MIC28500 Datasheet, PDF (17/29 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control™ Synchronous DC-DC Buck Regulator

Micrel, Inc.
Application Information
Setting the Switching Frequency
The MIC28500 is an adjustable-frequency, synchronous
buck regulator featuring a unique digitally modified
adaptive on-time control architecture. The switching
frequency can be adjusted between 100kHz and 500kHz
by changing the resistor divider connected network
consisting of R18 and R19.
Figure 5. Switching Frequency Adjustment
The following formula gives the estimated switching
frequency:
f_SW
_ ADJ
=
f_O
×
R19
R18 + R19
Eq. 2
Where f_O = Switching Frequency when R18 is 100k and
R19 being open, f_O should be typically 500kHz. For more
precise setting, it is recommended to use the following
graph.
500
450
400
350
300
250
200
150
100
50
0
10.00
Switching Frequency
R18 = 100k, I_OUT =1A
VIN = 48V
VIN = 75V
100.00
1000.00
R19 (k Ohm)
10000.00
Figure 6. Switching Frequency vs. R19
The evaluation board design is optimized for a switching
frequency of 250kHz. If the switching frequency is
MIC28500
programmed to either lower end or higher end, the
design needs optimization.
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
peak-to-peak inductor ripple current. Generally, higher
inductance values are used with higher input voltages.
Larger peak-to-peak 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 peak-to-peak 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 3:
L=
V_OUT × (V_IN(max) − V_OUT )
V_IN(max) × f_sw × 20% × I_OUT(max)
Eq. 3
where:
f_SW = switching frequency, 300kHz
20% = ratio of AC ripple current to DC output current
V_IN(max) = maximum power stage input voltage
The peak-to-peak inductor current ripple is:
ΔI_L(pp)
=
V_OUT × (V_IN(max) − V_OUT )
V_IN(max) × f_sw × L
Eq. 4
The peak inductor current is equal to the average output
current plus one half of the peak-to-peak inductor current
ripple.
I_L(pk) =I_OUT(max) + 0.5 × ΔI_L(pp)
Eq. 5
The RMS inductor current is used to calculate the I²R
losses in the inductor.
I_L(RMS) =
I_OUT(max) ²
+
ΔI_L(PP) ²
12
Eq. 6
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance. The
June 2011
17
M9999-060311-B