LTC3410B_15 Datasheet, PDF (11/16 Pages) Linear Technology – 2.25MHz, 300mA Synchronous Step-Down Regulator in SC70

LTC3410B
APPLICATIO S I FOR ATIO
Thermal Considerations
In most applications the LTC3410B does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3410B 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 LTC3410B 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 tempera-
ture rise is given by:
T_R = (P_D)(θ_JA)
where P_D is the power dissipated by the regulator and
θ_JAis the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, T_J, is given by:
T_J = T_A + T_R
where T_A is the ambient temperature.
As an example, consider the LTC3410B in dropout at an
input voltage of 2.7V, a load current of 300mA and an
ambient temperature of 70°C. From the typical perfor-
mance graph of switch resistance, the R_DS(ON) of the
P-channel switch at 70°C is approximately 1.0Ω.
Therefore, power dissipated by the part is:
P_D = I_LOAD2 • R_DS(ON) = 90mW
For the SC70 package, the θ_JA is 250°C/ W. Thus, the
junction temperature of the regulator is:
T_J = 70°C + (90)(250) = 92.5°C
which is well below the maximum junction temperature
of 125°C.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (R_DS(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, V_OUT immediately shifts by an amount
equal to (∆I_LOAD • ESR), where ESR is the effective series
resistance of C_OUT. ∆I_LOAD also begins to charge or
discharge C_OUT, which generates a feedback error signal.
The regulator loop then acts to return V_OUT to its steady-
state value. During this recovery time V_OUT can be moni-
tored for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
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 C_OUT, causing a rapid drop in V_OUT. 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 • C_LOAD).
Thus, a 10µF capacitor charging to 3.3V would require a
250µs rise time, limiting the charging current to about
130mA.
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