How to use high-current integrated-switch power-regulator
ICs. By Frederik Dostal, National Semiconductor - EDN
Switching regulators with integrated power transistors provide
simplicity, require few components, and make compact step-down
power supplies. Few of these devices can supply more than 5A load
current, so, when designing with these high-current
integrated-switch regulators, you must consider thermal management,
bypassing of the supply voltage, and board layout.
At load currents of 5A or more, these topics become critical,
and you must understand them before selecting a power-management
architecture.
Simplicity and size are the biggest advantages of combining the
power switch with the control circuitry on one IC. In addition, you
can use the MOSFET the semiconductor company selected to go with
the control circuitry, and you can minimize the required PCB
(printed-circuit-board) area because the power supply requires only
one IC instead of a controller IC and one or more power-switch
ICs.
A disadvantage of an integrated design, however, is that
high-power output currents are present on the sensitive control
die. Also, the thermal dissipation of the power switch
instantaneously heats up the control circuitry.
With these advantages and drawbacks in mind, you can decide
whether to use an integrated power switch. Figure 1
shows a step-down dc/dc converter with an integrated power switch,
or regulator IC. Designs with an external power switch comprise a
controller IC plus a discrete power switch, usually a MOSFET.
For thermal considerations, you must determine through direct
measurement how much electrical power turns into heat inside the
regulator IC. Alternatively, you can assume total power-supply
efficiency and then break down the losses into conduction losses
and switching losses. You then assign them to the components on the
PCB (Reference 1).
A switching regulator at 5A of output current typically dissipates
2W of power.
To prevent the silicon from exceeding its maximum temperature
limit, you must dissipate the power away from the silicon through
the package to the PCB and, ultimately, to the surrounding air. You
must optimise PCB layout to achieve the best flow of heat. Fans
effectively cool circuits, but, for most applications, using a fan
is unacceptable due to cost, noise, and maintenance issues. Getting
by without a fan often requires the use of convection air cooling
or large heat sinks.
A thermal plot of an IC with 15V input, 3.3V output, and a 5A
load current shows hot spots (Figure 2). The hottest area is
freewheeling diode D1. To aid in the thermal dissipation of the
diode, choose a Schottky diode with a low package thermal
resistance. You might replace the SMC-packaged diode of
Figure 2
with a D-Pak or D2Pak that has lower thermal resistance than the
SMC package.
Choosing a package requires a trade-off between pin count and
the thermal resistance. Standard packages have either many pins and
higher thermal resistance or fewer pins and lower thermal
resistance. A good package for high power dissipation is the TO-263
thin package, which has a large exposed pad like the classic TO-263
package but is much thinner.
Thermal considerations
When you incorporate thermal considerations into the design of a
PCB, your primary goal is to efficiently conduct energy away from
the heat source. If you effectively achieve this goal, the whole
board has an even temperature distribution. Your next concern is
moving heat from the PCB into the surrounding air or adjacent
materials, such as the product casing. The more copper a PCB has,
the better the heat transfer away from any hot spots is. Copper
also helps heat transfer away from the board. More layers within
the PCB enable better heat transfer than does one or two layers. A
common PCB standard uses two ounces of copper per square foot of
board area. You are better off with thicker copper, larger copper
areas, and more PCB layers.
Use vias between the hot spots and the bottom of the board to
effectively conduct the heat away. You should also fill the vias
with solder, which does not have the best thermal conductivity but
conducts heat much better than does air. However, not all
board-manufacturing processes allow for solder-filled vias.
Manufacturers often place small vias next to each other for good
thermal transfer. The most thermally effective vias are those that
are as close to the heat source as possible - often right below a
thermal pad of the regulator IC. Unfortunately, not all
manufacturing processes allow such a placement. You should spread
the heat-generating components around the PCB to avoid hot spots,
but the electrical considerations call for close placements. You
must find a compromise.
Electrical considerations
It is difficult to keep the voltage supply for the regulator’s
internal rails clean. Many circuit blocks in the IC, such as the
internal bandgap reference and the comparators for the feedback
loop, need low noise to perform correctly. In a buck regulator, the
input trace is a noisy node because it quickly switches from full
current to no current. Integrated power regulators often use
separate pins for the supply voltage of the internal rails and as
the input to the main power switch. With packages having low
thermal resistance, the IC may have only one supply pin for the
internal voltage and the power stage. In such cases, you must
filter the input-voltage pin to keep the switching noise low. Use a
high-quality ceramic bypass capacitor and connect it close to the
input-voltage and ground pins. This rule is among the most
important for step-down voltage regulators, and it is especially
important for regulator ICs with integrated power switches.
The second important rule is to keep ac traces as short as
possible. Circuits in which the current flow changes as the power
switch changes state are ac traces. It is important to keep these
traces especially short to minimize trace-inductance-generated
voltage offsets. The shorter these traces are, the less voltage
offset the IC generates across them, and the resulting system noise
is lower. You can find the ac traces by printing the schematic of a
circuit three times. Use a pen on one of the printouts to draw
along the traces in which current flows when the power switch is
on. Use the second printout and mark where current flows when the
power switch is off. On the third printout, mark all the traces
that you marked on the previous two printouts but not on both of
them. This approach yields a plot with all the ac traces.
Several ac current flows occur in a buck regulator, including
the most critical traces: those during the on-state of the switch,
the off-state of the switch, and between the two switch states
(Figure 3).
Small circuit loops and thick traces minimize the parasitic trace
inductance. In contrast to thermal design, the electrical
requirement is to keep components, especially along the ac traces,
as close together as possible.
To decide whether you can use an integrated power-MOSFET
step-down regulator, you must do more than look at the maximum
input-voltage range and load current. Thermal considerations are
important because they can rule out the implementation of an
integrated power switch. Integrated-switch power-management
circuits can well serve systems with forced airflow with low
ambient temperatures or short peaks at maximum load current.
Frederik Dostal is an application engineer at
National Semiconductor’s design
centre (Phoenix). He joined the company in 2001, and his
responsibilities have included support for Europe and Central
Europe and coverage of many automotive accounts. Dostal’s current
position involves product development and support for switching
regulators and controllers. He holds a degree in electrical
engineering from the Friedrich-Alexander-Universität (Erlangen,
Germany) and is a member of the IEEE.