MIC2125/6

DS20005459B-page 20

 2015 Microchip Technology Inc.

5.0

APPLICATION INFORMATION

5.1

Setting the Switching Frequency

The

MIC2125/6

are

adjustable-frequency,

synchronous buck controllers featuring a unique

adaptive ON–time control architecture. The switching

frequency can be adjusted between 200 kHz and

750 kHz by changing the resistor divider network

consisting of R19 and R20.

FIGURE 5-1:

Switching Frequency

Adjustment.

Equation 5-1 gives the estimated switching frequency.

EQUATION 5-1:

For more precise setting, it is recommended to use

Figure 5-2.

FIGURE 5-2:

Switching Frequency vs.

R20

5.2

MOSFET Selection

Voltage rating, on-resistance, and total gate charge are

important parameters for MOSFET selection.

The voltage rating for the high-side and low-side

MOSFETs are essentially equal to the power stage

of the MOSFETs to account for voltage spikes due to

circuit parasitic elements.

The power dissipated in the MOSFETs is the sum of

EQUATION 5-2:

EQUATION 5-3:

The total high-side MOSFET switching loss is:

EQUATION 5-4:

Turn-on and turn-off transition times can be

approximated by:

EQUATION 5-5:

VDD/PVDD

SW

FB

5V

2.2μF

x3

AGND

MIC2125/26

BST

4.7μF

PGND

VIN

FREQ

CS

R20

R19

f

SW ADJ



f

O

R20

R19

R20

+

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



=

Where:

Switching Frequency when R19 is

750 kHz.

P

SW

P

CONDUCTION

P

AC

+

=

P

CONDUCTION

I

SW RMS



2

R

DS ON





=

Where:

On-Resistance of the MOSFET

RMS current of the MOSFET

P

AC

0.5

V

IN

I

LOAD

t

R

t

F

+

 f

SW









=

Where:

Switching Transition Times

Load Current

Switching Frequency

t

R

Q

SW HS



R

HSD PULL

UP

–



R

HS GATE



+





V

DD

V

TH

–

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