LTC3404 LINER [Linear Technology], LTC3404 Datasheet - Page 12

• Manufacturer Part Number: LTC3404
• Description: 1.4MHz High Efficiency Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3404
Thermal Considerations
In most applications the LTC3404 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3404 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 175 C, both power
switches will be turned off and the SW node will become
high impedance.
To avoid the LTC3404 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 LTC3404 in dropout at an
input voltage of 3V, a load current of 500mA, and an
ambient temperature of 70 C. From the typical perfor-
mance graph of switch resistance, the R
12
T
D
T
A
R
J
is the power dissipated by the regulator and q
is the ambient temperature.
= T
= (P
A
D
+ T
)(
R
U
JA
)
U
J
, is given by:
OPTIONAL
R
C
R2
W
C
C
C2
C1
R1
DS(ON)
Figure 7. LTC3404 Layout Diagram
1
2
3
4
U
C
RUN
I
V
GND
TH
IN
FB
+
LTC3404
of the
SYNC/MODE
PLL LPF
JA
V
SW
IN
8
7
6
5
P-channel switch at 70 C is approximately 0.7 . There-
fore, power dissipated by the part is:
For the MSOP package, the
junction temperature of the regulator is:
which is 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
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 stabil-
ity problem. The internal compensation provides adequate
compensation for most applications. But if additional
compensation is required, the I
external compensation using R
Figure 7. (The 47pF capacitor, C
noise decoupling.)
P
T
J
D
L1
= 70 C + (0.175)(150) = 96 C
= I
LOAD
BOLD LINES INDICATE
HIGH CURRENT PATHS
LOAD
OUT
+
2
• R
, which generates a feedback error signal.
OUT
• ESR), where ESR is the effective series
C
OUT
DS(ON)
.
OUT
V
OUT
+
–
I
LOAD
immediately shifts by an amount
= 0.175W
3404 F07
V
+
–
IN
also begins to charge or
JA
C2
TH
is 150 C/ W. Thus, the
, is typically needed for
C
, C
pin can be used for
C1
OUT
OUT
as shown in
can be moni-
to its steady-
DS(ON)
).