LTC3545 LINER [Linear Technology], LTC3545 Datasheet - Page 14

• Manufacturer Part Number: LTC3545
• Description: Triple 800mA Synchronous Step-Down Regulator-2.25MHz
• Manufacturer: LINER [Linear Technology]

LTC3545/LTC3545-1
1. The quiescent current is due to two components: the
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 PV
of PV
proportional to frequency. Both the DC bias and gate charge
losses are proportional to PV
be more pronounced at higher supply voltages.
2. I
internal switches, R
tinuous 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
The R
be obtained from the Typical Performance Characteristics
curves. Thus, to obtain I
R
output current.
Other losses when in switching operation, including C
and C
generally account for less than 2% total additional loss.
Thermal Considerations
The LTC3545/LTC3545-1 requires the package backplane
metal to be well soldered to the PC board. This gives the
QFN package exceptional thermal properties, making
it diffi cult in normal operation to exceed the maximum
junction temperature of the part. In most applications the
LTC3545/LTC3545-1 do not dissipate much heat due to
their high effi ciency. In applications where the LTC3545/
LTC3545-1 are running at high ambient temperature
with low supply voltage and high duty cycles, such as in
dropout, the heat dissipated may exceed the maximum
14
APPLICATIONS INFORMATION
L
R
2
and multiply the result by the square of the average
SW
R losses are calculated from the resistances of the
IN
OUT
DS(ON)
that is typically larger than the DC bias current and
= (R
IN
ESR dissipative losses and inductor core losses,
to ground. The resulting dQ/dt is the current out
DS(ON)
DS(ON)TOP
for both the top and bottom MOSFETs can
and the duty cycle (DC) as follows:
SW
)(DC) + (R
, and external inductor R
2
R losses, simply add R
IN
and thus their effects will
DS(ON)BOT
)(1 – DC)
L
. In con-
SW
to
IN
junction temperature of the part if it is not well thermally
grounded. If the junction temperature reaches approxi-
mately 150°C, the power switches will be turned off and
the SW nodes will become high impedance.
To prevent the LTC3545/LTC3545-1 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 one channel of the LTC3545/
LTC3545-1 in dropout at an input voltage of 2.5V, a load
current of 800mA, and an ambient temperature of 85°C.
From the typical performance graph of switch resistance,
the R
timated as 0.42Ω. Therefore, power dissipated by the
channel is:
The θ
temperature rise due to this power dissipation is:
And a junction temperature of:
which is below the maximum junction temperature of
125°C. This would not be the case if all three channels
were operating at 800mA in dropout. Then T
limiting the allowed ambient temperature in this scenario
to less than 70°C.
T
T
P
T
T
R
J
R
J
D
= T
DS(ON)
JA
= 85°C + 18°C = 103°C
= P
= θ
= I
A
D
for the 3mm × 3mm QFN package is 68°C/W. The
A
LOAD
JA
D
is the ambient temperature.
is the power dissipated by the regulator and θ
+ T
• θ
• P
of the P-channel switch at 85°C can be es-
2
R
JA
D
• R
= 18°C
DS(ON)
= 0.27W
J
, is given by:
R
= 55°C,
35451fb
JA