MIC2194_05 Datasheet, PDF (9/10 Pages) Micrel Semiconductor – 400kHz SO-8 Buck Control IC

MIC2194
MIC2194
Voltage
Amplifier
V_OUT
R1
Pin 2
R2
V_REF
1.245V
Figure 5
The output voltage is determined by:
V_OUT =
V_REF
× 1+
R1
R2
Where: V_REF for the MIC2194 is typically 1.245V.
Lower values of R1 are preferred to prevent noise from
appearing on the FB pin. A typically recommended value is
10kΩ. 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 with the following
formula:
R2= ^{VREF × R1}
V_OUT – V_REF
Efficiency 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:
• The V_IN supply current, which includes the current
required to switch the external MOSFET.
• 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.
• Use a ferrite material for the inductor core, which has
less core loss than an MPP or iron power core.
Micrel
Under heavy output loads the significant contributors to
power loss are (in approximate order of magnitude):
• Resistive on time losses in the MOSFET
• Switching transition losses in the MOSFET
• Inductor resistive losses
• Current sense resistor losses
• Input capacitor resistive losses (due to the capacitors
ESR)
To minimize power loss under heavy loads:
• Use low on-resistance MOSFETs. Use low threshold
logic level MOSFETs when the input voltage is below
5V. Multiplying the gate charge by the on-resistance
gives a figure of merit, providing a good balance
between low load and high load efficiency.
• Slow transition times and oscillations on the voltage
and current waveforms dissipate more power during
the turn on and turn off of the MOSFET. A clean
layout will minimize parasitic inductance and capaci-
tance in the gate drive and high current paths. This
will allow the fastest transition times and waveforms
without oscillations. 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 bandwidth, slower transient response
and possible instability under certain conditions.
• Lowering the current sense resistor value will de-
crease 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.
April 2005
9
MIC2194