ltc3405es6 Linear Technology Corporation, ltc3405es6 Datasheet - Page 11

• Manufacturer Part Number: ltc3405es6
• Description: 1.5mhz, 300ma Synchronous Step-down Regulator In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Other losses including C
losses and inductor core losses generally account for less
than 2% total additional loss.
Thermal Considerations
In most applications the LTC3405 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3405 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 LTC3405 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:
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 LTC3405 in dropout at an
input voltage of 2.7V, a load current of 300mA and an
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
T
R
J
L
R
= T
and multiply the result by the square of the average
= (P
SW
D
A
DS(ON)
A
is the power dissipated by the regulator and θ
is the ambient temperature.
D
+ T
= (R
)(θ
R
JA
DS(ON)TOP
for both the top and bottom MOSFETs can
)
U
)(DC) + (R
U
IN
2
J
R losses, simply add R
, is given by:
and C
DS(ON)
W
DS(ON)BOT
OUT
and the duty cycle
ESR dissipative
)(1 – DC)
U
SW
to
JA
ambient temperature of 70°C. From the typical perfor-
mance graph of switch resistance, the R
P-channel switch at 70°C is approximately 0.94Ω. There-
fore, power dissipated by the part is:
For the SOT-23 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
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 + (0.0846)(250) = 91.15°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)
. ∆I
OUT
LOAD
immediately shifts by an amount
= 84.6mW
also begins to charge or
JA
is 250°C/ W. Thus, the
OUT
. No regulator can
OUT
OUT
LTC3405
DS(ON)
can be moni-
to its steady-
DS(ON)
11
of the
LOAD
3405fa
).
).