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Posted on 01 September 2019

Power Supply Design Made Easy

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Using Integrated Switching Regulators

When it came to power supply design, most Engineers would scratch their head thinking, “Where do I start?” Decision must be decided concerning which typology – buck, boost, flyback, half-bridge, fullbridge, etc. How about control scheme, voltage mode, current mode, constant-on, or R3? What is the behavior as various frequencies? This will dictate what inductance and capacitance to meet output ripple and load transient.

By Tu Bui – Application Engineering Manager, Intersil

 

What compensation types must be used to insure stability across line, load, temperature, etc.? Selecting the “right” MOSFETs is not a trivial task either. Will the driver is capable to handle the Gate capacitance of the MOSFET? How will the parasitic capacitances and Rdson influence the overall power dissipation?

The power supply is not finished yet. The PCB layout designers may came back saying that there is not enough space to accommodate all those components selected. Where to place the controller, MOSFET, input capacitors, inductor, output capacitors, control circuitries, etc.? How about grounding scheme? Where to connect PGND and AGND? How to minimize AC loop for best EMI performance or eliminating noise interaction? Where is the heatsink and what direction of the airflow? How many vias should be used? Again, designing a power supply switching regulator is no simple task.

Using an Intersil’s Integrated FET DC/DC regulator can make the designing of a buck power converter much easier. Most of the challenging decisions are already made and optimized within the IC, such as MOSFET(s) sizing, drivers, current sense elements and limit, loop compensation, thermal compensation, and thermal shutdown protection. High switching frequency above 1MHz allows the use of small inductor and ceramic capacitors that are readily available from many vendors. Finally, there are evaluation boards and recommended PCB layouts for most of the Intersil Integrated FET solutions available for customer’s reference.

Advantages of Integrated FET DC/DC Converters

Figure 1 is the complete typical application circuit for the 4A converter using the ISL8014. Only a few external components required. Figure 2 is the block diagram of the ISL8014 Integrated FET silicon. A lot of features and functions are included in one package which made power supply designing so much effortless.

Typical Application Schematic (4A Integrated FET Power Converter)

Internal Block Diagram (4A Integrated FET Power Converter)

Internal MOSFET

Notice the highside power P-channel MOSFET from VIN to LX and the lowside N-channel MOSFET from LX to PGND; thus, there is no need for wasting time finding the right MOSFET. These internal MOSFET along with its drivers are designed for wide range of applications such as switching frequency, load current, input voltage, temperature range, etc.

The driver rise and fall time are set to approximately 3ns, optimizing between EMI noise and power dissipation. The non-overlap time, turning ON/OFF of the highside and lowside MOSFETs or deadtime, are well controlled to avoiding shoot-thru phenomena. No additional Schottky diode from LX to PGND needed for efficiency improvement. See the switching waveform in figure 3a and figure 3b.

LX Switching Waveform (Falling Transition)

LX Switching Waveform (Rising Transition)

Discontinuous and Continuous Mode

Intersil offer wide varieties of integrated regulator to choose from. For low cost sensitive application, there is standard buck which operates in discontinuous mode (DCM) at light low and require an external power Schottky diode. On the other hand, there are many synchronous buck regulators available that do not required additional Schottky diode and can operates in both continuous mode (CCM) and/or DCM.

Internal and External Loop Compensation

Most of Intersil’s low input voltage are internally compensated which saved designer from the task of ensuring stability for every operational conditions. The values are chosen to support most typical applications described in the datasheets. For the wider input and/or higher output current rating regulators, the compensation is external for maximum flexibility. Clear instructions and design guidelines are given in the datasheets.

Overcurrent Protection with Thermal Compensation

Every Intersil’s integrated regulators employ an overcurrent protection feature. The sensing circuitry is located next to the highside power P-channel MOSFET monitoring the peak current. This prevents external noise that required additional filter and slow down the protection response time, usually when the MOSFETs are not integrated within the IC. In an event when there is too much current drawn, the comparator would tripped and terminate the highside MOSFET. In addition to high noise immunity overcurrent protection scheme, there is also thermal compensation to maintain relatively constant limit value across temperature range. Most MOSFET Rdson changed at about 0.5% per ºC. In an external MOSFET solution especially when using it to sense the current, it is difficult to adjust for thermal changes without additional cost and/or complexity. The integrated regulator can easily adjust internally for the MOSFET changed. The thermal coupling between the power devices and control section are tight compared to the external configuration. Figure 4 showed a comparison between a thermally compensated device and an uncompensated one.

Output Current Overloading Threshold for a Typical 4A Device

Other Advanced Control Functions

Intersil Integrated FET DC/DC converters offer advanced control functions as show in Table 1.

Table 1 - part 1

Intersil Integrated FET DC-DC converters

Design Example Using Integrated FET

Let use an ISL8014 as an example referencing figure 1. Let assume the requirement is Vin=5V input and Vo=1.8V output with ripple less than 18mV. The design procedure as follows:

Select the switching frequency, Fs. The natural switching frequency is 1MHz otherwise one can synchronize to a higher frequency, up to 4MHz. Let use 1MHz for simplicity. Calculate the inductor, L. Where ΔI is the peak to peak ripple current thru the inductor. It is recommended to set the ΔI at about 30% of the maximum output current. In the ISL8014, the maximum output current is 4A. Therefore ΔI=1.2A

Equation 1

Sizing the equivalent series resistor of the output capacitor, Resr.

Equation 2

The minimum recommended output capacitor is 44μF. Ceramic technology is a good choice since it got low Resr. Each 22μF in 0805 package is about 5mΩ. Therefore. 2 x 22μF is a good choice.

Selecting the feedback resistors divider using this relation. Where VFB equal 0.8V in the datasheet:

Equation 3

The input capacitor is not as critical. 2 x 22 μF is also a good choice for C1.

The next step is layout. Refer to the datasheet of ISL8014.
Place the IC.
Place the inductor next to LX node of the IC.
Place C2 next to the other side of the inductor, L, and PGND of the IC.
Place C1 next to VIN pin of the IC.
Place R3 next to SGND and VFB.
Place R2 and C3 next to R3.
Add about 6 vias under the IC’s power pad for thermal relief.
Add about 4 vias for PGND connections to each of C1 and C2.
Fill the second layer with PGND connection.
See the layout example of the ISL8014.

Layout example of the ISL8014

Conclusion

Intersil Integrated FET regulator offered wide features and functions selection which made its solution much simpler to use, especially with internal loop compensation. In low input range applications, recommended output inductor and capacitors are also listed. A straight forward design procedure and layout design was discussed. Most of the designer may use these simple steps to obtain the desired result. For the Integrated FET with external compensation configurations, detailed analysis can be reference within its datasheet.

 

 

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