MIC22601 Datasheet, PDF (15/18 Pages) Micrel Semiconductor – 4 MHz, 6A Integrated Switch Synchronous Buck Regulator

Micrel, Inc.
Current limit
The MIC22601 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 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 on 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.
Figure 4. Current Limit Detail
Thermal considerations
The MIC22601 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_A + P_D · Rθ_JA
MIC22601
Where
• P_DISS is the power dissipated within the MLF^®
package and is typically 1.8W at 6A load. This
has been calculated for a 0.47µH inductor and
details can be found in table 1 below for
reference.
V_INÆ
V_OUT
3
3.5
4
4.5
5
@5AÈ
1
1.67 1.71 1.76 1.81 1.85
1.2
1.68 1.72 1.77 1.81 1.86
1.8
1.70 1.74 1.79 1.74 1.84
2.5
1.72 1.76 1.80 1.85 1.89
3.3
1.78 1.82 1.86 1.91
Table 1. Power dissipation (W) for 5A output
• Rθ_JA is a combination of junction to case thermal
resistance (Rθ_JC) and Case to Ambient thermal
resistance (Rθ_CA), since thermal resistance of
the solder connection from the ePAD to the PCB
is negligible; Rθ_CA is the thermal resistance of
the ground plane to ambient. So Rθ_JA = Rθ_JC +
Rθ_CA.
• T_A is the Operating Ambient temperature.
Example
The Evaluation board has 2 copper planes contributing
to an Rθ_CA of approximately 25^°C/W. The worst case
Rθ_JC of the MLF^® 4x4 is 14^°C/W.
Rθ_JA = Rθ_JC + Rθ_CA
Rθ_JA = 14 + 25 = 39^°C/W
To calculate the junction temperature for a 50^°C
ambient:
T_J = T_AMB+P_DISS. Rθ_JA
T_J = 50 + (1.8 x 39)
T_J = 120^°C
This is below our maximum of 125^°C.
May 2009
15
M9999-050509-A