Is a buck regulator the best option for my needs?

Question

I'm looking at using an MC33063 buck/boost inverter in a QFN package to step down 5-30V to 4.2V. This will be used to power an MCU, some modules, LEDs cosuming about 200mA max, optionally to charge a small LiPo, so the 1.5A is more than enough. What I would like to know is if this is the correct way to go considering space is of the essence and I need to use as little extra parts as possible, ideally low part heights. The additional diode, resistors and caps I saw in example schematics could probably be made to fit. Everything is SMD. Can I expect to be able to go as low as 5Vin or more likely around 6V?

1.5A isn't mandatory, but I would rather not go down to 0.5A, just in case, so 1A is a good compromise.

Answer 1

This will be used to power an MCU, some modules, LEDs cosuming about 200mA max

Linear technology parts are not cheap (I haven't checked on this part) but it has a low parts count. The picture below directly converts to 3V3 but, by using resistors connected to Vfb, higher regulation voltages can be obtained: -

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GOALPOST CHANGE:

1.5A isn't mandatory, but I would rather not go down to 0.5A, just in case, so 1A is a good compromise.

Or, plug your numbers into the LT parametric engine here. I chose to tick the radio box marked "synchronous" because then you don't need to worry about the flyback diode.

Answer 2

A buck regulator suits your need BUT the 33063 may not.
Input voltage range is OK (they work down to about 3V in) but power dissipation may be a problem at higher voltages.

1.5A is the switch voltage so will be 2x to 4x Ioutmax at minimum Vin.
Assume 300 mA load at 4.2V = 1.26W. Regulator dissipation will be ~=
1.26W x Z/(1-Z) where z = efficincy & 0 < Efficiency < 1

At low Vin & Vin > Vout 33063 efficiency may be poor due to drop across non saturating darlingtpon switch. 33063 data sheet here
The example in 9.2.1 on pages 9 on has a similar Vin and power rating and only 62% efficiency. While that's an inverting converter the reasons for low Z largely still apply.
At say 66% efficiency and 1.2 W out power in regulator = 1.2 x (1-0.67/)/.67 ~+ 600 mW.

Whether that's acceptable in a QFN pkg needs checking - but it's annoyingly high.

Also, the 33063 has a nominal fmax of 100 kHz (page 7) and 7.9 fig1 suggests you MAY get 200 kHz+ in a gopod day downhill with the wind behind you. But at say 100 kHz the inductor size is large by modern standards.

The 33063/34063 is often still an excellent choice in many applications, but this is not one of them. There are many MUCH higher efficiency ICs, usually with synchronous rectification, and MUCH higher switching frequencies - 500 kHz to maybe 2 MHz + - allowing much smaller inductors. In trhis case something else will be a better choice.

Answer 3

Rathyer than use a buck-boost converter, like the MC33063 you referenced, I suggest using a buck converter like the L5973AD. It is available for $3.08 from Digi-Key.

Unlike LDO regulators, in general with a buck regulator you should be able to operate it with the input voltage as low as the desired output voltage, in your case 4.2V.

The L5973AD has a switch current limit of 2A, so it can deliver over 1.5A to the load. With the input and output voltages specified (Vin=5V, Vout=4.2V), the efficiency is approximately 87%. The package can dissipate 2.25W, with the temperature below 60°C.

Answer 4

As already said above a synchronous step-down converter would probably be a good choice. Have a look at the MP9942 datasheet from Monolithic Power Systems. It's available at mouser, quite cheap and comes in a small package.