MIC28500 Datasheet, PDF (18/29 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control™ Synchronous DC-DC Buck Regulator

Micrel, Inc.
MIC28500
high frequency operation of the MIC28500 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 Equation 7:
P_INDUCTOR(Cu) = I_L(RMS)2 × R_WINDING
Eq. 7
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))
Eq. 8
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 equivalent series resistance (ESR). Voltage and RMS
current capability are two other important factors for
selecting the output capacitor. Recommended capacitor
types are ceramic, low-ESR aluminum electrolytic, OS-
CON and POSCAP. 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)
Eq. 9
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 in
Equation 10:
ΔV_OUT(pp) =
( ) _{⎜⎜⎝⎛}
C
ΔI_L(PP)
OUT ^{× f}SW
× 8 ^{⎟⎟⎠⎞2}
+
ΔI_L(PP) × ESR _COUT
2
Eq. 10
where:
C_OUT = output capacitance value
f_SW = switching frequency
As described in the “Theory of Operation” subsection in
Functional Description, the MIC28500 requires at least
20mV peak-to-peak ripple at the FB pin to make the g_m
amplifier and the error comparator behave properly. Also,
the output voltage ripple should be in phase with the
inductor current. Therefore, the output voltage ripple
caused by the output capacitors value should be much
smaller than the ripple caused by the output capacitor
ESR. If low-ESR capacitors, such as ceramic capacitors,
are selected as the output capacitors, a ripple injection
method should be applied to provide the enough
feedback voltage ripple. Please refer to the “Ripple
Injection” subsection for more details.
The voltage rating of the capacitor should be 20%
greater for aluminum electrolytic or OS-CON. The output
capacitor RMS current is calculated in Equation 11:
I_{COUT (RMS)}
=
ΔI_L(PP)
12
Eq. 11
The power dissipated in the output capacitor is:
P_{DISS(COUT )} = I_{COUT (RMS)2} × ESR_COUT
Eq. 12
Input Capacitor Selection
The input capacitor for the power stage input V_IN should
be selected for ripple current rating and voltage rating.
Tantalum input capacitors may fail when subjected to
high inrush currents, caused by turning the input supply
on. A tantalum input capacitor’s voltage rating should be
at least two times the maximum input voltage to
maximize reliability. Aluminum electrolytic, OS-CON, and
multilayer polymer film capacitors can handle the higher
inrush currents without voltage de-rating. The input
voltage ripple will primarily depend on the input
capacitor’s ESR. The peak input current is equal to the
peak inductor current, so:
June 2011
18
M9999-060311-B