MP28200 – 5.5V, 200mA, 1.5MHz, ULTRA-LOW I
Q
, STEP-DOWN CONVERTER
MP28200 Rev.1.0 www.MonolithicPower.com 14
10/10/2016 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
APPLICATION INFORMATION
Inductor Selection
Most applications work best with a 1µH to
2.2µH inductor. Select an inductor with a DC
resistance less than 200m to optimize
efficiency.
High-frequency, switch-mode power supplies
with a magnetic device have strong electronic
magnetic inference for the system. Any
unshielded power inductor should be avoided
since it has poor magnetic shielding. Metal alloy
or multiplayer chip power shield inductors are
recommended for the application since they can
decrease influence effectively. Table 2 lists
some recommended inductors.
Table 2: Recommended Inductors
Inductance
Manufacturer
P/N
Package Manufacturer
2.2H
DFE201612P-
2R2M
2016 Tokyo
2.2H
74479775222A 2012 Wurth
For most designs, the inductance
value can be calculated with Equation (1):
OUT IN OUT
1
IN L OSC
V(VV)
L
VIf
(1)
Where I
L
is the inductor ripple current.
Choose the inductor current to be
approximately 30% of the maximum load
current. The maximum inductor peak current
can be calculated with Equation (2):
L
L(MAX) LOAD
I
II
2
(2)
Input Capacitor Selection
The input capacitor reduces the surge current
drawn from the input and the switching noise
from the device. Select an input capacitor with a
switching frequency impedance less than the
input source impedance to prevent high-
frequency switching current from passing to the
input source. Use low ESR ceramic capacitors
with X5R or X7R dielectrics with small
temperature coefficients. For most applications,
a 10F capacitor is sufficient.
The input capacitor requires an adequate ripple
current rating since it absorbs the input
switching current.
Estimate the RMS current in the input capacitor
with Equation (3):
OUT OUT
C1 LOAD
IN IN
VV
II 1
VV
(3)
The worst-case scenario occurs at VIN = 2V
OUT
,
shown in Equation (4):
LOAD
C1
I
I
2
(4)
For simplification, choose an input capacitor
with an RMS current rating greater than half of
the maximum load current.
The input capacitor can be electrolytic, tantalum,
or ceramic. When using electrolytic or tantalum
capacitors, add a small, high-quality, 0.1F,
ceramic capacitor as close to the IC as possible.
When using ceramic capacitors, ensure that
they have enough capacitance to provide a
sufficient charge to prevent excessive voltage
ripple at the input. The input voltage ripple
caused by capacitance can be estimated with
Equation (5):
LOAD OUT OUT
IN
IN
SIN
IV V
V1
fC1V V
(5)
Output Capacitor Selection
The output capacitor limits the output voltage
ripple and ensures a stable regulation loop.
Select an output capacitor with low impedance
at the switching frequency. Use ceramic
capacitors with X5R or X7R dielectrics. For
most applications, a 10µF capacitor is sufficient.
Estimate the V
OUT
ripple with Equation (6):
OUT OUT
OUT ESR
S1 IN S
VV
1
V1R
fL V 8fC2
(6)
Where L
1 is the inductor value, and RESR is the
equivalent series resistance (ESR) value of the
output capacitor. When using ceramic
capacitors, the capacitance dominates the
impedance at the switching frequency and
causes most of the output voltage ripple. For
simplification, the output voltage ripple can be
estimated with Equation (7):
OUT OUT
OUT
2
S1 IN
VV
V1
8f L C2 V
(7)
The characteristics of the output capacitor also
affect the stability of the regulation system.
