumentation, is given in Fig. 2.

Fig. 2. Schematic of LM27762EVM module PCB [9]

Since the main causes of voltage ripples in the LM27762 microcircuit are accounted for by charge pumping and the regulating system of negative output voltage magnitude, they were the main focus of this paper. To investigate into those causes, measurements of temporal dependencies of voltage at positive UflyDC+ and negative UflyDC_ poles of 1 µF flying capacitor C1, with open oscilloscope inputs, were used, as well as alternating component  Uout∼ of voltage at 4.7 µF capacitor Ccpout (see Figs. 1 and 2) with closed oscilloscope input. Voltage at this capacitor is the voltage present at the charge pump regulating system output. It comes to the input of the negative output voltage LDO.

Voltage waveforms were recorded with oscilloscope GDS-72202 byGW Instek, with the use of FreeWave software. As a trigger, voltage Ufly. was normally applied. To ensure high quality of signal recording, the functions of digital filtering and single start, available in the GDS72202 oscilloscope, were used.

In this paper, LM27762 microcircuit operation in three different modes was studied. One of the modes was implemented on standard module LM27762EVM in the as-supplied condition, when 249 kΩ and 499 kΩ resistors are used in the regulating system as a voltage divider for the feedback circuit, as shown in Fig. 2. Such divider provides a voltage of 1.8 V at both outputs. In order to implement a typical operation mode of the microcircuit, voltage of 3.5 V was used for this mode as the input voltage.

Measurements in the other two modes were aimed at studying microcircuit operation in the limit cases under high input and output voltages. Those modes, even though called limit ones, do not go beyond the limits specified in the technical documentation [8]. One of them was implemented under input voltage of 5.5 V and output voltage of −4.9 V, and the other under input voltage of 5.0 V and output voltage which decreased from −4.70 to −4.07 V with the increase of load current. The two modes were implemented by replacing resistors in the feedback circuit for resistors with resistance of 82 kΩ and 249 kΩ.

Experiment results

Mode Uin = 3.5 V, Uout = –1.8 V. The mode is characterised by a great difference between the input and output voltage magnitudes. Given in Fig. 3 as an example are some of the measurement results for relatively low load currents, equal to 15 and 50 mA. In the left-hand field of the figure, zero levels of signals are shown. Here and below, odd numbers correspond to voltages across the positive or negative pole of the flying capacitor, and even ones, to the AC component of voltage at the charge pumping system output. A digital cut-off filter with frequency of 10 MHz was used in these measurements.

Fig. 3. Signal waveforms (2 µs/div) of voltages UflyDC+ (1 V/div) and Uout∼ (20 mV/div) under different load currents: 1, 2 – 15 mА; 3, 4 – 50 mА

For the load current of 15 mA, five discharge – charge cycles of the flying capacitor and their respective steps at Uout∼ can be plainly seen. Under load current of 50 mA, the number
of discharge – charge cycles of the flying capacitor increased to 15, while the interval between charge transfer bursts decreased significantly. With further increase of the load current, time interval between flying capacitor discharge – charge pulse bursts becomes shorter still, and at a certain value of current it disappears altogether.

Fig. 4 shows a comparison between signal waveforms under the currents of 100 mА and 250 mА. Under these currents the flying capacitor discharge – charge cycles occur with a constant frequency equal to 2 MHz.

Fig. 4. Signal waveforms (500 ns/div) of voltages UflyDC+ (1 V/div) and Uout∼ (20 mV/div) under different load currents: 1, 2 – 100 mА; 3, 4 – 250 mА

Along with recording of the UflyDC+ signal waveforms across the flying capacitor positive pole, the UflyDC– waveforms were recorded in the paper across the negative pole as well. Fig. 5
shows the waveforms for load current of 50 mA. It can be seen that the results well conform to the data given in Fig. 3 (curves 3 and 4). Fig. 6 shows a comparison between signal waveforms under load currents of 50 and 100 mА (burst pumping and constant-frequency pumping).

Fig. 5. Signal waveforms (2 µs/div) of voltages: 1 − UflyDC– (1 V/div); 2 − Uout∼ (20 mV/div) under load current of 50 mА

 

Fig. 6. Signal waveforms (2 µs/div) of voltages UflyDC– (1 V/div) and Uout∼ (20 mV/div) under different load currents: 1, 2 – 50 mА; 3, 4 – 100 mА

Mode Uin = 5.5 V, Uout = –4.9 V. The measurement results showed that under load current of 15 mА the bursts contain just three cycles of flying capacitor discharge – charge pulses (Fig. 7), whereas under load current of 50 mА, 5 cycles were observed, with the Uout∼ decrease over the pumping period amounting to 125 and 160 mV, respectively.

Fig. 7. Signal waveforms (500 ns/div) of voltages: 1 − UflyDC+ (2 V/div); 2 − Uout∼ (50 mV/div) under load current of 15 mА

Under load current of 100 mA, there is a different number of discharge – charge cycles in different bursts. This is demonstrated in Fig. 8. The mode is close to the state of transition from
burst pumping to constant-frequency pumping.

Fig. 8. Signal waveforms (2 µs/div) of voltages: 1 − UflyDC+ (2 V/div); 2 − Uout∼ (50 mV/div) under load current of 100 mА

Fig. 9 shows a comparison between signal waveforms under the currents of 153 and 220 mA. The pulses follow with the same frequency, but under load current of 220 mA their amplitude is somewhat greater.

Fig. 9. Signal waveforms (500 ns/div) of voltages UflyDC+ (2 V/div) and Uout∼ (50 mV/div) under different load currents: 1, 2 – 153 mА; 3, 4 – 220 mА

Since in many applications of combined DC-DC converters, which combine charge pumping with LDO, the output voltage ripples are of great importance (see Fig. 2), measurements of such ripples were taken during charge pumping system operation both in the burst mode (low load currents) and in the mode of constant-frequency pumping (high load currents). Two examples of the obtained results are shown in Figs. 10 and 11. It can be seen that in the burst mode the span of output voltage ripples is about 18 mV, with repetition period of about 7 µs, and in the constant-frequency (2 MHz) mode it makes 7 mV.

Fig. 10. Signal waveforms (1 µs/div) of voltages UflyDC– (2 V/div) and Uout∼ (20 mV/div) under load current of 90 mA

 

 

Fig. 11. Signal waveforms (2 µs/div) of voltages UflyDC– (2 V/div) and Uout∼ (20 mV/div) under load current of 157 mA

Mode Uin = 5.0 V, Uout = –4.7 V. According to the data provided by the LM27762 microcircuit manufacturer [8], with output voltage of −5 V and load current of 100 mА, voltage drop at VLDO– is to amount to 30 mV. Considering this, a mode close to the limit one was selected for the study. It is in this very case, with the same resistors in the feedback circuit and in the absence of load, that voltage settling at the output was equal not to −4.9 V, as was in the previous mode, but to −4.7 V.

The measurements taken showed that in the absence of load there were just three discharge – charge transfer cycles in the bursts, with the time interval between two consecutive bursts making about 150 µs.

Under load current of 50 mA, there were six discharge – charge transfer cycles, with the time interval between the bursts of about 5 µs. Under load current of 147 mА, there were no bursts, the discharge – charge cycles followed continuously, and voltage at the microcircuit output decreased to −4.44 V. Under load current of 239 mА, voltage decreased to −4.11 V. Fig. 12 shows a comparison between signal waveforms for load currents of 147 mА and 239 mА.

Fig. 12. Signal waveforms (500 ns/div) of voltages UflyDC+ (2 V/div) and Uout∼ (50 mV/div) under different load currents: 1, 2 – 147 mA; 3, 4 – 239 mА

Analysis of the obtained experimental results

The experimental findings of the study undertaken on the inverting DC-DC converter of LM27762 microcircuit make it possible to conclude that the information on its operation provided in technical documentation [8] is incomplete and at times incorrect. From the microcircuit description it is impossible to visualise its operation algorithm in different modes. Such information, available in [8], is contained in just two short remarks. The first one is a footnote to the table with electrical characteristics, reading “When VIN = 5.5 V charge pump may enter PWM mode in hot conditions” [8]. The second remark relates to operation with pulse-frequency modulation (PFM): “To minimize quiescent current during light load operation, LM27762 allows PFM or pulseskipping operation. By allowing the charge pump to switch less when the output current is low, the
quiescent current drawn from the power source is minimized. The frequency of pulsed operation is not limited and can drop into the sub-2-kHz range when not loaded. As the load increases, the frequency of pulsing increases until it transitions to constant frequency. The fundamental switching frequency in LM27762 is 2 MHz” [8]. No registered signal waveforms are given in [8].

The results obtained in this paper show that transition to the PWM mode under the input voltage of 5.5 V is not observed. In none of the microcircuit operation modes PFM is used in the charge pumping system. The discharge – charge cycle frequency is 2 MHz everywhere. Under relatively low load currents, at any values of the input and output voltages (excluding situations when the input voltage exceeds output voltage by a magnitude smaller than voltage drop at LDO), the charge pumping system operates in the burst mode, with the discharge – charge cycle frequency in a burst being equal to 2 MHz. The number of pulses in a burst depends on the load current and the upper and lower actuation threshold of the regulating system, which compares feedback voltage with the reference value. The obtained results demonstrate that the difference between the upper and the lower actuation thresholds is great.
In the results for the input voltage of 3.5 V and output voltage of −1.8 V, as shown in Fig. 3, under load currents of 15 and 50 mА, it amounts to about 75 mV. In the results for the input voltage of 5.5 V and output voltage of −4.9 VB, as shown in Fig. 7, under load current of 15 mА, it amounts to about 125 mV. Under load current of 50 mA, it is about 160 mV. These are very high values, but the use of LDO following charge pumping system makes it possible to reduce ripples to low values of the order of 18 mV, which undoubtedly is a great achievement.

It should be mentioned that, apparently, the mode of a burst of pulses in charge pumping for low load currents was first applied by Linear Technology in converter LTC3245 [6][7], receiving the name Burst Mode® Operation. Application of a pumping pulse burst under low currents in [6] is implemented on the base of joint use of a conventional system of pumping regulation by the upper and lower limits, together with control and regulation of charge transferred per one cycle. When LTC3245 is in the Burst Mode® Operation, under light loads, the maximum charge is transferred per one cycle. Regulation of charge current in the burst makes it possible to hold output voltage ripples within 50 mV without the use of LDO.

The LM27762 manufacturer does not refer to the algorithm of its operation under low currents as the burst mode, but it follows from the measurements taken by the authors of this paper that this is exactly the mode used in this microcircuit. However, voltage ripples at the charge pumping system output are, firstly, much higher in case of LM27762 than LTC3245, and, secondly, they depend on the load current. The charge transferred per one cycle in the burst mode may depend on a number of factors, in particular, on the resistance of switches, characteristics of the capacitors applied, voltage magnitude at the inverter input, and other. Therefore, control over the amount of transferred charge, or the current during this transfer, is of utmost importance. It would appear that there is no such control in LM27762. It is merely the continuous control of the output voltage magnitude that is maintained here. The incomplete last discharge – charge cycles obtained in this paper and shown in some of the figures (see Figs. 5, 6, 8) can be explained exactly by this.

An important item associated with operation of the LM27762 microcircuit is its transition, with the load current increase, from the burst mode to the mode of pumping with the constant clock frequency of 2 MHz. Two issues arise here. The first is the transition condition, the second is due to what the constant output voltage is maintained when load current increases. From the findings of this paper it can be seen that with an increase of load current during operation in the burst mode, inter-burst time intervals decrease and the system smoothly transits to the mode of constant-frequency pumping.

As concerns maintaining constant voltage at the output in case of constant pumping frequency, the recorded waveforms of Uout∼ demonstrate that with the increasing load current the amplitude of this voltage ripples increases only slightly. Here, maintaining the output voltage constant is provided by LDO. When voltage margin at the LDO input is small, as it was in the mode with Uin = 5.0 V and Uout = –4.7 V, with load current increasing, the output voltage decreases.

In conclusion it should be pointed out that the paper reflects on the results of the first stage of experimental research of combined bipolar DC-DC converters with charge pumping. This stage covers generation of voltage with negative polarity. At a later stage, it is planned to obtain the necessary results for generated positive voltage and undertake a sufficiently complete simulation of converter operation as a whole.

The research findings presented in the paper were obtained within the framework of the government task order No. 8.5577.2017/8.9 for carrying out a project on the subject “Investigation into noise characteristics and ripples of microcircuits of mobile secondary power supplies”, set by the Ministry of Education and Science of the Russian Federation.

About the Authors

V. K. Bityukov
Federal State Budget Educational Institution of Higher Education “MIREA - Russian Technological University”
Russian Federation


V. A. Petrov
Federal State Budget Educational Institution of Higher Education “MIREA - Russian Technological University”
Russian Federation


A. A. Sotnikova
Federal State Budget Educational Institution of Higher Education “MIREA - Russian Technological University”
Russian Federation


Review

For citations:


Bityukov V.K., Petrov V.A., Sotnikova A.A. Operation of an inverting DC-DC converter with charge pumping and LDO in the LM 27762 microcircuit. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2019;(1):35-43. https://doi.org/10.38013/2542-0542-2019-1-35-43

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