MIC2165_1011 Datasheet, PDF (16/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring HyperLight Load®

Micrel, Inc.
MIC2165
P_SW = P_CONDUCTION + P_AC
(7)
P_CONDUCTION = I_SW(RMS) ² × R_DS(ON)
(8)
P_AC = P_{AC(off )} + P_AC(on)
(9)
where:
R_DS(ON) = on-resistance of the MOSFET switch
D = Duty Cycle = V_OUT / V_IN
Making the assumption that the turn-on and turn-off
transition times are equal; the transition times can be
approximated by:
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 in Equation 12.
L=
( ) V_OUT × V_{ΙΝ(max)} − V_OUT
V_{ΙΝ(max)} × f_SW × 20% × I_OUT(max)
(12)
where:
f_SW = switching frequency
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:
t_T
=
C_ISS
×
V_DD
+ C_OSS
I_G
× V_IN
(10)
ΔI_L(PP)
=
V_OUT × (V_IN(max) − V_OUT )
V_IN(max) × f_SW × L
(13)
where:
C_ISS and C_OSS are measured at V_DS = 0
I_G = gate-drive current
The peak inductor current is equal to the average output
current plus one half of the peak-to-peak inductor current
ripple.
The total high-side MOSFET switching loss is:
I_L(PK) = I_OUT(max) + 0.5 × ΔI_L(PP)
(14)
P_AC = (V_IN + V_D )×I_PK × t _T × f_SW
(11)
where:
t_T = Switching transition time
V_D = Diode drop
f_SW = Switching Frequency
The high-side MOSFET switching losses increase with the
switching frequency and the input voltage V_IN. 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
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
(15)
Maximizing efficiency requires the proper selection of core
material and minimizing the winding resistance. The high
frequency operation of the MIC2165 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. Copper loss
in the inductor is calculated by in Equation 16:
September 2010
16
M9999-092410-E