LTC3409 LINER [Linear Technology], LTC3409 Datasheet - Page 12

• Manufacturer Part Number: LTC3409
• Description: 600mA Low Vin Buck Regulator in 3mm x 3mm DFN
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3409
1. The V
2. I
12
DC bias current as given in the Electrical Characteristics
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
out of V
rent. In continuous mode, I
Q
bottom switches. Both the DC bias and gate charge losses
are proportional to V
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 Characteris-
tics. Thus, to obtain I
and multiply the result by the square of the average
output current.
2
T
R losses are calculated from the resistances of the
R
and Q
SW
0.0001
IN
DS(ON)
0.001
IN
0.01
quiescent current is due to two components: the
0.1
= (R
IN
1
B
to ground. The resulting dQ/dt is the current
0.1
that is typically larger than the DC bias cur-
are the gate charges of the internal top and
2.5V
4.2V
3.6V
DS(ON)TOP
for both the top and bottom MOSFETs can
IN
IN
IN
BURST
PULSE SKIP
2.5V
U
1
4.2V
IN
LOAD CURRENT (mA)
IN
SW
IN
3.6V
2
)(DC) + (R
and thus their effects will be more
Figure 2
R losses, simply add R
U
, and external inductor R
IN
10
GATECHG
DS(ON)
100
W
DS(ON)BOT
and the duty cycle
= (Q
3409 F02
1000
T
+ Q
)(1 – DC)
U
B
SW
) where
to R
L
. In
L
Other losses including C
losses and inductor core losses generally account for less
than 2% total additional loss.
Thermal Considerations
In most applications the LTC3409 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3409 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 maximum
junction temperature of the part. If the junction tempera-
ture reaches approximately 150°C, both power switches
will be turned off and the SW node will become high
impedance.
To avoid the LTC3409 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 LTC3409 in dropout at an
input voltage of 1.6V, a load current of 600mA and an
ambient temperature of 75°C. From the typical perfor-
mance graph of switch resistance, the R
P-channel switch at 75°C is approximately 0.48Ω. There-
fore, power dissipated by the part is:
For the DD8 package, the θ
temperature of the regulator is:
which is well below the maximum junction temperature of
125°C.
T
T
P
T
R
J
J
D
= T
= 75°C + (0.1728)(43) = 82.4°C
= (P
= I
D
A
LOAD
A
is the power dissipated by the regulator and θ
is the ambient temperature.
D
+ T
)(θ
2
R
JA
• R
)
DS(ON)
= 172.8mW
JA
IN
J
is 43°C/W. Thus, the junction
, is given by:
and C
OUT
ESR dissipative
DS(ON)
of the
3409f
JA