Simplify Power Supply Design with Integrated Switching Regulators

When it comes to power supply design, most engineers are scratching their heads and often ask, "Where do I start?" First, the power supply topology must be determined, including buck, boost, flyback, half-bridge, and full-bridge. The control scheme, voltage mode, current mode, fixed on-time, etc. must also be determined. Other questions include: (1) What are the frequency characteristics of the power supply? This will determine what inductor and capacitor should be used to meet the output ripple and load transient response requirements. (2) In order to ensure the stability of the entire circuit under various load and temperature conditions, what compensation scheme should be used? (3) Selecting the "right" MOSFET is also not a trivial matter. Can the drive circuit control the gate capacitance of the MOS FET ? How will parasitic capacitance and Rds(on) affect the total power consumption?

But there are more questions to answer. A PCB designer may come and tell you that there is not enough space on the PCB board to accommodate all the selected components. Where should the controller be placed? Or where should the MOSFETs, input capacitors, inductor, output capacitors, control circuitry, and so on be placed? What grounding scheme should be used? Where do PGND and AGND connect? How can AC loops be minimized for best electromagnetic interference (EMI) performance or to eliminate noise interference? Where should the heat sink be placed? What is the direction of airflow? How many vias should be used?

The above problems show that the design of power switching regulators is not a simple task. But Intersil's integrated FET DC/DC regulators make the design of step-down power converters easy. Most of the difficult problems have been solved inside these IC chips, and various configurations have been optimized, such as MOSFET size, drive circuit, current sensing element and current limiting, loop compensation, temperature compensation and overheat protection. The switching frequency is as high as 1MHz, so small inductors and ceramic capacitors can be used, which are standard products of many manufacturers. Finally, for most solutions, Intersil also provides evaluation circuit boards and recommended PCB designs for customer reference.

Advantages of Integrated FET DC/DC Converters

Figure 1 is a typical application circuit of a complete 4A converter using the ISL8014 chip. This circuit requires very few external components. Figure 2 is a block diagram of the ISL8014 integrated FET silicon chip. The integration of many features and functions on the same chip makes power supply design very easy.

Figure 1: Typical application diagram of a 4A integrated FET power converter.

1. Built-in MOSFET

Please note that Figure 2 has a high-side power P-channel MOSFET from the VIN pin to the LX pin, and a low-side N-channel MOSFET from the LX pin to the PGND pin, so there is no need to waste time looking for the right MOSFET. These built-in MOSFETs together with the driver circuit can meet a wide range of application requirements in terms of switching frequency, load current, input voltage, temperature range, etc.

Figure 2: Block diagram of the internal circuit of a 4A integrated FET power converter.

The rise and fall time of the driver circuit is about 3ns, achieving the best balance between EMI noise and power consumption. The non-overlap time, the on/off transition time (or dead time) of the high-side and low-side MOSFETs are well controlled to avoid shoot-through. No additional Schottky diode is required between the LX and PGND pins to improve efficiency. The switching waveforms are shown in Figures 3a and 3b.

Figure 3a: LX switching waveform (buck).

Figure 3b: LX switching waveform (boost).

2. Intermittent mode and continuous mode

There are many integrated regulators available to designers. For products where cost is not an overly important consideration, Intersil offers a standard buck regulator that operates in discontinuous mode (DCM) at light loads and requires an external power Schottky diode. On the other hand, there are many synchronous buck regulators that do not require an external Schottky diode and can operate in continuous mode (CCM) and/or DCM.

3. Internal and external loop compensation

Most low input voltage regulators offered by Intersil feature internal compensation, and the designer does not need to guarantee stability for every operating condition. The parameters selected support most typical applications listed in the data sheet. For regulators with a wider range of rated input current or higher rated output current, external compensation is used to gain greater flexibility. The product data sheet provides clear instructions and design guidelines.

4. Overcurrent protection with temperature compensation

The Intersil integrated regulators listed in Table 1 feature overcurrent protection. The peak current is monitored in the region of the high-side power P-channel MOSFET. This protects against external noise that might require additional filtering and longer protection response times, as is often the case with ICs without integrated MOSFETs.

If the current drawn is too large, the comparator flips and the high-side MOSFET is turned off. In addition to strong noise immunity and overcurrent protection, the temperature compensation function of the regulator can also maintain relatively constant limits throughout the allowable temperature range. For every 1°C change in temperature, the Rds(on) of most MOSFETs will change by 0.5%. For solutions using external MOSFETs, especially when using external MOSFETs to sense current, it is difficult to adjust according to temperature changes without adding additional cost and/or circuit complexity. Integrated regulators can easily adjust internally according to changes in MOSFETs. Compared with external circuits, the thermal coupling between the power device and the control part is tighter. A comparison of devices with temperature compensation and devices without temperature compensation is shown in Figure 4.

Figure 4: Output current overload threshold for a typical 4A device.

5. Other advanced control functions

Intersil's integrated FET DC/DC converters also feature advanced control functions, as shown in Table 1.

Design Example Using Integrated FET

This design example takes the ISL8014 in Figure 1 as an example, assuming that the required input is Vin=5V, the output is Vo=1.8V, and the output voltage ripple is less than 18mV. The design steps are as follows:

(1) Determine the switching frequency Fs. The normal switching frequency is 1 MHz, and it can be increased by synchronization to a maximum of 4 MHz. For simplicity, 1 MHz is used here.

(2) Calculate the inductor L. ΔI is the peak-to-peak ripple current flowing through the inductor. It is recommended to set ΔI to about 30% of the maximum output current. The maximum output current of ISL8014 is 4A, so ΔI = 1.2A.

(3) Determine the equivalent series resistance R _ESR of the output capacitor .

(4) The minimum recommended output capacitor value is 44μF. Ceramic capacitors are a good choice because of their low R _{ESR . Each 22μF capacitor in a 0805 package has an R ESR} of 5mΩ, so 2×22μF can be selected.

(5) Calculate the feedback resistor divider voltage using the following formula, where VFB _is 0.8V as specified in the datasheet:

(6) The input capacitance is not important, C1 can be set to 2×22μF.

(7) Next is PCB design. Please refer to the ISL8014 data sheet, which can be downloaded from www.intersil.com. The key PCB design steps include: inserting the IC on the circuit board; inserting the inductor adjacent to the IC's LX node; inserting C2 adjacent to the other end of the inductor L and the IC's PGND pin; inserting C1 next to the IC's VIN pin; inserting R3 next to the SGND and V _FB pins; inserting R2 and C3 next to R3; drilling about 6 vias under the IC's power pad for heat dissipation; drilling about 4 vias to connect PGND to C1 and C2 respectively; filling the second layer with PGND connections. The PCB design of ISL8014 is shown in Figure 5.

Figure 5: PCB design.

Conclusion

Intersil's integrated FET regulators have a wide selection of features and functions that make them easier to use, especially when using built-in loop compensation. This article lists the recommended output inductors and capacitors for low input voltage applications and discusses simple design steps and layout design. With these simple steps, most designers can get the results they need. For integrated FETs using external compensation, the data sheet is almost always accompanied by a specific and detailed analysis.

Table 1: Characteristics of Intersil's integrated FET DC/DC converters.