LTC3405A-1.5 LINER [Linear Technology], LTC3405A-1.5 Datasheet - Page 10

• Manufacturer Part Number: LTC3405A-1.5
• Description: 1.5V, 1.8V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3405A-1.5/LTC3405A-1.8
2. I
Other losses including C
losses and inductor core losses generally account for less
than 2% total additional loss.
Thermal Considerations
In most applications, the LTC3405A series parts do not
dissipate much heat due to their high efficiency. But, in
applications where they run 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 keep the LTC3405A series parts 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.
10
internal switches, R
continuous mode, the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. Thus, the series
resistance looking into the SW pin is a function of both
top and bottom MOSFET R
(DC) as follows:
The R
be obtained from the Typical Performance Charateristics
curves. Thus, to obtain I
R
output current.
T
2
R
L
R losses are calculated from the resistances of the
R
and multiply the result by the square of the average
= (P
SW
D
DS(ON)
is the power dissipated by the regulator and
D
= (R
)(
JA
DS(ON)TOP
for both the top and bottom MOSFETs can
)
U
SW
)(DC) + (R
U
, and external inductor R
IN
2
R losses, simply add R
and C
DS(ON)
W
DS(ON)BOT
OUT
and the duty cycle
ESR dissipative
)(1 – DC)
U
SW
L
. In
to
JA
The junction temperature, T
where T
As an example, consider the LTC3405A-1.8 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
at 70 C is approximately 0.94 and the R
N-channel synchronous switch is approximately 0.75 .
The series resistance looking into the SW pin is:
Therefore, power dissipated by the part is:
For the SOT-23 package, the
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
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.
T
R
P
T
J
J
D
SW
= T
= 70 C + (0.0792)(250) = 89.8 C
= I
= 0.95 (0.67) + 0.75 (0.33) = 0.88
A
A
LOAD
is the ambient temperature.
+ T
LOAD
OUT
2
R
• R
, which generates a feedback error signal.
OUT
• ESR), where ESR is the effective series
SW
.
OUT
= 79.2mW
I
LOAD
immediately shifts by an amount
DS(ON)
J
, is given by:
also begins to charge or
JA
of the P-channel switch
is 250 C/ W. Thus, the
OUT
OUT
sn3405a1518 3405a1518fs
DS(ON)
can be moni-
to its steady-
DS(ON)
of the
).