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MIC28510_12 Datasheet, PDF (18/30 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control Synchronous DC/DC Buck Regulator | |||
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Micrel, Inc.
Application Information
Setting the Switching Frequency
The MIC28510 is an adjustableâfrequency, synchronous
buck regulator featuring a unique digitally-modified,
adaptive onâtime control architecture. The switching
frequency can be adjusted between 100kHz and 500kHz
by changing the resistor divider network consisting of
R18 and R19.
Figure 5. Switching Frequency Adjustment
The following formula gives the estimated switching
frequency:
fSW _ ADJ
ï½
fO
ï´
R19
R18 ï« R19
Eq. 3
where fO = switching frequency when R18 is 100k and
R19 being open, fO is typically 450kHz. For more precise
setting, it is recommended to use the graph illustrated
in Figure 6:
MIC28510
Inductor Selection
Values for inductance, peak, and RMS currents are
required to select the output inductor. The input and
output voltages and the inductance value determine the
peakâtoâpeak inductor ripple current. Generally, higher
inductance values are used with higher input voltages.
Larger peakâtoâpeak ripple currents will increase the
power dissipation in the inductor and MOSFETs. Larger
output ripple currents will also require more output
capacitance to smooth out the larger ripple current.
Smaller peakâtoâpeak ripple currents require a larger
inductance value and therefore a larger and more
expensive inductor. A good compromise between size,
loss and cost is to set the inductor ripple current to be
equal to 20% of the maximum output current. The
inductance value is calculated by Equation 4:
L
ï½
VOUT ï´ (VIN(max) ï VOUT )
VIN(max) ï´ fsw ï´ 20% ï´ IOUT(max)
Eq. 4
where:
fSW = Switching frequency
20% = Ratio of AC ripple current to DC output current
VIN(MAX) = Maximum power stage input voltage
The peakâtoâpeak inductor current ripple is:
ïIL(pp)
ï½
VOUT ï´ (VIN(max) ï VOUT )
VIN(max) ï´ fsw ï´ L
Eq. 5
The peak inductor current is equal to the average output
current plus one half of the peakâtoâpeak inductor
current ripple.
IL(pk) =IOUT(max) + 0.5 ï´ ÎIL(pp)
Eq. 6
The RMS inductor current is used to calculate the I2R
losses in the inductor:
IL(RMS) ï½
IOUT(max) 2
ï«
ÎIL(PP) 2
12
Eq. 7
Figure 6. Switching Frequency vs. R19
March 2012
18
M9999-030912-A
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