MMM5062 datasheet(16/22 Pages) FREESCALE

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Micrel, Inc.

MIC22950

February 2010

M9999-021910-B

Current Limit

The MIC22950 is protected against overload in two

stages. The first is to limit the current in the P-Channel

switch; the second is over- temperature shutdown.

Current is limited by measuring the current through the

high-side MOSFET during its power stroke and

immediately switching off the driver when the preset limit

is exceeded.

The circuit in Figure 4 describes the operation of the

current-limit circuit. Since the actual RDSON of the

P-Channel

MOSFET

varies

part-to-part,

over-

temperature and with input voltage, simple I.R voltage

detection is not employed. Instead, a smaller copy of the

Power MOSFET (Reference FET) is fed with a constant

current which is a directly proportional to the factory set

current limit. This sets the current limit as a current ratio

and thus, is not dependant upon the RDSON value.

Current limit is set to nominal value. Variations in the

scale factor K between the Power PFET and the

reference PFET used to generate the limit threshold

account for a relatively small inaccuracy.

Figure 4. Current Limit Detail

Thermal Considerations

The MIC22950 is packaged in the MLF

® 5mm x 5mm, a

package that has excellent thermal performance

equaling that of the larger TSSOP packages. This

maximizes heat transfer from the junction to the exposed

pad (ePAD) which connects to the ground plane. The

size of the ground plane attached to the exposed pad

determines the overall thermal resistance from the

junction to the ambient air surrounding the printed circuit

board. The junction temperature for a given ambient

temperature can be calculated using:

TJ = TA + PDISS × RθJA

Where:

PDISS is the power dissipated within the MLF

® package

and is typically 1.5W at 10A load. This has been

calculated for a 1µH inductor and details can be found in

Table 1.

RθJA is a combination of junction-to-case thermal

resistance

(RθJC)

and

case-to-ambient

thermal

resistance (RθCA), since thermal resistance of the solder

connection from the ePAD to the PCB is negligible; RθCA

is the thermal resistance of the ground plane to ambient.

So RθJA = RθJC + RθCA.

TA is the Operating Ambient temperature.

VIN

VOUT

@ 10A

3.5

4.5

1.66

1.67

1.68

1.7

1.73

1.2

1.68

1.69

1.71

1.72

1.74

1.8

1.76

1.77

1.78

1.8

2.5

1.85

1.84

1.85

1.86

3.3

1.92

1.91

1.92

Table 1. Power Dissipation (W) for 10A Output

Example

The

Evaluation

board

has

two

copper

planes

contributing to an RθJA of approximately 25

oC/W. The

worst case RθJC of the MLF

® is 11oC/W. If we look at a

typical application of 3.6V to 1.8V @ 10A, the estimated

Power dissipation in the MLF

® package taken from Table

1 will be 1.76W:

RθJA = RθJC + RθCA

RθJA = 11 + 25 = 36

oC/W

To calculate the junction temperature for a 50

ambient:

TJ = TA+PDISS. RθJA

TJ = 50 + (1.76 x 36)

TJ = 113

This is below our maximum of 125

oC.

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