MIC2103 datasheet(22/36 Pages) MICREL

22 / 36 page

Micrel, Inc.

MIC2103/04

August 2012

M9999-080712-A

Application Information

Setting the Switching Frequency

The MIC2103/04 are adjustable-frequency, synchronous

buck controllers featuring a unique adaptive on-time

control architecture. The switching frequency can be

adjusted between 200kHz and 600kHz by changing the

resistor divider network consisting of R19 and R20.

Figure 5. Switching Frequency Adjustment

The following formula gives the estimated switching

frequency:

ADJ







(Eq. 4)

Where fO = Switching Frequency when R19 is 100k and

R20 being open, fO is typically 550kHz. For a more

precise setting, it is recommended to use the following

graph:

Switching Frequency

100

200

300

400

500

600

10.00

100.00

1000.00

10000.00

R20 (k Ohm)

R19 = 100k, IOUT =10A

VIN = 48V

VIN =75V

Figure 6. Switching Frequency vs. R20

MOSFET Selection

The MIC2103/04 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 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

MIC2103/04 gate-drive circuit. At 200kHz switching

frequency, the gate charge can be a significant source of

power dissipation in the MIC2103/04. At low output load,

this power dissipation is noticeable as a reduction in

efficiency. The average current required to drive the

high-side MOSFET is:

side]

G[high

(avg)





(Eq. 5)

where:

IG[high-side](avg) = Average high-side MOSFET gate

current

QG = Total gate charge for the high-side MOSFET taken

from the manufacturer’s data sheet for VGS = VDD.

fSW = Switching Frequency

The low-side MOSFET is turned on and off at VDS = 0

because an internal body diode or external freewheeling

diode is conducting during this time. The switching loss

for the low-side MOSFET is usually negligible. Also, the

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