MIC2128 datasheet(19/32 Pages) MICROCHIP

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DS20005620A-page 19

MIC2128

5.0

APPLICATIONS INFORMATION

5.1

Setting the Switching Frequency

The MIC2128 is an adjustable-frequency, synchronous

buck controller featuring a unique adaptive on-time

control architecture. The switching frequency can be

adjusted between 270 kHz and 800 kHz by changing

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 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 MIC2128 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 and 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

MIC2128

VIN

4.5V to 75V

SW _ADJ

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



200

300

400

500

600

700

800

500

5000

2 (k

)

IN = 75V

IN = 48V

OUT = 5V

1 = 100 k

OUT = 5A

IN = 24V

MIC2128

SOFT-

START

0.6V

VREF

Comparator

COMPENSATION

VOUT

OUT

RE F

------







Where:

VREF

=0.6V

OUT

REF

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

–

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

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