LTC3410-1.875 LINER [Linear Technology], LTC3410-1.875 Datasheet - Page 11

• Manufacturer Part Number: LTC3410-1.875
• Description: 2.25MHz, 300mA Synchronous Step-Down Regulator in SC70
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Thermal Considerations
In most applications the LTC3410-1.875 does not dissi-
pate much heat due to its high efficiency. But, in applica-
tions where the LTC3410-1.875 is running at high ambient
temperature with low supply voltage, 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 prevent the LTC3410-1.875 from exceeding the maxi-
mum 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
θ
the ambient temperature.
The junction temperature, T
where T
As an example, consider the LTC3410-1.875 with an input
voltage of 2.7V, a load current of 300mA and an ambient
temperature of 70°C. From the typical performance
graph of switch resistance, the R
P-channel switch at 70°C is approximately 1.05Ω and
the R
proximately 0.75Ω. The series resistance looking into the
SW pin is:
Therefore, power dissipated by the part is:
JA
T
T
R
P
is the thermal resistance from the junction of the die to
R
J
D
SW
= T
DS(ON)
= (P
= I
= 1.05Ω (0.69) + 0.75Ω (0.31) = 0.96Ω
A
D
A
LOAD
D
is the ambient temperature.
+ T
is the power dissipated by the regulator and
)(θ
of the N-channel synchronous switch is ap-
2
R
JA
• R
)
DS(ON)
U
= 86.4mW
U
J
, is given by:
W
DS(ON)
U
of the
For the SC70 package, the θ
junction temperature of the regulator is:
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
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 (∆I
resistance of C
charge C
regulator loop then acts to return V
value. During this recovery time V
overshoot or ringing that would indicate a stability prob-
lem. 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 dis-
charged 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.
T
J
= 70°C + (0.0864)(250) = 91.6°C
OUT
OUT
, causing a rapid drop in V
LOAD
, which generates a feedback error signal. The
OUT
• ESR), where ESR is the effective series
. ∆I
OUT
LOAD
immediately shifts by an amount
also begins to charge or dis-
LTC3410-1.875
JA
OUT
is 250°C/ W. Thus, the
OUT
OUT
can be monitored for
to its steady-state
. No regulator can
DS(ON)
11
34101875f
LOAD
).
).