Hi, I'm Roland van Roy
Field Application Manager at Richtek
I'd like to explain you a bit about power management components
and how to select the right component
for your application
Power conversion is all about input and output conditions
Component selection depends a lot on input voltage
output voltage and output current
Let’s have a look at some examples.
The power source for your application
could be a Li-Ion battery
or a small 12V adapter
or maybe an larger 24V/48V industrial supply
Or maybe your application is fed directly from the AC line voltage
Power management components for the battery could be a battery charger
and a battery gauge that shows the remaining battery charge
The 12V adapter probably uses a flyback controller
and the industrial supply will probably have a power factor controller as well.
Now let’s look at the final application
A wireless module contains circuits that are sensitive to noise.
To provide a stable 3V supply
a low noise linear regulator is most suitable here
To provide the stable 5V for the USB port from battery
you need a boost converter
and maybe a power switch for short-circuit protection
To power a 3.3V sensor from battery
you need a buck-boost
to either step down or step up the voltage
at different battery voltage conditions
Most System-on-Chip devices require low voltage
but can draw large currents
A high current buck step down converter is the right choice
DDR memory also requires low voltage at high currents
and may need a DDR terminator
to terminate the data-bus lines
To drive an 8Ω speaker with amplified audio signal
you need an audio power amplifier
Most LCD panels are backlit by LED strings
which need to be driven by DC input LED drivers
and a retrofit LED lamp
will require a driver circuit that transforms
the AC line voltage into
a stable constant current for the LED string
As you can see
each application has different conditions and considerations
for selecting a specific power management component
Richtek can provide integrated power IC solutions for many different applications
In the following section
we’ll discuss the different topologies
and selection criteria in more detail
If you need to drop a higher input voltage
to a lower output voltage
at relatively moderate power levels
you can use a low dropout linear regulator
LDOs are simple, cheap
and provide a clean low noise supply
They can also react quickly to load transients
However, they have low efficiency
and the power dissipation in the device
can lead to thermal limitations
LDOs are normally used in applications with low current
or applications with relatively low voltage drop
They are also suitable for applications
that are sensitive to noise
If you need to step down a high input voltage
to a low output voltage at higher current levels
a switch-mode step down converter
or buck converter is a better choice
Due to the switching action
buck converters have high efficiency
and they are very flexible
in providing different conversion ratios
at various power levels
Buck converters will have some switching ripple on the output
and they have a bit more external components
compared to an LDO
But when your application requires
high currents or high Vin/Vout ratios
a buck converter is the best choice
In applications where the output voltage
is higher that the input voltage
you need to step-up the voltage
This requires boost converter
Boost converters are also switch-mode converters
and have good efficiency
and can be used in in applications
with various step-up ratios
But also here, the output voltage will contain some switching noise
Whenever the output voltage is higher that the input voltage
you’ll need a boost converter
Here is a situation where
the input voltage is not fixed, but can change
either being higher or lower than the output voltage
For example a battery
where the voltage can fluctuate during discharge
and charge conditions
In this case, you will need a buck-boost converter
This converter will automatically configure itself
as a buck or a boost converter
depending on the input and output voltage conditions
to provide a stable output voltage
Integrated buck-boost converters have high efficiency
and are quite flexible in handling various input voltage conditions
The switching action will result in
some switching ripple on the output voltage
Buck-boost converters are often used
in systems powered by batteries
or any application
where the input voltage can be higher or lower than the output voltage
To summarize
When you need to step down the input voltage
you choose an LDO when the application current is low
or when the step-down ratio is low
and when providing power to noise sensitive applications
You would choose a buck for applications
with higher current or higher step down ratios
When you need to step-up the input voltage
you need a boost converter
It is basically used in all applications where Vout is higher than Vin
When input voltage can vary
either above or below the output voltage
you need a buck-boost converter
It is often used in battery operated equipment
to keep stable output voltage
over the full battery voltage range
Besides voltage regulators
you may also need power switches
to enable a specific supply rail
Power switches are basically electrically controllable on/off switches
They often have build-in over current protection
which makes the application short-circuit proof
Power switches are often used to provide power and protection for USB ports
Voltage supervisor ICs or reset ICs
are used to monitor the supply line
of micro controllers or processors
When the supply voltage drops below a certain level
the voltage supervisor IC will initiate a reset to the processor
to avoid data corruption due to too low processor supply voltage
When the supply voltage recovers, the reset signal is removed
usually with some time delay
to ensure stable processor initialization
If your application contains a rechargeable battery
you most likely will also need a circuit that will charge the battery
Battery charger ICs are normally powered from an adapter
and will provide accurate charging voltage
and charging current control for the
specific battery cell in your application
They will also monitor the battery temperature
to ensure safe charging conditions
Some battery charger ICs also include power path control
to allow the system to be run from either adapter or battery
You can also add a battery gauge IC
which will measure the battery charge and discharge condition
and provide accurate battery state of charge information to your system
Let’s briefly discuss AC/DC converter applications
The main function of an AC/DC converter is to
transform the 50 or 60Hz AC line voltage
into a stable DC output voltage
and at the same time provide mains isolation
Take for example a small switch-mode AC adapter
The circuit will consist of a rectifier stage
and a flyback converter,
which is basically a buck-boost converter
with a split inductor to form a transformer
The flyback controller will control the primary switching MOSFET
in order to regulate the output voltage at the secondary winding
It takes its feedback information from the secondary output voltage
to maintain stable output at varying AC line
and output load conditions
Power supplies with power ratings above 75W like open frame industrial supplies
need to meet specific power factor requirements
In these systems a PFC boost stage is added
which provides a stable supply to the flyback stage
and controls the boost input current
to meet power factor and AC input current harmonics requirements
LEDs are light emitting diodes
They need to be driven with a constant current for a certain light output
If you want to vary the light output
you need a circuit that varies the current through the LED string
The forward voltage of most high brightness LED ranges from 3 to 3.5V
The number of LEDs in the string
will determine the total LED string voltage
Depending on the input voltage and the LED string voltage
you may need a buck
a boost
or a buck-boost LED driver
Richtek has a wide range of LED drivers for various applications
from DC input LCD backlight LED drivers
to AC input retrofit LED lamps
I hope this short tutorial has helped you a bit
in finding a better component for your power management application
for more information
please visit the Richtek website at:
www.richtek.com