MIC22700_10 Datasheet, PDF (11/21 Pages) Micrel Semiconductor – 1MHz, 7A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
Application Information
The MIC22700 is a 7A Synchronous step down regulator
IC with a fixed 1 MHz, voltage mode PWM control
scheme. The other features include tracking and
sequencing control for controlling multiple output power
systems, power on reset.
Component selection
Input Capacitor
A minimum 22µF ceramic is recommended on each of
the PVIN pins for bypassing. X5R or X7R dielectrics are
recommended for the input capacitor. Y5V dielectrics,
aside from losing most of their capacitance over
temperature, they also become resistive at high
frequencies. This reduces their ability to filter out high
frequency noise.
Output Capacitor
The MIC22700 was designed specifically for the use of
ceramic output capacitors. 100µF can be increased to
improve transient performance. Since the MIC22700 is
in voltage mode, the control loop relies on the inductor
and output capacitor for compensation. For this reason,
do not use excessively large output capacitors. The
output capacitor requires either an X7R or X5R
dielectric. Y5V and Z5U dielectric capacitors, aside from
the undesirable effect of their wide variation in
capacitance over temperature, become resistive at high
frequencies. Using Y5V or Z5U capacitors can cause
instability in the MIC22700.
Inductor Selection
Inductor selection will be determined by the following
(not necessarily in the order of importance):
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC22700 is designed for use with a 0.47µH to
4.7µH inductor.
Maximum current ratings of the inductor are generally
given in two methods: permissible DC current and
saturation current. Permissible DC current can be rated
either for a 40°C temperature rise or a 10% loss in
inductance. Ensure the inductor selected can handle the
maximum operating current. When saturation current is
specified, make sure that there is enough margin that
the peak current will not saturate the inductor. The ripple
can add as much as 1.2A to the output current level. The
RMS rating should be chosen to be equal or greater than
the Current Limit of the MIC22700 to prevent
overheating in a fault condition. For best electrical
MIC22700
performance, the inductor should be placed very close to
the SW nodes of the IC. For this reason, the heat of the
inductor is somewhat coupled to the IC, so it offers some
level of protection if the inductor gets too hot. It is
important to test all operating limits before settling on the
final inductor choice.
The size requirements refer to the area and height
requirements that are necessary to fit a particular
design. Please refer to the inductor dimensions on their
datasheet.
DC resistance is also important. While DCR is inversely
proportional to size, DCR can represent a significant
efficiency loss. Refer to the “Efficiency Considerations”
below for a more detailed description.
Enable/DLY Capacitor
Enable/DLY sources 1µA out of the IC to allow a startup
delay to be implemented. The delay time is simply the
time it takes 1µA to charge C_DLY to 1.25V. Therefore:
T _DLY
= ^{1.24 ⋅ CDLY}
1× 10 ^−6
Efficiency considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power consumed.
Efficiency
%
=
⎜⎜⎝⎛
V_OUT
V_IN
× I_OUT
× I_IN
⎟⎟⎠⎞ ^{× 100}
Maintaining high efficiency serves two purposes. It
decreases power dissipation in the power supply,
reducing the need for heat sinks and thermal design
considerations and it decreases consumption of current
for battery powered applications. Reduced current draw
from a battery increases the devices operating time,
critical in hand held devices.
There are mainly two loss terms in switching converters:
static losses and switching losses. Static losses are
simply the power losses due to VI or I²R. For example,
power is dissipated in the high side switch during the on
cycle. Power loss is equal to the high side MOSFET
RDS_(ON) multiplied by the RMS Switch Current squared
(I_SW2). During the off cycle, the low side N-Channel
MOSFET conducts, also dissipating power. Similarly, the
inductor’s DCR and capacitor’s ESR also contribute to
the I²R losses. Device operating current also reduces
efficiency by the product of the quiescent (operating)
current and the supply voltage. The current required to
drive the gates on and off at a constant 1MHz or 4MHz
frequency and the switching transitions make up the
switching losses.
March 2010
11
M9999-031810-B