LTC3406B-2ES5#TRPBF Linear Technology, LTC3406B-2ES5#TRPBF Datasheet - Page 10

• Manufacturer Part Number: LTC3406B-2ES5#TRPBF
• Description: IC SYNC BUCK REG 600MA TSOT23-5
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3406B-2ES5#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 5.5 V
• Current - Output: 600mA
• Frequency - Switching: 2.25MHz
• Voltage - Input: 2.5 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: TSOT-23-5, TSOT-5, TSOP-5
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
LTC3406B-2
1. The V
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 LTC3406B-2 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3406B-2 is running at high ambient tem-
perature with low supply voltage and high duty cycles,
such as in dropout, 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.
10
the DC bias current as given in the electrical character-
istics and the internal main switch and synchronous
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
internal power MOSFET switches. Each time the gate is
switched from high to low to high again, a packet of
charge, dQ, moves from V
dQ/dt is the current out of V
the DC bias current. In continuous mode, I
f(Q
internal top and bottom switches. Both the DC bias and
gate charge losses are proportional to V
their effects will be more pronounced at higher supply
voltages.
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.
2
L
R losses are calculated from the resistances of the
R
T
and multiply the result by the square of the average
SW
+ Q
IN
DS(ON)
= (R
B
quiescent current is due to two components:
) where Q
DS(ON)TOP
for both the top and bottom MOSFETs can
U
T
and Q
SW
)(DC) + (R
U
, and external inductor R
IN
2
R losses, simply add R
B
IN
and C
IN
are the gate charges of the
DS(ON)
that is typically larger than
to ground. The resulting
W
DS(ON)BOT
OUT
and the duty cycle
ESR dissipative
)(1 – DC)
IN
U
GATECHG
and thus
SW
L
. In
to
=
To avoid the LTC3406B-2 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 LTC3406B-2 in dropout at an
input voltage of 2.7V, a load current of 600mA and an
ambient temperature of 70°C. From the typical perfor-
mance graph of switch resistance, the R
P-channel switch at 70°C is approximately 0.52Ω. There-
fore, power dissipated by the part is:
For the SOT-23 package, the θ
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 stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
T
T
P
T
R
J
J
D
= T
= 70°C + (0.1872)(250) = 116.8°C
= (P
= I
D
A
A
LOAD
is the power dissipated by the regulator and θ
is the ambient temperature.
D
+ T
)(θ
LOAD
OUT
2
R
JA
• 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
= 187.2mW
J
, is given by:
also begins to charge or
JA
is 250°C/ W. Thus, the
OUT
OUT
DS(ON)
can be moni-
to its steady-
sn3406b2 3406b2fs
DS(ON)
of the
JA
).