MIC23303 Datasheet, PDF (15/21 Pages) Micrel Semiconductor – 4MHz PWM 3A Buck Regulator with HyperLight Load™ and Power Good

Micrel, Inc.
From this simple circuit we can calculate V_X if we know
I_SOURCE, V_Z, and the resistor values, R_XY and R_YZ, using the
equation:
MIC23303
Since effectively all of the power losses (minus the
inductor losses) in the converter are dissipated within the
MIC23303 package, P_DISS can be calculated thus:
( ) V_X = I_SOURCE × R _XY + R _YZ + V_Z
Thermal circuits can be considered using these same rules
and can be drawn similarly replacing current sources with
power dissipation (in Watts), resistance with thermal
resistance (in °C/W) and voltage sources with temperature
(in °C).
Figure 6. Thermal Circuit Description
Now replacing the variables in the equation for V_X, we can
find the junction temperature (T_J) from power dissipation,
ambient temperature, and the known thermal resistance of
the PCB (Rθ_CA) and the package (Rθ_JC).
( ) T_J = P_DISS × Rθ_JC + Rθ_CA + T_AMB
As can be seen in the diagram, total thermal resistance
Rθ_JA = Rθ_JC + Rθ_CA. Hence this can also be written:
P_DISS
=

^POUT

×(1
η
− 1)_ − I_OUT ²

× DCR
Where:
η = Efficiency taken from efficiency curves and DCR =
Inductor DCR.
Rθ_JC and Rθ_JA are found in the Operating Ratings section
of the datasheet. Where the reel board area differs from
1in square, Rθ_CA (the PCB thermal resistance) values for
various PCB copper areas can be taken from Figure 7
below. This graph is taken from Designing with Low
Dropout Voltage Regulators, which is available from the
Micrel website (LDO Application Hints).
Example:
A MIC23303 is intended to drive a 2A load at 1.8V and is
placed on a printed circuit board which has a ground plane
area of at least 25mm square.
The voltage source is a Li-ion battery with a lower
operating threshold of 3V and the ambient temperature of
the assembly can be up to 50°C.
Summary of variables:
I_OUT = 2A
V_OUT = 1.8V
V_IN = 3V to 4.2V
T_AMB = 50°C
Rθ_JA = 61°C/W from datasheet
η @ 2A = 85% (worst case @ 5V from Figure 3)
( ) T_J = P_DISS × Rθ_JA + T_AMB
September 6, 2013
15
090613-2.0