MIC2176 Datasheet, PDF (18/31 Pages) Micrel Semiconductor – Wide Input Voltage, Synchronous Buck

Micrel, Inc.
Making the assumption that the turn-on and turn-off
transition times are equal; the transition times can be
approximated by:
t_T
=
C_ISS × V_IN
+ C_OSS × V_HSD
I_G
(12)
where:
C_ISS and C_OSS are measured at V_DS = 0
I_G = Gate-drive current
The total high-side MOSFET switching loss is:
P_AC = (V_HSD + V_D ) × I_PK × t _T × f_SW
(13)
where:
t_T = Switching transition time
V_D = Body diode drop (0.5V)
f_SW = Switching Frequency
The high-side MOSFET switching losses increase with
the switching frequency and the input voltage V_HSD. 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.
MIC2176
The inductance value is calculated by Equation 14:
L = ^{VOUT × (VIN(max) − VOUT )}
V_IN(max) × f_sw × 20% × I_OUT(max)
(14)
where:
f_SW = Switching frequency, 300kHz
20% = Ratio of AC ripple current to DC output current
V_IN(max) = Maximum power stage input voltage
The peak-to-peak inductor current ripple is:
ΔI_L(pp)
=
V_OUT × (V_IN(max) − V_OUT )
V_IN(max) × f_sw × L
(15)
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)
(16)
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
(17)
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance. The
high frequency operation of the MIC2176 requires the
use of ferrite materials for all but the most cost sensitive
applications. 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.
November 2010
18
M9999-111710-A