MIC23450_13 Datasheet, PDF (15/22 Pages) Micrel Semiconductor – 3MHz, PWM, 2A Triple Buck Regulator with HyperLight Load® and Power Good

Micrel, Inc.
From that, the loss in efficiency due to inductor
resistance can be calculated as follows:
Efficiency
Loss
=

^{1 −}


V_OUT × I_OUT
V_OUT × I_OUT + P_DCR
 ^{× 100}
Eq. 6
Efficiency loss due to DCR is minimal at light loads and
gains significance as the load is increased. Inductor
selection becomes a trade-off between efficiency and
size in this case.
Thermal Considerations
As most applications will not require 2A continuous
current from all outputs at all times, it is useful to know
what the thermal limits will be for various loading
profiles.
The allowable overall package dissipation is limited by
the intrinsic thermal resistance of the package (Rθ_(J-C))
and the area of copper used to spread heat from the
package case to the ambient surrounding temperature
(Rθ_(C-A)). The composite of these two thermal resistances
is Rθ_(J-A), which represents the package thermal
resistance with at least 1 square inch of copper ground
plane. From this figure, which for the MIC23450 is
30°C/W, we can calculate maximum internal power
dissipation as shown in Equation 7:
PD_MAX
=
T_JMAX − T_AMB
Rθ _(J−A)
Eq. 7
where:
T_JMAX = Maximum junction temp (125°C)
T_AMB = Ambient temperature
Rθ_(J-A) = 30°C/W
As can be expected, the allowable dissipation tends
towards zero as the ambient temperature increases
towards the maximum operating junction temperature.
The graph of PD_MAX vs. Ambient temperature could be
drawn quite simply using this equation. However, a more
useful measure is the maximum output current per
regulator vs. ambient temperature. For this, we must first
create an ‘exchange rate’ between power dissipation per
regulator (P_DISS) and its output current (I_OUT).
MIC23450
An accurate measure of this function can utilize the
efficiency curve, as illustrated in Equation 8:
η=
P_OUT
P_OUT + P_LOSS
P_LOSS
=
P_OUT (1− η)
η
Eq. 8
where:
η = Efficiency
P_OUT = I_OUT.V_OUT
To arrive at the internal package dissipation P_DISS, one
would need to remove the inductor loss P_DCR which is
not dissipated within the package. This however, does
not give a worst case figure, since efficiency is typically
measured on a nominal part at nominal temperatures.
The I_OUT to P_DISS function we use therefore is a
synthesized P_DISS which accounts for worst case values
at maximum operating temperature, as shown in
Equation 9:
P_DISS
=
I_OUT
2


R_{DSON_P}
×
V_OUT
V_IN
+ R_{DSON_N} × _{}1−
V_OUT
V_IN



Eq. 9
where:
R_{DSON_P} = Maximum R_DSON of the high side, P-Channel
switch at T_JMAX
R_{DSON_N} = Maximum R_DSON of the low side, N-Channel
switch at T_JMAX
V_OUT = Output Voltage,
V_IN
= Input Voltage
Since ripple current and switching losses are small with
respect to resistive losses at maximum output current,
they can be considered negligible for the purpose of this
method, but could be included if required.
November 5, 2013
15
110513-1.1