ltc3406ab Linear Technology Corporation, ltc3406ab Datasheet - Page 10

• Manufacturer Part Number: ltc3406ab
• Description: 1.5mhz, 600ma Synchronous Step-down Regulator In Thinsot
• Manufacturer: Linear Technology Corporation

LTC3406AB
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of
the losses in LTC3406AB circuits: V
and I
the effi ciency loss at very low load currents whereas the
I
load currents. In a typical effi ciency plot, the effi ciency
curve at very low load currents can be misleading since
the actual power lost is of no consequence as illustrated
in Figure 2.
1. The V
APPLICATIONS INFORMATION
10
2
R loss dominates the effi ciency loss at medium to high
the DC bias current as given in the electrical charac-
teristics 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
than the DC bias current. In continuous mode, I
= f(Q
the 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.
2
R losses. The V
T
IN
+ Q
quiescent current is due to two components:
0.0001
Figure 2. Power Lost vs Load Current
0.001
B
0.01
) where Q
0.1
1
0.1
V
IN =
IN
3.6V
1.0
quiescent current loss dominates
OUTPUT CURRENT (mA)
T
and Q
IN
10.0
B
IN
to ground. The resulting
are the gate charges of
that is typically larger
IN
100.0
V
V
V
quiescent current
OUT =
OUT =
OUT =
3406B F08
1.2V
1.8V
2.5V
1000.0
IN
and thus
GATECHG
2. I
Other losses including C
and inductor core losses generally account for less than
2% total additional loss.
Thermal Considerations
In most applications the LTC3406AB does not dissipate
much heat due to its high effi ciency. But, in applications
where the LTC3406AB 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.
To avoid the LTC3406AB 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 temperature rise is
given by:
where P
is the thermal resistance from the junction of the die to
the ambient temperature.
internal switches, R
continuous mode, the average output current fl owing
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
obtained from the Typical Performance Characteristics
curves. Thus, to obtain I
R
output current.
T
2
R
L
R losses are calculated from the resistances of the
R
= (P
and multiply the result by the square of the average
SW
D
DS(ON)
is the power dissipated by the regulator and θ
D
= (R
)(θ
JA
DS(ON)TOP
for both the top and bottom MOSFETs can be
)
SW
IN
)(DC) + (R
and C
, and external inductor R
2
R losses, simply add R
DS(ON)
OUT
ESR dissipative losses
DS(ON)BOT
and the duty cycle
)(1 – DC)
SW
3406abfa
L
. In
to
JA