MIC22700 datasheet(15/21 Pages) MICREL

15 / 21 page

Micrel, Inc.

MIC22700

July 2009

M9999-072209-B

An alternative method here shows an example of a VDDQ

& VTT solution for a DDR memory power supply. Note

that POR is taken from Vo1 as POR2 will not go high.

This is because POR is set high when FB > 0.9

⋅VREF. In

this example, FB2 is regulated to ½⋅VREF.

Current Limit

The MIC22700 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 IR 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 MIC22700 is packaged in the MLF

® 4mm x 4mm, 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 = TAMB + PDISS · RθJA

Where

•

PDISS is the power dissipated within the MLF

package and is typically 1.5W at 7A load. This

has been calculated for a 1µH inductor and

details can be found in table 1 below for

reference.

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