MIC23155 Datasheet, PDF (12/19 Pages) Micrel Semiconductor – 3MHz PWM 2A Buck Regulator with HyperLight Load and Power Good

Micrel Inc.
Application Information
The MIC23155 is a high performance DC-DC step down
regulator offering a small solution size. Supporting an
output current up to 2A in a tiny 2.5mm x 2.5mm Thin
MLF^® package, the IC requires only four external
components while meeting today’s miniature portable
electronic device needs. Using the HyperLight Load^®
switching scheme, the MIC23155 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.
Output Capacitor
The MIC23155 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.
Inductor Selection
When selecting an inductor, it is important to consider
the following factors:
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC23155 is designed for use with a 0.47µH to
2.2µH inductor. For faster transient response, a 0.47µH
inductor will yield the best result. For lower output ripple,
a 2.2µH inductor is recommended.
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% to 20% 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 so
that the peak current does not cause the inductor to
saturate. Peak current can be calculated as follows:
MIC23155
I_PEAK
=
⎡
⎢^IOUT
⎣
+
V_OUT
⎜⎜⎝⎛
1
− V_OUT /V_IN
2×f ×L
⎟⎟⎠⎞⎥⎦⎤
As shown by the calculation above, the peak inductor
current is inversely proportional to the switching
frequency and the inductance. The lower the switching
frequency or the inductance, the higher the peak current.
As input voltage increases, the peak current also
increases.
The size of the inductor depends on the requirements of
the application. Refer to the Typical Application Circuit
and Bill of Materials for details.
DC resistance (DCR) is also important. While DCR is
inversely proportional to size, DCR can represent a
significant efficiency loss. Refer to the Efficiency
Considerations.
The transition between Continuous Conduction Code
(CCM) to HyperLight Load^® mode is determined by the
inductor ripple current and the load current.
The diagram shows the signals for High Side switch
Drive (HSD) for Ton control, the Inductor current, and
the Low Side switch Drive (LSD) for T_OFF control.
In HLL mode, the inductor is charged with a fixed Ton
pulse on the high side switch. After this, the low side
switch is turned on and current falls at a rate V_OUT/L. The
controller remains in HLL mode while the inductor falling
current is detected to cross approximately -50mA. When
the LSD (or T_OFF) time reaches its minimum and the
inductor falling current is no longer able to reach the
threshold, the part is in CCM mode.
Once in CCM mode, the T_OFF time will not vary.
Therefore, it is important to note that if L is large enough,
the HLL transition level will not be triggered.
That inductor is:
L_MAX
=
V_OUT ⋅135ns
2 ⋅ 50mA
April 2011
12
M9999-041811-A