MIC2174 Datasheet, PDF (15/24 Pages) Micrel Semiconductor – 300kHz, Synchronous Buck Controller 300kHz, Synchronous Buck Controller

Micrel, Inc.
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 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 are selected as the output
capacitors, such as ceramic capacitors, a ripple injection
method is applied to provide the enough FB voltage
ripples. Please refer to the “Ripple Injection” subsection
for more details.
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)
MIC2174
Voltage Setting Components
The MIC2174 requires two resistors to set the output
voltage as shown in Figure 5.
Figure 5. Voltage-Divider Configuration
The output voltage is determined by the equation:
V_OUT
=
V_REF
× (1+
R1)
R2
(26)
where, V_REF = 0.8V. A typical value of R1 can be
between 3kΩ and 10kΩ. If R1 is too large, it may allow
noise to be introduced into the voltage feedback loop. If
R1 is too small in value, it will decrease the efficiency of
the power supply, especially at light loads. Once R1 is
selected, R2 can be calculated using:
R2 = ^{VREF × R1}
V_OUT − V_REF
(27)
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.
I_D(avg) = I_OUT × 2 × 30ns × f_SW
(28)
The reverse voltage requirement of the diode is:
V_DIODE(rrm) = V_HSD
The power dissipated by the Schottky diode is:
P_DIODE = I_D(avg) × V_F
(29)
where, V_F = forward voltage at the peak diode current.
The external Schottky diode is not necessary for the
circuit operation since the low-side MOSFET contains a
parasitic body diode. The external diode will improve
efficiency and decrease the high frequency noise. If the
MOSFET body diode is used, it must be rated to handle
the peak and average current. The body diode has a
relatively slow reverse recovery time and a relatively
high forward voltage drop. The power lost in the diode is
September 2009
15
M9999-090409-B