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Rev. E, Sep 2005
ML3406
Application Information (3)
Thermal Considerations
In most applications the ML3406 does not dissipate much
heat due to its high efficiency. But, in applications where
the ML3406 is running at high ambient temperature with
low supply voltage and high duty cycles, such as in
dropout, the heat dissipated may exceed the maximum
junction
temperature
of
the
part.
If
the
junction
temperature reaches approximately 150°C, both power
switches will be turned off and the SW node will become
high impedance.
To avoid the ML3406 from exceeding the maximum
junction temperature, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated exceeds the
maximum
junction
temperature
of
the
part.
The
temperature rise is given by:
TR = (PD)(ѲJA)
Where PD is the power dissipated by the regulator and ѲJA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, TJ, is given by:
TJ = TA + TR
Where TA is the ambient temperature.
As an example, consider the ML3406 in dropout at an
input voltage of 2.7V, a load current of 600mA and an
ambient
temperature
of
70°C.
From
the
typical
performance graph of switch resistance, the RDS(ON) of the
P-channel switch at 70°C is approximately 0.52Ω.
Therefore, power dissipated by the part is:
PD = ILOAD
2
· RDS(ON) =
187.2mW
For the SOT-23 package, the ѲJA is 250°C/W. Thus, the
junction temperature of the regulator is:
TJ = 70°C + (0.1872)(250) = 116.8°C
Which is below the maximum junction temperature of
125°C
Note that at higher supply voltages, the junction
temperature is lower due to reduced switch resistance
(RDS(ON)).
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When a
load step occurs, VOUT immediately shifts by an amount
equal to (
ILOAD ESR), where ESR is the effective series
resistance of COUT,
ILOAD also begins to charge or
discharge COUT, which generates a feedback error signal.
The regulator loop then acts to return VOUT to its
steady-state value. During this recovery time VOUT can be
monitored for overshoot or ringing that would indicate a
stability problem. For a detailed explanation of switching
control loop theory.
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with COUT, causing a rapid drop in VOUT. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25 CLOAD).
Thus, a 10µF capacitor charging to 3.3v would require a
250µs rise time, limiting the charging current to about
130mA.
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy materials
are small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
electrical characteristics. The choice of which style inductor
to use often depends more on the price vs size
requirements than on what the ML3406 requires to
operate. Table 1 shows some typical surface mount
inductors that work well in ML3406 applications.
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