Pam2841, Application information – Diodes PAM2841 User Manual
Page 9
PAM2841
Document number: DSxxxxx Rev. 1 - 5
9 of 16
September 2013
© Diodes Incorporated
PAM2841
A Product Line of
Diodes Incorporated
Application Information
Inductor Selection
The selection of the inductor affects steady state operation as well as transient behavior and loop stability. These factors make it the most
important component in power regulator design. There are three important inductor specifications, inductor value, DC resistance and saturation
current. Considering inductor value alone is not enough.
The inductor value determines the inductor ripple current. Choose an inductor that can handle the necessary peak current without saturation, the
inductor DC current given by:
Iin_DC = V
OUT
*I
OUT
/(V
IN
*η) η = efficiency.
Inductor values can have ±20% tolerance with no cur rent bias . When the inductor current approaches saturation level, its inductance can
decrease 20% to 35% from the 0A value depending on how the inductor vendor defines saturation current. Using an inductor with a smaller
inductance value causes discontinuous PWM when the inductor current ramps down to zero before the end of each switching cycle. This
reduces the boost converter's maximum output current, causes large input voltage ripple and reduces efficiency. Large inductance value
provides much more output current and higher conversion efficiency. For these reasons, an inductor within 4.7µH to 22µH value range is
recommended.
Schottky Diode Selection
The high switching frequency of the PAM2841 demands a high-speed rectification for optimum efficiency. Ensure that the diode average and
peak current rating exceeds the average output current and peak inductor current. In addition, the diode's reverse breakdown voltage must
exceed the open protection voltage.
Input and Output Capacitor Selection
Input Capacitor
At least a 1µF input capacitor is recommended to reduce the input ripple and switching noise for normal operating conditions. Larger value and
lower ESR (Equivalent Series Resistance) may be needed if the application require very low input ripple. It follows that ceramic capacitors are a
good choice for applications. Note that the input capacitor should be located as close as possible to the device.
Output Capacitor
The output capacitor is mainly selected to meet the requirement for the output ripple and loop stability. This ripple voltage is related to the
capacitor's capacitance and its equivalent series resistance (ESR). A output capacitor of 1μF minimum is recommended and maybe need a
larger capacitor. The total output voltage ripple has two components: the capacitive ripple caused by the charging and discharging on the output
capacitor, and the ohmic ripple due to the capacitor's equivalent series resistance (ESR):
V
RIPPLE
= V
RIPPLE(C)
+ V
RIPPLE(ESR)
V
RIPPLE(C)
≈ ½*(L/C
OUT
*((V
OUT(MAX)
– V
IN(MIN)
)))*(I
2
PEAK
– I
2
OUT
)
V
RIPPLE(ESR)
= I
PEAK
*R
ESR(COUT)
Where I
PEAK
is the peak inductor current.
Multilayer ceramic capacitors are an excellent choice as they have extremely low ESR and are available in small footprints. Capacitance and
ESR variation with temperature should be considered for best performance in applications with wide operating temperature ranges.