DC-DC Boost Up Converter
Dave uses MIC2253 (previously from Micrel but now Microchip owns it) dc-dc boost up converter to do this task but however, MIC2253 available only MLF package which require a hot air flow station to do the soldering. If you have instrument to make this happen, MIC2253 is actually very suitable to use as dc power supply backend, and of course it is cheap as well.
[Update and Revise 17/10/2018]
Deleted LT3489 simulation results due to chosen MIC2288 from Microchip for cheaper price tag. MIC2288 is a 1.2MHz DC-DC boost converter that available in package SOT-23. It provides switching current limits to 1A at normal condition and maximum output voltage of 34V. It has input voltage range from 2.5v to 10v and it fits my plan to accommodate two series connected 18650 lithium batteries that provide 7.4v. Below are steps to explain important parameters and calculations to get the converter work under your expectation.
For MIC2288, feedback voltage at normal condition is 1.24v, output voltage can be adjusted using this equation.
Vout = VFB * ((R1/R2) + 1)
Calculate maximum switch current
To determine maximum output current, first we need to calculate the duty cycle that operate in minimum input voltage because this leads to the maximum switch current. Assuming series connected batteries operating voltage range from minimum 7.0v to maximum 7.4v. Note that from MIC2288 datasheet maximum duty cycle is 0.9 at normal condition.
D =1 – ( (Vin(min) * η)/ Vout )
- Vin(min) = Minimum input voltage
- η = efficiency ( assuming 80%)
- Vout = Output voltage (assuming maximum 18.5v)
From duty cycle I now can determine inductor ripple current. This section calculation is based on 10uH inductor, if you are wondering which inductance fits your application, first look at your selected component datasheet recommendation, then check out “Inductor Selection” below. Inductor ripple current can be obtained using this formula:
ΔIL = (Vin(min) * D)/ (fs * L)
- fs = Converter PWM frequency (1.2MHz for MIC2288)
- L = Inductance
ΔIL = 0.407
Next it has to be determined if the selected IC can deliver maximum output current. Maximum output current really depends on converter current limit, MIC2288 is a cheaper IC due to it has lower current limit, which is only 1A. If you plan to make your device to have better power performance, you need to use MIC2253 or other switch that has capability of higher current limit.
Iout(max) = (Ilim – ΔIL/2) * (1-D)
- Ilim = Minimum value of current limit of the integrated switch (1A for MIC2288)
Iout(max) = 0.241A
Wait, the story is not yet finished. Another factor will impact the selection of converter, zener diode and inductor is, maximum switch current which is the peak current that these components must withstand.
ISW(max) = ( ΔIL/2) + (Iout(max)/(1-D))
ISW(max) = 0.998A
Higher inductance tends to have higher maximum output current based on equation (3) due to it can reduce ripple current. But smaller inductance has advantage of smaller footprint and saving PCB space. It really depends on what are you making and your specification requirements. If there is no recommended range of inductance in the datasheet, equation below helps to sort it out.
L = (Vin * (Vout – Vin )/ (ΔIL * fs * Vout )
where ΔIL can’t apply equation (3) due to unknown L, but it practically settled using equation below. A good estimation for inductor ripple current is about 20% to 40% of the output current. Thus,
ΔIL = (0.2 to 0.4) * Iout(max) * (Vout/Vin)
Make sure you diode forward current can withstand maximum output current.
IF = Iout(max)