MIC22600 Datasheet, PDF (15/17 Pages) Micrel Semiconductor – 1MHz 6A Integrated Switch Synchronous Buck Regulator

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.
MIC22600
Current Limit
The MIC22600 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 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 9A 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.
September 2007
Figure 4. Current Limit Detail
Thermal Considerations
The MIC22600 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:
T_J = T_AMB + P_DISS · Rθ_JA
Where
• P_DISS is the power dissipated within the MLF^®
package and is typically 1.5W at 6A load. This
has been calculated for a 1µH inductor and
details can be found in table 1 below for
reference.
15
M9999-092007-A