LTC3564 LINER [Linear Technology], LTC3564 Datasheet - Page 12

• Manufacturer Part Number: LTC3564
• Description: 2.25MHz, 1.25A Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

LTC3564
APPLICATIO S I FOR ATIO
Other losses including C
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:
where P
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, T
where T
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-
12
The R
be obtained from the Typical Performance Characteris-
tics curves. Thus, to obtain I
to R
average output current.
T
T
R
J
= T
= (P
L
D
A
DS(ON)
A
and multiply the result by the square of the
is the power dissipated by the regulator and θ
is the ambient temperature.
D
+ T
)(θ
R
JA
for both the top and bottom MOSFETs can
)
U
U
IN
J
, is given by:
and C
2
R losses, simply add R
W
OUT
ESR dissipative
U
SW
JA
mance graph of switch resistance, the R
P-channel switch at 70°C is approximately ~0.2Ω. There-
fore, power dissipated by the part is:
For the SOT-23 package, the θ
junction temperature of the regulator is:
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
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
equal to (ΔI
resistance of C
discharge C
The regulator loop then acts to return V
state value. During this recovery time V
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
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
Thus, a 10μF capacitor charging to 3.3V would require a
250μs rise time, limiting the charging current to about
130mA.
P
T
J
D
= 70°C + (0.288)(215) = 131.9°C
= I
OUT
LOAD
, causing a rapid drop in V
LOAD
OUT
2
• R
, which generates a feedback error signal.
OUT
• ESR), where ESR is the effective series
DS(ON)
. ΔI
OUT
LOAD
immediately shifts by an amount
= 288mW
also begins to charge or
JA
is 215°C/ W. Thus, the
OUT
. No regulator can
OUT
OUT
DS(ON)
can be moni-
to its steady-
DS(ON)
of the
LOAD
3564f
).
).