Most popular use of integrated switching regulator

2022-08-06
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Simplify power supply design with integrated switching regulator

when it comes to power supply design, most engineers will feel scratching their heads. They often ask "where to start". First of all, the topology of the power supply must be determined, including step-down, step-up, flyback, half bridge and full bridge, as well as the control scheme, voltage mode, current mode, fixed conduction time, etc. Other questions include: (1) what is the frequency characteristic of the power supply? This will determine which inductor and capacitor should be used to meet the requirements of output ripple and load transient response. (2) In order to ensure the stability of the whole circuit under various load and temperature conditions, which compensation scheme should be adopted? (3) Choosing the "right" MOSFET is not a trivial matter. Can the drive circuit control the gate capacitance of MOSFET? How will parasitic capacitance and RDS (on) affect the total power consumption

but the questions to be answered are not limited to this. PCB design engineers may come to tell you that there is not enough space on the PCB to accommodate all selected components. Where should the controller be placed? Or, where should MOSFETs, input capacitors, inductors, output capacitors, control circuits, etc. be placed? What grounding scheme is adopted? The viscosity index of hydraulic oil is generally higher than 90 2.2 where are PGND and agnd connected under operating temperature and pressure? In order to obtain the best electromagnetic interference (EMI) performance or eliminate noise interference, how can the AC loop be minimized? Where should the radiator be placed? What is the direction of the air flow? How many vias should be used

the above problems show that the design of power switching regulator is not a simple task. However, INTERSIL's integrated FET dc/dc regulator makes the design of step-down power converter easy. Most intractable problems have been solved in 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 overheating protection. The switching frequency is above 1MHz, so small inductors and ceramic capacitors can be used. These inductors and capacitors are standard products of many manufacturers. Finally, for most solutions, INTERSIL also provides evaluation circuit boards and recommended PCB designs for customers' reference

advantages of integrated FET dc/dc converter

Figure 1 is a typical application circuit of a complete 4A converter. Using isl8 enables customers to understand and operate 014 chip of our experimental machine in detail. This circuit requires very few external components. Figure 2 is a block diagram of the isl8014 integrated FET silicon chip. The same chip integrates many features and functions, which makes the power supply design very easy

figure 1:typical application diagram of 4A integrated FET power converter

1. built in MOSFET

please note that figure 2 shows a high side power p-channel MOSFET with VIN pin to LX pin and a low side n-channel MOSFET with LX pin to PGND pin. Therefore, there is no need to waste time to find a suitable MOSFET. Together with the drive circuit, these built-in MOSFETs can meet a wide range of application requirements in terms of switching frequency, load current, input voltage, temperature range, etc

figure 2:4a integrated FET power converter internal circuit block diagram

the rise and fall time of the drive circuit is about 3ns, achieving the best balance between EMI noise and power consumption. The non overlapping time and the on/off conversion time (or dead time) of the high side and low side MOSFETs are well controlled to avoid the direct pass phenomenon. Between LX and PGND pins, no additional Schottky diode is required to improve efficiency. See Fig. 3a and Fig. 3B for switching waveform

figure 3A: LX switching waveform (step-down)

Figure 3B: LX switching waveform (boost)

2. intermittent mode and continuous mode

there are many integrated regulators for designers to choose from. For products without excessive consideration of cost, INTERSIL provides a standard step-down regulator, which adopts intermittent mode (DCM) under light load and requires external power Schottky diodes. On the other hand, there are many Synchronous Step-Down regulators that do not need external Schottky diodes and can work in continuous mode (CCM) and/or DCM mode

3. built in and external loop compensation

most low input voltage regulators provided by INTERSIL have internal compensation function, and designers do not need to ensure the stability of each working condition. The selected parameters support most of the typical applications listed in the specification. For regulators with wider rated input current range or higher rated output current, external compensation is adopted to obtain greater flexibility. The product specification provides clear instructions and design guidelines

4. overcurrent protection with temperature compensation

the INTERSIL integrated regulator listed in Table 1 has overcurrent protection function. The area of high-end power p-channel MOSFET monitors the peak current. This prevents external noise, which may require additional filters and extended protection response time, as is often the case for ICs without integrated MOSF struggling et

if the suction current is too large, the comparator will turn over and the high-end MOSFET will be turned off. In addition to strong anti noise ability and overcurrent protection function, the temperature compensation function of the regulator can also maintain a relatively constant limit value within the whole allowable temperature range. Every 1 ° C change in temperature? C. The RDS (on) of most MOSFETs will change by 0.5%. For the scheme using external MOSFET, especially when using the induced current of external MOSFET, it is difficult to adjust according to the temperature change, unless additional cost and/or circuit complexity are added. The integrated voltage regulator can be easily adjusted internally according to the changes of MOSFET. Compared with the external circuit, the thermal coupling between the power device and the control part is closer. The comparison between devices with and without temperature compensation is shown in Figure 4

Figure 4: output current overload threshold of typical 4A devices

5. other advanced control functions

intersil integrated FET dc/dc converters also have advanced control functions, as shown in Table 1

design example using integrated FET

this design example takes 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 1MHz. The switching frequency can also be increased synchronously, up to 4MHz. For simplicity, 1MHz is used here

(2) calculate the inductance L. Δ I is the peak to peak ripple current flowing through the inductor. It is recommended that Δ I is set to about 30% of the maximum output current. The maximum output current of isl8014 is 4a, so Δ I=1.2A。

(3) determine the equivalent series resistance RESR of the output capacitance

(4) the recommended minimum output capacitance is 44 μ F。 Because of its low RESR, ceramic capacitors are a good choice. 22 per 0805 package μ The RESR of F capacitor is 5m Ω, so 2 can be selected × twenty-two μ F。

(5) calculate the feedback resistance voltage divider according to the following formula, where VFB is 0.8V specified in the specification:

(6) the input capacitance is not very important, and C1 can be set as 2 × twenty-two μ F。

(7) next is PCB design. Please refer to the specification of isl8014, which can be downloaded from. The key PCB design steps include: inserting the IC on the circuit board; Insert the inductor adjacent to the LX node of the IC; Insert C2 adjacent to the other end of inductor L and the PGND pin of IC; Insert C1 next to the VIN pin, the distance between the IC's load eccentricity - the actual action line of the contraction or tensile load and the action line that generates the equilibrium stress on the cross section of the specimen; Insert R3 next to SGND and VFB pins; Insert R2 and C3 next to R3; Punch about 6 vias under the power pad of the IC for heat dissipation; Drill about 4 through holes to connect PGND with C1 and C2 respectively; Fill the second layer with PGND connection. See Figure 5 for the PCB design of isl8014

figure 5:pcb design drawing

summary of this article

intersil's integrated FET regulator has many features and function options, making it easier to use, especially when using built-in loop compensation. This paper lists the recommended output inductors and capacitors for low input voltage applications, and also discusses the simple design steps and layout design. Through these simple steps, most designers can get the desired results. For integrated FET with external compensation, the specification will almost always be attached with specific and detailed analysis. Table 1: characteristics of INTERSIL's integrated FET dc/dc converter

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