“In battery-powered low-voltage applications such as portable and wearable devices, there are often functions that require higher voltages to operate, such as RF transceivers, precision analog circuits, white LED backlight driving, biasing avalanche photodiodes (APDs) circuit, etc. This requires the use of a DC-DC boost converter to up-convert to the required voltage, allowing the device to work both energy-efficiently and efficiently.

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Author: Hard City Allchips

In battery-powered low-voltage applications such as portable and wearable devices, there are often functions that require higher voltages to operate, such as RF transceivers, precision analog circuits, white LED backlight driving, biasing avalanche photodiodes (APDs) circuit, etc. This requires the use of a DC-DC boost converter to up-convert to the required voltage, allowing the device to work both energy-efficiently and efficiently.

Boost Regulator Features

To meet some specific higher voltage requirements in low voltage applications, step-up DC-DC regulators convert low input voltages to high output voltages. Typical circuit components include: inductors, power MOSFETs, rectifier diodes, control ICs, input and output capacitors.

Figure 1: Basic Boost Regulator Configuration

A common retrofit configuration typically uses two MOSFETs, with a second MOSFET that replaces the rectifier diode and turns on when the power switch is off. MOSFETs have a lower voltage drop, which drastically reduces power dissipation while increasing the efficiency of the regulator.

In addition, some regulators include protection against overtemperature, output short circuits, open load conditions, and input overcurrent conditions.

Peripheral component selection

The conversion efficiency is an important indicator to measure the DC-DC boost circuit, and the power loss is mainly caused by the parasitic series resistance (ESR) of the Inductor, the forward voltage drop of the Schottky diode, the on-resistance of the power tube and the switch. loss of these four aspects. Of course, the chip itself also has static power consumption, which will affect the conversion efficiency under low load conditions, so the on-resistance of the power tube inside the chip must also be very small. At the same time, a suitable driving circuit should be designed inside the chip to ensure that the switching edge of the power tube is very steep, so as to reduce the power consumption during switching.
The difference in the choice of inductor and Schottky diode will affect the conversion efficiency, and the difference in the choice of capacitor and inductance will affect the output ripple. By choosing the appropriate inductor, capacitor and Schottky diode, high conversion efficiency, low ripple and low noise can be obtained.

1. Inductor selection

The inductor is a key element of the boost converter: it stores energy during the on-time of the power switch and transfers the stored energy to the output during the off-time through the output rectifier diode.
Designers must balance low inductor current ripple with high efficiency. For a given physical size, lower inductance inductors will have higher saturation current and lower series resistance, but lower inductance will result in higher peak currents, resulting in lower energy efficiency, higher ripple, and higher noise .
The inductance value of the inductor is related to the minimum inductance value Lmin, current ripple, etc. When calculating the specific inductance value, pay attention to the duty cycle (D) parameter, the specific size is: D = (Vout-Vin)/Vout.

First, it is necessary to ensure the minimum inductance value Lmin required to make the DC-DC boost work normally in continuous current mode.

This formula is derived in the continuous current mode, ignoring other conditions such as parasitic resistance, diode conduction voltage drop, and the actual value is larger. If the value of the inductance is less than Lmin, the magnetic saturation of the inductance may occur, resulting in a great decrease in the efficiency of the DC-DC circuit, and even the stable voltage cannot be output normally.

Second, considering the current ripple problem through the inductor, also in the continuous current mode ignoring the parasitic parameters,

When L is too small, the current ripple on the inductor will be too large, causing the maximum current through the inductor, the Schottky diode and the power tube in the chip to be too large. Since the power tube is not ideal, the power loss on the power tube will increase when the current is particularly large, resulting in a decrease in the conversion efficiency of the entire DC-DC circuit.

Third, in general, when efficiency is not considered, the load capacity that a small inductor can drive is stronger than that of a large inductor. However, under the same load conditions, the current ripple and the maximum current value of the large inductance are small, so the large inductance can make the circuit start up at a lower input voltage (the above are the conclusions derived under the same parasitic resistance conditions) .

In order to reduce the size of the external inductor, the operating frequency can be increased. For example, at a working frequency of 350KHz, only an inductance above 3.3uH is needed to ensure normal operation, but if the output terminal needs to output a large current (for example, the output current is greater than 50mA), in order to improve the work efficiency, it is recommended to use a larger inductance.

In the case of heavy load, the series resistance of the inductor will greatly affect the conversion efficiency. Assuming that the resistance of the inductor is rL and the load resistance is Rload, the power loss in the inductor is roughly calculated as follows:

Comprehensive consideration, it is recommended to use 27uH,

2. Output capacitor selection

The output capacitor reduces load ripple and helps provide a stable output voltage during load transients. When considering the ESR of the capacitor, the ripple of the output voltage is:

In order to reduce the output ripple, a relatively large output capacitor value is required. However, if the output capacitor is too large, the response time of the system will be too slow, so it is recommended to use a 100uF capacitor. If less ripple is required, larger capacitors are required.
When the output is connected to a large load, the ripple caused by ESR will become the most important factor. At the same time, ESR will increase the efficiency loss and reduce the conversion efficiency. Therefore, it is recommended to use tantalum capacitors with low ESR, or use multiple or X7R ceramic capacitors in parallel. Other types of capacitors may have higher ESR, reducing converter efficiency.

3. Diode

The rectifier diode has a great influence on the DC-DC efficiency. Although ordinary diodes can also make the DC-DC circuit work normally, it will reduce the efficiency by 5~10%. Therefore, it is recommended to use a diode with low forward voltage and short response time. Turkey diodes, such as 1N5817, 1N5819, 1N5821, 1N5822, etc.
In terms of specific parameters, the average forward rated current of the diode must be equal to or higher than the maximum output current, the repetitive peak forward rated current must be equal to or higher than the peak inductor current, and the reverse breakdown voltage must be higher than the rated voltage of the internal power switch.

For example, the MCP1665 has a 36V internal switch capable of delivering up to 1A. Therefore, Microchip recommends using the STPS2L40VU Schottky diode from STMicroelectronics, which has a reverse breakdown voltage of 40V and a forward current of 2A.

4. Input capacitance

If the input power supply is stable, even without the input filter capacitor, the DC-DC circuit can output the current and voltage with low ripple and low noise. However, when the power supply is far away from the DC-DC circuit, it is recommended to add a filter capacitor of more than 10uF to the input end of the DC-DC to reduce the output noise.

DC-DC boost regulators have high-speed switching characteristics and are very sensitive to PCB layout: parasitic inductance and capacitance can cause high output ripple, poor output regulation, excessive electromagnetic interference (EMI), and even high voltage spikes due to resulting in failure. Therefore, peripheral components should be close to the IC die, ground nodes should be close to the IC power ground pins to minimize loop area, and power ground, signal ground, and thermal pads should also be connected together at a single low-impedance ground point.