MIC28511 Datasheet, PDF (22/34 Pages) Microchip Technology – 60VIN, 3A Synchronous Buck Regulator

MIC28511
5.0 APPLICATION INFORMATION
5.1 Output Voltage Setting
Components
The MIC28511 requires two resistors to set the output
voltage as shown in Figure 5-1.
5.2 Setting the Switching Frequency
The MIC28511 switching frequency can be adjusted by
changing the resistor divider network from V_IN.
g_M AMP
V_REF
R1
FB
R2
FIGURE 5-1:
Configuration.
Voltage Divider
The output voltage is determined by Equation 5-1.
EQUATION 5-1:
V_OUT
=
V_F
B



1
+
RR_{----12--}
Where:
V_FB
0.8V
A typical value of R1 used on the standard evaluation
board is 10 kΩ. 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 light loads. Once R1 is
selected, R2 can be calculated using Equation 5-2:
EQUATION 5-2:
R2 = ----^V----^F---^B----^-----^R----1-----
V_OUT – V_FB
DS20005520A-page 22
FIGURE 5-2:
Adjustment.
Switching Frequency
Equation 5-3 gives the estimated switching frequency.
EQUATION 5-3:
Where:
f₀
f_SW
=
f₀



R^{----1---7--R---+-1---7-}R^{----1---9-}
Switching frequency when R17 is open;
typically 680 kHz.
Figure 5-3 shows the switching frequency versus the
resistor R17 when R19 = 100 kΩ.
700
R19 = 100kŸ, I_OUT = 1A
600
500
V_IN = 60V
400
V_IN = 12V
300
200
100
0
10
FIGURE 5-3:
R17.
100
1000
10000
R17 (kŸ)
Switching Frequency vs.
5.3 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
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