MIC23451 Datasheet, PDF (15/20 Pages) Micrel Semiconductor – 3MHz, 2A Triple Synchronous Buck Regulator

Micrel, Inc.
Thermal Considerations
Most applications will not require 2A continuous current
from all outputs at all times, so it is useful to know what
the thermal limits are 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 MIC23451 is
20°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) = 20°C/W
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. This requires creating
an ‘exchange rate’ between power dissipation per
regulator (P_DISS) and its output current (I_OUT).
An accurate measure of this function can use the
efficiency curve, as illustrated in Equation 8:
η=
P_OUT
P_OUT + P_LOSS
( ) _PLOSS
=
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,
remove the inductor loss P_DCR, which is not dissipated
within the package. This does not give a worst case
figure because efficiency is typically measured on a
nominal part at nominal temperatures. The I_OUT to P_DISS
function used in this case is a synthesized P_DISS, which
accounts for worst case values at maximum operating
temperature, as shown in Equation 9.
MIC23451
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
Because 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.
Using the function describing P_DISS in terms of I_OUT,
substitute P_DISS with Equation 7 to form the function of
maximum output current I_OUTMAX vs. ambient temperature
T_AMB (Equation 10):
I_OUTMAX =
T_JMAX − T_AMB
Rθ _(J−A)
R _{DSON_P}
×
V_OUT
V_IN
+ R_{DSON_N} × _{}1 −
V_OUT
V_IN

Eq. 10
The curves shown in the “Typical Characteristics” section
are plots of this function adjusted to account for 1, 2, or 3
regulators running simultaneously.
HyperLight Load Mode
Each regulator in the MIC23451 uses a minimum on and
off time proprietary control loop (patented by Micrel).
When the output voltage falls below the regulation
threshold, the error comparator begins a switching cycle
that turns the PMOS on and keeps it on for the duration
of the minimum-on-time. This increases the output
voltage. If the output voltage is over the regulation
threshold, then the error comparator turns the PMOS off
for a minimum-off-time until the output drops below the
threshold. The NMOS acts as an ideal rectifier that
conducts when the PMOS is off. Using an NMOS switch
instead of a diode allows for lower voltage drop across
the switching device when it is on. The asynchronous
switching combination between the PMOS and the
NMOS allows the control loop to work in discontinuous
mode for light load operations. In discontinuous mode,
the MIC23451 works in pulse-frequency modulation
(PFM) to regulate the output. As the output current
increases, the off-time decreases, which provides more
energy to the output. This switching scheme improves the
efficiency of MIC23451 during light load currents by
switching only when it is needed. As the load current
November 5, 2013
15
Revision 1.2