LTC3564_15 Datasheet, PDF (12/20 Pages) Linear Technology – 2.25MHz, 1.25A Synchronous Step-Down Regulator

LTC3564
APPLICATIO S I FOR ATIO
The R_DS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Characteris-
tics curves. Thus, to obtain I²R losses, simply add R_SW
to R_L and multiply the result by the square of the
average output current.
Other losses including C_IN and C_OUT ESR dissipative
losses and inductor core losses which generally account
for less than 2% total additional loss.
Thermal Considerations
In most applications the LTC3564 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3564 is running at high ambient tempera-
ture with low supply voltage and high duty cycles, such
as in dropout, the heat dissipated may exceed the maxi-
mum 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 LTC3564 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_JA
is 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 LTC3564 in dropout at an
input voltage of 2.7V, a load current of 1.2A 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 ~0.2Ω. There-
fore, power dissipated by the part is:
P_D = I_LOAD2 • R_DS(ON) = 288mW
For the SOT-23 package, the θ_JA is 215°C/ W. Thus, the
junction temperature of the regulator is:
T_J = 70°C + (0.288)(215) = 131.9°C
which is above 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 (ΔI_LOAD • ESR), where ESR is the effective series
resistance of C_OUT. ΔI_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.
3564f
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