MIC25400 Datasheet, PDF (14/27 Pages) Micrel Semiconductor – 2A Dual Output PWM Synchronous Buck Regulator IC

Micrel, Inc.
Application Information
Component Selection
Inductor
The value of inductance is determined by the peak to
peak inductor current. Higher values of inductance
reduce the inductor current ripple at the expense of a
larger inductor. Smaller inductance values allow faster
response to output current transients but increase the
output ripple voltage and require more output
capacitance.
The inductor value and saturation current are also
controlled by the method of overcurrent limit used (see
explanation in the previous section). The minimum value
of inductance for the MIC25400 is 4.7µH.
The peak-to-peak ripple current may be calculated using
the formula below.
I_PP
=
V_OUT ⋅ (η ⋅ V_IN(max) − V_OUT )
η ⋅ V_IN(max) ⋅ f_S ⋅ L
Where:
I_PP is the peak to peak inductor ripple current
L is the value of inductance
f_S is the switching frequency of the regulator
η is the efficiency of the power supply
Efficiency values from the Functional Characteristics
section can be use for these calculations.
The peak inductor current in each channel is equal to the
average output current plus one half of the peak-to-peak
inductor ripple current.
I_PK = I_OUT + 0.5 × I_PP
The RMS inductor current is used to calculate the I²R
losses in the inductor.
I_INDUCTORRMS = _IOUT ⋅
1+
1
3
⎜⎛
⎜
⎝
I_PP
I_OUT
⎟⎞²
⎟
⎠
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance. The
high frequency operation of the MIC25400 requires the
use of ferrite materials. Lower cost iron powder cores
may be used but the increase in core loss will reduce the
efficiency of the power supply. This is especially
noticeable at low output power. The inductor winding
resistance decreases efficiency at the higher output
current levels. The winding resistance must be
minimized although this usually comes at the expense of
a larger inductor.
The power dissipated in the inductor equals the sum of
the core and copper losses. Core loss information is
usually available from the magnetics vendor.
MIC25400
Input Capacitor
A 10μF ceramic is suggested on each of the V_IN pins for
bypassing. X5R or X7R dielectrics are recommended for
the input capacitor. Y5V dielectrics should not be used.
Besides losing most of their capacitance over
temperature, they also become resistive at high
frequencies, which reduce their ability to filter out high
frequency noise.
Output Capacitor
The MIC25400 regulator is designed for ceramic output
capacitors although tantalum and Aluminum Electrolytic
may also be used.
Output ripple voltage is determined by the magnitude of
inductor current ripple, the output capacitor’s ESR and
the value of output capacitance. When using ceramic
output capacitors, the primary contributor to output ripple
is the value of capacitance. Output ripple using ceramic
capacitors may be calculated using the equation below:
C _OUT
≥
I_PP
8 ⋅ ΔV_OUT
⋅ 2 ⋅ f_S
Where:
ΔV_OUT is the peak-to-peak output voltage ripple
I_PP is the peak-to-peak ripple current as see by the
capacitors
f_S is the switching frequency (1MHz nominal).
When using tantalum or Aluminum Electrolytic
capacitors, both the capacitance and ESR contribute to
output ripple. The total ripple is calculated below:
[ ] ΔV_OUT =
⎡
⎢⎣₈
⋅
I _PP
C_OUT ⋅
2
⋅
f_S
⎤²
⎥
⎦
+
I_PP ⋅ R _ESR
2
The output capacitor RMS current is calculated below:
I_COUTRMS
=
I_PP
12
The power dissipated in the output capacitors can be
calculated by the equation below:
( ) P_DISSCOUT = I_COUTRMS ² ⋅ R_ESR
Current limit resistor
The current limit circuit responds to the peak inductor
current flowing through the low-side FET. Calculating the
current setting resistor R_CS should take into account the
peak inductor current and the blanking delay of
approximately 100ns.
January 2011
14
M9999-020111-C