LTC3549 Datasheet, PDF (12/16 Pages) Linear Technology – 250mA Low VIN Buck Regulator in 2mm × 3mm DFN

LTC3549
APPLICATIO S I FOR ATIO
Thermal Considerations
In most applications the LTC3549 does not dissipate much
heat due to its high efficiency. But, in applications where the
LTC3549 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 LTC3549 from exceeding the maximum
junction temperature, the user will need to do a thermal
analysis. The goal of the thermal analysis is to determine
whether the operating conditions exceed the maximum
junction temperature of the part. The temperature 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 LTC3549 in dropout at an input
voltage of 1.6V, a load current of 250mA and an ambient
temperature of 75°C. In the Switch Resistance graph
shown in the Typical Performance Characteristics, the
R_DS(ON) of the P-channel switch at 75°C is approximately
0.8Ω. Therefore, power dissipated by the part is:
P_D = I_LOAD2 • R_DS(ON) = 50mW
For the DCB6 package, the θ_JA is 64°C/W. Thus, the junc-
tion temperature of the regulator is:
T_J = 75°C + (0.05)(64) = 78.2°C
which is well below the maximum junction temperature
of 125°C.
Note that at higher supply voltages, the junction temperature
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 dis-
charge 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 monitored
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 paral-
lel 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.
3549f
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