Buck regulator design based on LM2593HV regulator

    Abstract: LM2593HV is a new step-down (bump) regulator launched by National Semiconductor. It can output fixed voltages of 3.3V and 5V, and its output can also be adjusted as needed. Its maximum load current is 2A. This article introduces the structural characteristics, functional principles and issues that should be paid attention to in the selection of peripheral components of LM2593HV, and gives several typical application circuits.

    Keywords: voltage regulator regulator buck LM2593HV

1 Features of LM2593HV

LM2593HV is a monolithic IC with step-down (mutation) switching regulation launched by National Semiconductor. The output of this regulator is fixed at 3.3V and 5V, which can also be adjusted as needed. Its adjustable range is 1.2~57V, and it has excellent linearity and load regulation functions and can drive a 2A load. Under complete input and output conditions, the allowable error of its output current is ±4%, and the allowable error of the oscillation frequency is ±15%. In addition, LM2593HV also has external shutdown and self-protection functions, and can operate at 150kHz. changing frequency. The typical standby current in the low-power standby state is only 90μA, which can greatly reduce the power consumption of the system. The chip is also designed with over-temperature shutdown and two-level current-limiting protection functions. It can be used in simple and efficient voltage reduction (sudden change) regulation, linear regulator pre-regulation, patch exchange and positive and negative conversion and other regulating systems. The main features of LM2593HV are as follows:

    ●Has fixed 3.3V, 5V and adjustable voltage output;

●The adjustable output voltage range is 1.2~57V, and the maximum allowable error is ±0.4%;

●The output load current is 2A;

●The maximum output voltage can reach 60V;

●Built-in 150kHz oscillation fixed frequency;

●Contains shutdown and soft start switches;

●With adjustable output error flag;

●With error flag delay function;

●Has low power consumption mode, typical value is 90μA;

●Extremely efficient;

●With thermal shutdown and current limiting protection functions.

2 Pin functions of LM2593HV

LM2593HV adopts two standard 7-pin TO-220 and TO-263 surface mount package forms. The TO-220 package form also has different bent pins to choose from to adapt to the application needs of different occasions. Figure 1 shows Its outline package diagram.

LM2593HV contains circuits such as frequency compensation, fixed frequency oscillator, shutdown/soft start, error output flag, flag delay, overheating protection, current source and voltage source. The functions of each pin are as follows:

VIN (pin 1): positive power input terminal. Adding a suitable bypass capacitor to this pin can optimize its transient voltage and provide the required switching current to the chip.

OUTPUT (pin 2): Internal switch output. Within a working cycle of VOUT/VIN, the voltage on this pin can switch between (+VIN-VSTART) and -0.5V.

ERROR FLAG (pin 3): error flag output. When the chip's output voltage exceeds the adjustment range (at least 95% of the nominal value), and the open collector output of its internal transistor is low (≤0.6V), the chip is in a shutdown state, and its assigned shutdown current is 90μA; When this pin is high level (≥2.0V), the chip is in normal working condition. It has a typical switching threshold voltage of 1.3V and an internal maximum clamping voltage of 7V. If the voltage input to this pin is higher than the clamping voltage, the voltage must be divided by an external resistor to ensure that its current does not exceed 1mA. When the voltage input to this pin reaches 1.8V, the working cycle of the chip is minimum. Then as the voltage increases, the working cycle also increases, reaching the maximum (100%) when it reaches 2.8V or higher. Therefore, when Connecting a capacitor to this pin can achieve the soft-start function, and the chip's internal power source will charge the capacitor to the clamping voltage. When the voltage of this pin is lower than 1.3V, the charging current is 5μA, and when it is higher than 1.3V, it will drop to 1.6μA. This can be achieved with a smaller soft-start capacitor.

3. Selection of components

3.1 Inductor

During normal startup (not soft start, nor output short circuit), the current passing through the inductor may exceed the chip limit current ICLIM due to instantaneous changes. ICLIM should be larger than the load current ILOAD. In an overload state, it may also lead to saturation of the inductor. Normally, the chip protects itself by preventing the inductor current from exceeding ICLM. But if the DC input voltage exceeds 40V, the chip will be unable to do anything, so the current through the inductor will increase until the device is damaged. In order to ensure the reliability of the system, when the DC input voltage exceeds 40V, the instantaneous current of the inductor should be set to ICLIM instead of the saturation current of the inductor.

3.2 Input capacitance

A bypass capacitor should be connected between the ground and the VIN pin, and it should be as close to the chip as possible, and the pin should be as short as possible. This capacitor can be used to protect against large voltage transients at the input and can also provide instantaneous current for switch activation.

3.3 Output capacitor

The output capacitor COUT can be used to filter the output to improve the stability of the chip loop. COUT should be a low impedance, low ESR solid foil capacitor, and its ESR value should not be less than 100mΩ, otherwise it may cause a short circuit. However, if the ESR is too large, it may affect the system efficiency and output rated voltage. Therefore, the ESR must be selected carefully.

3.4 Clamping Diode

Because of the fast switching rate and low forward voltage drop of the LM2593HV, a Schottky diode is used to provide current to the inductor when the switch is off. The voltage rating of this diode should be greater than the DC input voltage (not output).

3.5 Shutdown/soft start

Current suppression at startup is very useful when internal current is limited. Soft start can be used instead of voltage blocking or delayed start, and very low output voltage ramps require larger soft-start capacitors. If only the shutdown function is used, this capacitor can be omitted.

4 General applications

4.1 Reverse regulator

Figure 2 is a positive and negative voltage conversion circuit with a common ground. LM2593HV can detect the reverse output voltage at the FEEDBACK pin and adjust it. The maximum output current of LM2593HV depends on the input and output voltages. Its switching peak current IPEAK is determined by the following formula (efficiency is 100%):

IPEAK=ILOAD(VIN+VOUT)/VIN+[(VINVOUT) ×[(VINVOUT×10 6)/2Lf(VIN+VOUT)]

In the formula: L unit is μA, f unit is Hz.

The maximum voltage passing through the regulator is the sum of the absolute junction values ​​of the input and output voltages, but it cannot exceed 60V. If the input voltage of 20V is converted into an output voltage of -5V, the voltage between the ground and the input pin is 25V. The maximum input voltage allowed by the LM2593HV is 60V.

Diode D1 in Figure 2 can be used to eliminate input voltage pulse noise under small or no load conditions. At low input, a Schottky diode can be used, and at high input a 1N5400 can be used.

This regulator requires a large input current when starting up, even at a small load, which should be comparable to the current of the LM2593HV (about 4.0A). Therefore, a soft-start circuit is used in Figure 2. Several shutdown methods are also given.

4.2 Undervoltage lockout

In some applications, it is required that the regulator does not change when the input voltage reaches a pre-fair value. Figure 3 is the undervoltage lockout circuit of the abrupt regulator, and Figure 4 is the connection circuit when it is reversed. Figure 3 uses Zener diode Z1 to set the threshold voltage when the switch is activated. When the input is less than the Zener voltage, resistors R1 and R2 set the SD/SS pin low and the chip starts to work. The undervoltage lockout threshold voltage is approximately 1.5V higher than the Zener voltage. Figure 4(a) shows a continuous threshold voltage for on and off. If a hysteresis circuit is used. Then the on voltage and off voltage in Figure 4(b) will be different. Its hysteresis is approximately equal to the output voltage value. Since the SD/SS pin registers a Zener clamp voltage of about 7V, when Q1 is open circuit, the current through R2 is about 1mA.

4.3 Negative voltage charge pump

Sometimes the circuit can also use a charge pump to meet the needs of small current and negative voltage output. The specific circuit is shown in Figure 5. The magnitude of this negative voltage is equal to the correct input voltage and provides an output current of 600mA. However, a minimum load of 1.2A must be ensured on the adjustable positive output to ensure normal operation of the charge pump. Resistor R1 is used to prevent the charging current of C1 from being greater than the limiting current of LM2593HV. This approach can also be used with other simple conversion systems.

5 Typical applications

LM2593HV has a wide range of applications, and only requires very few external components to achieve voltage stabilization, and the operation is also very convenient and simple. Figure 6 is a typical application circuit diagram of LM2593HV.