MIC2174 Datasheet, PDF (14/24 Pages) Micrel Semiconductor – 300kHz, Synchronous Buck Controller 300kHz, Synchronous Buck Controller

Micrel, Inc.
where:
t_T = Switching transition time
V_D = Body diode drop (0.5v)
f_SW = Switching Frequency (300kHz)
The high-side MOSFET switching losses increase with
the input voltage V_HSD due to the longer turn-on time and
turn-off time. The low-side MOSFET switching losses
are negligible and can be ignored for these calculations.
Inductor Selection
Values for inductance, peak, and RMS currents are
required to select the output inductor. The input and
output voltages and the inductance value determine the
peak-to-peak inductor ripple current. Generally, higher
inductance values are used with higher input voltages.
Larger peak-to-peak ripple currents will increase the
power dissipation in the inductor and MOSFETs. Larger
output ripple currents will also require more output
capacitance to smooth out the larger ripple current.
Smaller peak-to-peak ripple currents require a larger
inductance value and therefore a larger and more
expensive inductor. A good compromise between size,
loss and cost is to set the inductor ripple current to be
equal to 20% of the maximum output current. The
inductance value is calculated by the equation below.
L=
V_OUT × (V_HSD(max) − V_OUT )
V_HSD(max) × f_sw × 20% × I_OUT(max)
(13)
where:
f_SW = switching frequency, 300 kHz
20% = ratio of AC ripple current to DC output current
V_HSD(max) = maximum power stage input voltage
The peak-to-peak inductor current ripple is:
ΔI_L(pp)
=
V_OUT × (V_HSD(max) − V_OUT )
V_HSD(max) × f_sw × L
(14)
The peak inductor current is equal to the average output
current plus one half of the peak-to-peak inductor current
ripple.
I_L(pk) =I_OUT(max) + 0.5 × ΔI_L(pp)
(15)
The RMS inductor current is used to calculate the I²R
losses in the inductor.
I_L(RMS) =
I_OUT(max) ²
+
ΔI_L(PP) ²
12
(16)
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance. The
high frequency operation of the MIC2174 requires the
use of ferrite materials for all but the most cost sensitive
applications.
MIC2174
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 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 is equal to the sum of the core and copper
losses. At higher output loads, the core losses are
usually insignificant and can be ignored. At lower output
currents, the core losses can be a significant contributor.
Core loss information is usually available from the
magnetics vendor. Copper loss in the inductor is
calculated by the equation below:
P_INDUCTOR(Cu) = I_L(RMS)2 × R_WINDING
(17)
The resistance of the copper wire, R_WINDING, increases
with the temperature. The value of the winding
resistance used should be at the operating temperature.
P_WINDING(Ht) = R_{WINDING(20°C)} × (1 + 0.0042 × (T_H – T_20°C))
(18)
where:
T_H = temperature of wire under full load
T_20°C = ambient temperature
R_{WINDING(20°C)} = room temperature winding resistance
(usually specified by the manufacturer)
Output Capacitor Selection
The type of the output capacitor is usually determined by
its ESR (equivalent series resistance). Voltage and RMS
current capability are two other important factors for
selecting the output capacitor. Recommended capacitors
are tantalum, low-ESR aluminum electrolytic, OS-CON
and POSCAPS. The output capacitor’s ESR is usually
the main cause of the output ripple. The output capacitor
ESR also affects the control loop from a stability point of
view. The maximum value of ESR is calculated:
ESR _COUT
≤
ΔV_OUT(pp)
ΔI_L(PP)
(19)
where:
ΔVOUT(pp) = peak-to-peak output voltage ripple
ΔI_L(PP) = peak-to-peak inductor current ripple
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated below:
ΔV_OUT(pp) =
( ) _{⎜⎜⎝⎛}
ΔI_L(PP)
C_OUT × f_SW
× 8 ^{⎟⎟⎠⎞2}
+
ΔI_L(PP) × ESR _COUT
2
(20)
September 2009
14
M9999-090409-B