MIC22400 Datasheet, PDF (16/19 Pages) Micrel Semiconductor – 4A Integrated Switch Synchronous Buck Regulator with Frequency Programmable up to 4MHz

Micrel, Inc.
An alternative method here shows an example of a V_DDQ
& V_TT 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⋅V_REF. In
this example, FB₂ is regulated to ½⋅V_REF.
MIC22400
Current Limit
The MIC22400 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.
Figure 4 describes the operation of the current limit
circuit. Since the actual RDS_ON 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 RDS_ON value. Current
limit is set to 6A nominal. 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.
February 2008
Figure 4. Current Limit Detail
Thermal Considerations
The MIC22400 is packaged in the MLF^® 3mm 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:
T_J = T_AMB + P_DISS · Rθ_JA
Where
• P_DISS is the power dissipated within the MLF^®
package and is typically 0.89W at 3A load. This
has been calculated for a 1µH inductor and
details can be found in table 1 below for
reference.
16
M9999-022108-A