MIC2127A datasheet(22/38 Pages) MICROCHIP

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MIC2127A

DS20005676B-page 22

 2016 Microchip Technology Inc.

5.0

APPLICATIONS INFORMATION

5.1

Setting the Switching Frequency

The MIC2127A is an adjustable-frequency, synchro-

nous buck controller, featuring a unique adaptive

on-time control architecture. The switching frequency

can be adjusted between 270 kHz-800 kHz by chang-

ing the resistor divider network between VIN and AGND

pins consisting of R1 and R2, as shown in Figure 5-1.

FIGURE 5-1:

Switching Frequency

Adjustment.

Equation 5-1 shows the estimated switching frequency.

EQUATION 5-1:

fO is the switching frequency when R1 is 100 k and R2

being open; fO is typically 800 kHz. For more precise

setting, it is recommended to use Figure 5-2.

FIGURE 5-2:

Switching Frequency vs. R2.

5.2

Output Voltage Setting

The output voltage can be adjusted using a resistor

divider from output to AGND whose mid-point is

connected to the FB pin, as shown the Figure 5-3.

FIGURE 5-3:

Output Voltage Adjustment.

The

output

voltage

can be calculated using

Equation 5-2.

EQUATION 5-2:

The maximum output voltage that can be programmed

using the MIC2127A is limited to 30V, if not limited by

the maximum duty cycle (see Equation 4-2).

A typical value of R1 is less than 30 k. If R1 is too

large, it may allow noise to be introduced into the

voltage feedback loop. It also increases the offset

between the set output voltage and actual output

voltage because of the error amplifier bias current. 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 5-3.

EQUATION 5-3:

VIN

FREQ

AGND

MIC2127A

VIN

4.5V to 75V

SW _ADJ

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



200

300

400

500

600

700

800

500

5000

2 (k

)

IN = 48V

IN = 75V

IN = 24V

OUT = 5V

1 = 100 k

OUT = 5A

MIC2127A

SOFT-

START

0.6V

VREF

Comparator

COMPENSATION

VOUT

OUT

RE F

------







Where:

VREF

=0.6V

OUT

REF

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

–

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

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