MIC28511 datasheet(22/34 Pages) MICROCHIP

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MIC28511

DS20005520A-page 22

 2016 Microchip Technology Inc.

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.

FIGURE 5-1:

Voltage Divider

Configuration.

The output voltage is determined by Equation 5-1.

EQUATION 5-1:

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:

5.2

Setting the Switching Frequency

The MIC28511 switching frequency can be adjusted by

changing the resistor divider network from VIN.

FIGURE 5-2:

Switching Frequency

Adjustment.

Equation 5-3 gives the estimated switching frequency.

EQUATION 5-3:

Figure 5-3 shows the switching frequency versus the

resistor R17 when R19 = 100 kΩ.

FIGURE 5-3:

Switching Frequency vs.

R17.

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

M AMP

REF

OUT

-------







Where:

VFB

0.8V



OUT

–

-----------------------------

R17

R19

--------------------------







Where:

Switching frequency when R17 is open;

typically 680 kHz.

100

1000

10000

R17 (k )

IN = 60V

IN = 12V

R19 = 100k , I

OUT = 1A

700

600

500

400

300

200

100

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