April 2005 11 M9999-042205
MIC2184 Micrel, Inc.
fied free running frequency of the MIC2184. If the synchroniz-
ing frequency is lower, double pulsing of the gate drive
outputs will occur. When not used, the sync pin must be
connected to ground.
The maximum recommended output switching frequency is
600kHz. Synchronizing to higher frequencies may be pos-
sible, however, higher power dissipation in the internal gate
drive circuits will occur. The MOSFET gates require charge
to turn on the device. The average current required by the
MOSFET gate increases with switching frequency.
Soft Start
Soft start reduces the power supply input surge current at
start up by controlling the output voltage risetime. The input
surge appears while the output capacitance is charged up. A
slower output risetime will draw a lower input surge current.
Soft start may also be used for power supply sequencing.
The soft start voltage is applied directly to the PWM compara-
tor. A 5µA internal current source is used to charge up the soft
start capacitor. The capacitor is discharged when either the
enable pin voltage drops below the standby threshold or the
V
DD
voltage drops below its UVLO level.
The part switches at a low duty cycle when the soft start pin
voltage is zero. As the soft start voltage rises from 0V to 0.7V,
the duty cycle increases from the minimum duty cycle to the
operating duty cycle. The oscillator runs at the foldback
frequency (1/4 of the switching frequency) until the feedback
voltage rises above 0.3V. The risetime of the output is
dependent of the soft start capacitor output capacitance,
input and output voltage and load current.
Voltage Setting Components
The MIC2184 requires two resistors to set the output voltage
as shown in Figure 5.
V
REF
1.245V
Voltage
Amplifier
Pin 6
MIC2184
V
OUT
R1
R2
Figure 5
The output voltage is determined by the equation below.
VV
R
R
OUT
REF
=×+1
1
2
Where: V
REF
for the MIC2184 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.
R
VR
VV
REF
OUT
REF
2
1
=
×
±
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
A supply current
• The V
IN
P 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.
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.
