Application of POSCAP in Synchronous Rectification Step-Down DC/DC Converter

1 Introduction

POSCAP of SANYO is the abbreviation of "Polymer Organic Semiconductor Solid Electrolytic Capacitor". Most of the POSCAP anodes are sintered tantalum, and only a small part of the anodes are aluminum foil, so POSCAP tantalum capacitors are commonly used. POSCAP tantalum capacitors are particularly suitable for filter capacitors in power supply circuits, such as DC/DC converters in personal computers, portable computers and base stations, video recorders, digital cameras, navigation systems (GPS), PDAs, mobile phones, DVDs, etc. In order to help users develop new products, many DC/DC converter manufacturers provide typical application circuits with complete parameters in the data sheets, and users can assemble a complete power supply according to the circuit diagram. At the same time, evaluation boards are also specially provided to accelerate the development cycle. However, if the use conditions (such as switching frequency, input voltage, output voltage or output current) change and are different from the typical application circuit, the parameters of the relevant components in the circuit must also be changed accordingly. This article uses examples to introduce how to determine the value of the inductor and the value of the output capacitor after the use conditions change through simple calculations when the use conditions change, and select the appropriate POSCAP capacitor.

2 Characteristics and application examples of POSCAP tantalum capacitors

POSCAP tantalum capacitors are characterized by small size and large capacitance; low ESR (equivalent series resistance); large allowable ripple current; excellent noise absorption characteristics and frequency characteristics; good temperature characteristics; long life; no explosion or combustion in the event of a fault, and safer than ordinary tantalum electrolytic capacitors, so they are most suitable for use as output capacitors in high-efficiency, low-voltage, and high-current step-down DC/DC converters. In recent years, the operating voltage of electronic products has dropped from 5 V to 3 V, and even to below 1.0 V, while the output current has increased. For this reason, many high-efficiency, low-voltage, and high-current step-down DC/DC converters have been developed. The capacitance change of POSCAP tantalum capacitors in the range of 10 to 100 kHz is much smaller than that of ordinary tantalum capacitors, so for DC/DC converters with an operating frequency of 1 to 3 MHz, it is extremely beneficial to use POSCAP tantalum capacitors to reduce losses and ripple voltage.

The high-speed synchronous rectification step-down DC/DC converter circuit composed of National Semiconductor's LM2745 is shown in Figure 1. When the use conditions change, the inductor value and output capacitor value in the original typical application circuit may change, so it is necessary to select a suitable POSCAP capacitor.

The circuit has an input voltage of VIN = 3.3 V, VCC = 3.3 V, an output voltage of VOUT = 1.2 V, a maximum output current of IOUT(max) = 1.6 A, and a switching frequency of fsw = 1 000 kHz. In the figure, C01.2 is C01 and C02 in parallel, and C02 is a 0.1μF, 16 V multilayer ceramic capacitor. The specific steps for calculating its inductor L1 and C01 are as follows:

(1) Preliminary estimation of the ripple △I′OUT on the circuit

△I′OUT can be 20% to 40% of IOUT(max), but because the switching frequency is high, the output current IOUT(max) is large, and a coefficient of 20% is used, then:

△I′OUT=20%IOUT(max)=3.2 A

(2) Calculation of inductor L1

The L1 value is selected according to the similar standard value, that is, LACT = 0.22 Μh

(3) Complex calculation of ripple current △IOUT

According to the value of LACT, △IOUT is calculated. The specific formula is as follows:

(4) Select the appropriate POSCAP tantalum capacitor

According to the known VOUT and calculated △IOUT, select the appropriate output capacitor COUT according to the POSCAP sample. Two conditions must be met: VOUT = (0.8 ~ 0.9) VRATED (rated voltage of the capacitor); △IOUT (ALLOW) (maximum allowable ripple current of the capacitor) >> △IOUT. If the △IOUT (ALLOW) of the selected single capacitor cannot meet the requirement of being greater than △IOUT, N capacitors of the same model can be connected in parallel to make △IOUT (ALLOW) × N > △IOUT.

Therefore, according to the POSCAP sample, the 6TPD470M with a rated voltage of 6.3 V is selected. Its capacitance is 470 μF, ESR is 10 mΩ (max), and △IOUT (ALLOW) is 4400 mA, which meets the two conditions for selecting capacitance.

(5) Calculation of maximum output ripple voltage

After selecting the capacitor, we know its maximum ESR value, and the maximum output ripple voltage is:

△VOUT=△IOUT×ESR/N=3.4 A×10 mΩ=34 mV

If the calculated △VOUT cannot meet the requirements, choose a capacitor with lower ESR or increase the N value to solve the problem. The purpose of using a capacitor with large capacitance and small ESR is to reduce the output ripple voltage and improve the load transient response. If other electrolytic capacitors are used, they often require a larger capacitance or more parallel numbers. POSCAP tantalum capacitors give full play to their strengths.

The above calculation results are very close to the parameters in the data, and have certain practical value. However, considering the tolerance of the capacitor and the influence of temperature on ESR and capacitance, experiments are needed to verify or modify them.

3 Conclusion

In recent years, many DC/DC controller manufacturers have adopted POSCAP tantalum capacitors in their typical application circuits and evaluation boards. Drawing on the above application examples of POSCAP tantalum capacitors, engineers and technicians use the above formula to calculate the parameter values ​​of other types of DC/DC converter application circuit components when the use conditions change, and select appropriate POSCAP tantalum capacitors. However, it must be noted that POSCAP tantalum capacitors cannot be used with reverse polarity, cannot be used beyond the rated voltage, and must be limited to rapid charging or discharging. It must be confirmed that it is within the operating temperature range.