MIC2182_04 Datasheet, PDF (19/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller

MIC2182
The input voltage ripple will primarily depend on the input
capacitors ESR. The peak input current is equal to the peak
inductor current, so:
∆V_IN = I_{inductor(peak)} × R_{ESR(CIN )}
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 induc-
tor ripple current 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 × R_{ESR(CIN )}
Voltage Setting Components
The MIC2182-3.3 and MIC2182-5.0 ICs contain internal
voltage dividers that set the output voltage. The MIC2182
adjustable version requires two resistors to set the output
voltage as shown in Figure 13.
Error
Amp
R1
FB
7
R2
V_REF
1.245V
MIC2182 [adj.]
Figure 13. Voltage-Divider Configuration
The output voltage is determined by the equation:
V_O = V_REF × ^{}1+ _RR21^{}
Where: V_REF for the MIC2182 is typically 1.245V.
A typical value of R1 can be between 3k and 10k. 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 low output loads.
Once R1 is selected, R2 can be calculated using:
R2 = ^{VREF × R1}
V_O − V_REF
Voltage Divider Power Dissipation
The reference voltage and R2 set the current through the
voltage divider.
I_divider
=
V_REF
R2
The power dissipated by the divider resistors is:
P_divider = (R1+ R2) × I_divider2
Efficiency Calculation and Considerations
Efficiency is the ratio of output power to input power. The
difference is dissipated as heat in the buck converter. Under
light output load, the significant contributors are:
Micrel
• Supply current to the MIC2182
• MOSFET gate-charge power (included in the IC
supply current)
• Core losses in the output inductor
To maximize efficiency at light loads:
• Use a low gate-charge MOSFET or use the
smallest MOSFET, which is still adequate for
maximum output current.
• Allow the MIC2182 to run in skip mode at lower
currents.
• Use a ferrite material for the inductor core, which
has less core loss than an MPP or iron power
core.
Under heavy output loads the significant contributors to
power loss are (in approximate order of magnitude):
• Resistive on-time losses in the MOSFETs
• Switching transition losses in the MOSFETs
• Inductor resistive losses
• Current-sense resistor losses
• Input capacitor resistive losses (due to the
capacitors ESR)
To minimize power loss under heavy loads:
• Use logic-level, low on-resistance MOSFETs.
Multiplying the gate charge by the on-resistance
gives a Figure of merit, providing a good bal-
ance between low and high load efficiency.
• Slow transition times and oscillations on the
voltage and current waveforms dissipate more
power during turn-on and turnoff of the
MOSFETs. A clean layout will minimize parasitic
inductance and capacitance in the gate drive
and high current paths. This will allow the fastest
transition times and waveforms without oscilla-
tions. Low gate-charge MOSFETs will transition
faster than those with higher gate-charge
requirements.
• For the same size inductor, a lower value will
have fewer turns and therefore, lower winding
resistance. However, using too small of a value
will require more output capacitors to filter the
output ripple, which will force a smaller band-
width, slower transient response and possible
instability under certain conditions.
• Lowering the current-sense resistor value will
decrease the power dissipated in the resistor.
However, it will also increase the overcurrent
limit and will require larger MOSFETs and
inductor components.
• Use low-ESR input capacitors to minimize the
power dissipated in the capacitors ESR.
Decoupling Capacitor Selection
The 4.7µF decoupling capacitor is used to minimize noise on
the VDD pin. The placement of this capacitor is critical to the
proper operation of the IC. It must be placed right next to the
April 22, 2004
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M9999-042204