MIC2174_10 Datasheet, PDF (16/27 Pages) Micrel Semiconductor – Synchronous Buck Controller Featuring Adaptive On-Time Control 40V Input, 300kHz

Micrel, Inc.
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) =
( ) _{⎜⎜⎝⎛}
C
ΔI_L(PP)
OUT ^{× f}SW
× 8 ^{⎟⎟⎠⎞2}
+
ΔI_L(PP) × ESR _COUT
2
(20)
where:
D = duty cycle
C_OUT = output capacitance value
f_SW = switching frequency
As described in the “Theory of Operation” subsection in
Functional Description, the MIC2174/MIC2174C requires
at least 20mV peak-to-peak ripple at the FB pin to make
the gm amplifier and the error comparator to behavior
properly. Also, the output voltage ripple should be in
phase with the inductor current. Therefore, the output
voltage ripple caused by the output capacitor C_OUT
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 FB voltage ripple. Please refer to the
“Ripple Injection” subsection for more details.
MIC2174/MIC2174C
The voltage rating of the capacitor should be twice the
output voltage for a tantalum and 20% greater for
aluminum electrolytic or OS-CON. The output capacitor
RMS current is calculated below:
I_{COUT (RMS)}
=
ΔI_L(PP)
12
(21)
The power dissipated in the output capacitor is:
P_{DISS(COUT )} = I_{COUT (RMS)} ² × ESR _COUT
(22)
Input Capacitor Selection
The input capacitor for the power stage input V_HSD
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:
ΔV_IN = I_L(pk) × ESR_CIN
(23)
The input capacitor must be rated for the input current
ripple. The RMS value of input capacitor current is
determined at the maximum output current. Assuming
the peak-to-peak inductor current ripple is low:
I_CIN(RMS) ≈ I_OUT(max) × D × (1− D)
The power dissipated in the input capacitor is:
P_DISS(CIN) = I_CIN(RMS)2 × ESR_CIN
(24)
(25)
External Schottky Diode (Optional)
An external freewheeling diode, which is not necessary,
is used to keep the inductor current flow continuous
while both MOSFETs are turned off. This dead-time
prevents current from flowing unimpeded through both
MOSFETs and is typically 30ns. The diode conducts
twice during each switching cycle. Although the average
current through this diode is small, the diode must be
able to handle the peak current.
September 2010
16
M9999-091310-C