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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Starting the MAX756 voltage converter with reduced input voltage. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters In some small-sized electronic devices powered by galvanic cells or batteries, step-up voltage converters are used on the MAX756 chip and its analogues. Starting them with a connected load and reduced supply voltage can be difficult. This article is devoted to the solution of this problem. Modern step-up voltage converters make it possible to obtain the required output voltage at a very low input voltage, often less than 1 V. In the vast majority of cases, the load of the voltage converter is constantly connected to its output. This makes it difficult to start the converter and achieve its output voltage of the required value, especially when the supply voltage is close to the minimum allowable. Modern step-up voltage converters make it possible to obtain the required output voltage at a very low input voltage, often less than 1 V. In the vast majority of cases, the load of the voltage converter is constantly connected to its output. This makes it difficult to start the converter and achieve its output voltage of the required value, especially when the supply voltage is close to the minimum allowable. A simplified block diagram of the MAX756 voltage boost converter microcircuit [1] and its connection are shown in fig. 1.
The microcircuit contains a control unit for the output key field-effect transistor and this transistor VT1 itself. This is how many microcircuits of step-up voltage converters are arranged. In addition to the DA1 chip, the voltage converter contains a storage inductor L1, a Schottky diode VD1 and two oxide capacitors C1 and C2 at the input and output, respectively. The control unit receives power from the output of the converter and performs pulse-width regulation. When the transistor VT1 is open, the load connected to the output is powered by the capacitor C2, the diode VD1 is closed, the inductor L1 is connected to the power supply. The current through the inductor increases and it stores energy. After closing the transistor VT1, the EMF pulse of the self-inductance of the throttle is added to the supply voltage and charges the capacitor C1 through the open diode VD2. Thus, the energy accumulated by the inductor L1 is transferred to the load. When the power supply voltage is close to the minimum allowable, starting the voltage converter may be difficult, since the transistor VT1 does not open completely. The control device is powered by the output voltage, which, when the converter is started, is less than the supply voltage by the voltage drop across the diode VD1 and the active resistance of the inductor L1. The insufficiently open channel of the transistor VT1 has a greater resistance, which limits the peak value of the current pulses through the inductor L1. As a result, the converter, not being able to simultaneously provide the load current and charge the output capacitor C2, cannot reach the rated output voltage. The described situation suggests that at the time of starting the converter, it is necessary to disconnect the load from it, which will allow the converter to reach the nominal operating mode at idle. After the output voltage reaches a certain value, and the output capacitor is charged, the load can be connected. In the future, the converter will operate normally. The developers of Maxim have taken this path, showing in [2] how to start the MAX756 boost converter with a connected load and low supply voltages. The MAX756 chip allows you to get one fixed voltage of 3,3 V or 5 V at the output at maximum load currents of 300 or 200 mA, respectively. The minimum supply voltage at which the converter starts at idle is 0,7 V. The converter has an input voltage drop detector (LBI/LBO pins; Low Battery Input, Low Battery Output - input and output of the low input voltage detector, respectively). The MAX756 is designed specifically for use in portable battery-powered equipment, so the detector is used to alert when the voltage at the LBI input falls below a certain threshold value chosen by the chip designer to be 1,25 V. In this case, the LBO output is connected to a common wire through the open internal transistor of the microcircuit. If the voltage at the LBI input is higher than 1,25 V, the internal transistor is closed and the lBo output is in a high impedance state. The detector operation voltage can be set by an input voltage divider connected to the battery supplying the converter. The signal at the LBO output is used both to notify the user about the battery discharge, and to forcefully disconnect, for example, the battery from the device in order to prevent their excessive discharge. The low minimum trigger voltage of the MAX756 microcircuit (0,7 V) makes it possible to build on its basis voltage converters powered by a single galvanic cell with a voltage of 1,5 V or a Ni-Cd or Ni-MH battery with a voltage of 1,2 V. Unfortunately, in the latter case, the value of the internal reference voltage U selected by the microcircuit manufacturerп = 1,25 V does not make it possible to determine the moment the battery is discharged to a voltage of 1 V, below which battery manufacturers do not recommend discharging them. The circuit of the converter based on the MAX756 microcircuit, in which the difficulties of starting at a low supply voltage are eliminated by disconnecting the load for the start time [2], is shown in fig. 2. A typical inclusion of the MAX756 chip (DA1) was used. When the supply voltage is applied, the voltage at the LBI input of the microcircuit is below the switching threshold (1,25 V), the voltage at the LBO output is low, transistors VT1 and VT2 are closed.
After the voltage at the output of the converter reaches the value Uconn=UPete(R1+R2)/R2, transistors VT1 and VT2 open and the load is connected to the output of the converter. With the resistances of resistors R1 and R2 indicated in the diagram, the load is connected to the converter when a voltage of 3,75 V is reached at its output. Graphs of the dependence of the maximum load current on the starting voltage of the converter [2] are shown in fig. 3. Upper line - with load disconnection for the start time, lower line - without disconnection. The graphs show that at a supply voltage of 1 V, these values are 65 and 2,5 mA, respectively. And with a converter supply voltage of 0,8 V, the maximum load current at startup increases from 45 μA to 45 mA.
Shown in Fig. 2 scheme has the only drawback: the LBI / LBO input voltage drop detector cannot be used for its intended purpose: to signal a decrease in the supply voltage, usually battery, below a certain threshold. The scheme shown in fig. 4 is devoid of the disadvantage noted above. It differs from that proposed in the article [2] by the output part of the device. When power is applied to the converter, the voltage at its output is below the threshold value of the voltage drop detector DA2. There is a low voltage at the detector output (pin 3), transistors VT1.1 and VT1.2 are closed, and the load is disconnected from the converter output. When power is applied, the voltage across the output capacitor C3 begins to rise. When it reaches a value of 4,7 V, output 3 DA2 goes into a high-impedance state, the output voltage of the converter is supplied to the gate of transistor VT1.1 through resistor R1. In this case, transistors VT1.1 and VT1.2 open, connecting the load to the output of the converter.
On fig. 5 shows a simpler version of turning on the converter on the MAX756 chip, which starts with the load connected. At the same time, unused LBI / LBO pins allow using the input voltage drop detector of the converter microcircuit for its intended purpose. Unlike the diagram in Fig. 4, the connection of the load to the output of the converter is carried out not after reaching a certain value of the output voltage, but with a certain time delay after the power is applied. When power is applied to the converter, capacitor C4 is discharged, the voltage between the gate and source of transistor VT1 is zero, so the transistor is closed, the load connected to the output is de-energized. As the capacitor C4 is charged through the resistor R1, the voltage across it reaches the threshold value izip, at which the transistor VT1 opens, and the load is energized from the output of the converter.
The duration of the load connection delay time tB (in milliseconds) without taking into account the charging time of the output capacitor C3 of the converter is calculated using formula (1.10) from the book [3]: t3=R1 C4 ln(UO/( UO - ORzip)), where R1 is the resistance of the resistor R1 in kiloohms; C4 is the capacitance of the capacitor C4 in microfarads; UO - output voltage of the converter (in volts). When calculating, it should be borne in mind that the value of Izip for the indicated transistor [4] can be in the range of 1,5 ... the time of establishing the output voltage of the converter at the minimum allowable supply voltage. The step-up voltage converter MAX756 has a domestic analogue KR1446PN1. Instead of the ZVP2110A [4] transistor, you can use another one designed for a current of at least 200 mA, for example, ZVP2106, BSP315, MMBF2202PT1. And MMDF2P02E is an assembly of two p-channel field-effect transistors, of which in the device according to the circuit in fig. 2 uses one of them. It can also be replaced by the transistors listed above. We will replace the 2N3904 transistor with imported 2N3903, 2N4400, 2N4401 or domestic KT315, KT3102 with any letter index. Transistor assembly IRF7307 is interchangeable with IRF7317 or IRF7507. Diode 1N5817 can be replaced by 1N5819, 1 N5820. Literature
Author: V. Oleinik
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