MIC33153 Datasheet, PDF (11/17 Pages) Micrel Semiconductor – 4MHz PWM 1.2A Internal Inductor Buck Regulator with HyperLight Load and Power Good

Micrel Inc.
Application Information
The MIC33153 is a high performance DC-to-DC step
down regulator offering a small solution size. With the
HyperLight Load™ switching scheme, the MIC33153 is
able to maintain high efficiency throughout the entire
load range while providing ultra-fast load transient
response. The following sections provide additional
device application information.
Input Capacitor
A 2.2µF ceramic capacitor or greater should be placed
close to the VIN pin and PGND pin for bypassing. A
Murata GRM188R60J475ME84D, size 0603, 4.7µF
ceramic capacitor is recommended based upon
performance, size, and cost. A X5R or X7R temperature
rating is recommended for the input capacitor. Y5V
temperature rating capacitors, aside from losing most of
their capacitance over temperature, can also become
resistive at high frequencies. This reduces their ability to
filter out high frequency noise.
Output Capacitor
The MIC33153 is designed for use with a 2.2µF or
greater ceramic output capacitor. Increasing the output
capacitance will lower output ripple and improve load
transient response but could also increase solution size
or cost. A low equivalent series resistance (ESR)
ceramic output capacitor such as the Murata
GRM188R60J475ME84D, size 0603, 4.7µF ceramic
capacitor is recommended based upon performance,
size, and cost. Both the X7R or X5R temperature rating
capacitors are recommended. The Y5V and Z5U
temperature rating capacitors are not recommended due
to their wide variation in capacitance over temperature
and increased resistance at high frequencies.
Compensation
The MIC33153 is designed to be stable with a 4.7µF
ceramic (X5R) output capacitor.
Duty Cycle
The typical maximum duty cycle of the MIC33153 is
80%.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied:
Efficiency
%
=
⎜⎜⎝⎛
V_OUT
V_IN
× I_OUT
× I_IN
⎟⎟⎠⎞
× 100
MIC33153
Maintaining high efficiency serves two purposes. It
reduces power dissipation in the power supply, reducing
the need for heat sinks and thermal design
considerations and it reduces consumption of current for
battery powered applications. Reduced current draw
from a battery increases the devices operating time and
is critical in hand held devices.
There are two types of losses in switching converters;
DC losses and switching losses. DC losses are simply
the power dissipation of I²R. Power is dissipated in the
high side switch during the on cycle. Power loss is equal
to the high side MOSFET R_DSON multiplied by the Switch
Current squared. During the off cycle, the low side N-
channel MOSFET conducts, also dissipating power.
Device operating current also reduces efficiency. The
product of the quiescent (operating) current and the
supply voltage represents another DC loss. The current
required driving the gates on and off at a constant 4MHz
frequency and the switching transitions make up the
switching losses.
Figure 3. Efficiency Under Load
Figure 3 shows an efficiency curve. From no load to
100mA, efficiency losses are dominated by quiescent
current losses, gate drive and transition losses. By using
the HyperLight Load™ mode, the MIC33153 is able to
maintain high efficiency at low output currents.
September 2010
11
M9999-092910-A