MIC2103 datasheet(26/42 Pages) MICROCHIP

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MIC2103/4

DS20005899A-page 26

 2017 Microchip Technology Inc.

5.0

APPLICATION INFORMATION

5.1

Setting the Switching Frequency

The

MIC2103/4

are

adjustable-frequency,

synchronous buck controllers that feature a unique

adaptive on-time control architecture. The switching

frequency can be adjusted between 200 kHz and

600 kHz by changing the resistor divider network

consisting of R19 and R20.

FIGURE 5-1:

Switching Frequency

Adjustment.

The following formula gives the estimated switching

frequency:

EQUATION 5-1:

For a more precise setting, it is recommended to use

the following graph:

FIGURE 5-2:

Switching Frequency vs.

R20.

5.2

MOSFET Selection

The MIC2103/4 controllers work from input voltages of

4.5V to 75V and have an internal 5V VDD LDO. This

internal VDD LDO provides power to turn on the

external N-channel power MOSFETs for the high-side

and low-side switches. For applications where VDD <

5V, it is necessary that the power MOSFETs used are

sub-logic level and are in full conduction mode for VGS

of 2.5V. For applications when VDD > 5V; logic-level

MOSFETs, whose operation is specified at VGS = 4.5V

must be used.

There are different criteria for choosing the high-side

and low-side MOSFETs. These differences are more

significant at lower duty cycles. In such an application,

the high-side MOSFET is then required to switch as

quickly as possible in order to minimize transition

losses, whereas the low-side MOSFET can switch

slower, but must handle larger RMS currents. When the

duty cycle approaches 50%, the current carrying

capability of the high-side MOSFET starts to become

critical.

It is important to note that the on-resistance of a

MOSFET increases with increasing temperature. A

75°C rise in junction temperature will increase the

channel resistance of the MOSFET by 50% to 75% of

the resistance specified at 25°C. This change in

resistance must be accounted for when calculating

MOSFET power dissipation and in calculating the value

of current limit. Total gate charge is the charge required

to turn the MOSFET on and off under specified

operating conditions (VDS and VGS). The gate charge

is supplied by the MIC2103/4 gate-drive circuit. At

200 kHz switching frequency, the gate charge can be a

significant source of power dissipation in the

VDD/PVDD

VDD

2.2μF

AGND

MIC2103/04

BST

1μF

PGND

VIN

FREQ

R20

R19

SW_ADJ

R20

R19

R20

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



Where:

fO = Switching frequency when R19 is 100 kΩ and

R20 is open. Typically 550 kHz.

100

200

300

400

500

600

10.00

100.00

1000.00

10000.00

R20 (k )

R19 = 100k, IOUT =10A

VIN = 48V

VIN =75V

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