MIC2102YML datasheet(24/37 Pages) MICREL

24 / 37 page

Micrel, Inc.

MIC2101/02

August 2012

M9999-080712-A

Application Information

Setting the Switching Frequency

The MIC2101/02 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:

R20

R19

R20

SW_ADJ







Eq. 4

Where fO = Switching Frequency when R19 is 100k and

R20 being open, fO is typically 600kHz. For more precise

setting, it is recommended to use the following graph:

Switching Frequency

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

10.00

100.00

1000.00

10000.00

R20 (k Ohm)

R19 = 100k, IOUT =12A

VIN = 12V

VIN =38V

Figure 6. Switching Frequency vs. R20

MOSFET Selection

The MIC2101/02 controllers work from input voltages of

4.5V to 38V and has 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 required to switch as quickly

as possible 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

MIC2101/02 gate-drive circuit. At 600kHz switching

frequency, the gate charge can be a significant source of

power dissipation in the MIC2101/02. 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[HIGHSIDE](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

gate-drive current for the low-side MOSFET is more

accurately calculated using CISS at VDS = 0 instead of

gate charge.

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