ltc3405a-1.375 Linear Technology Corporation, ltc3405a-1.375 Datasheet - Page 9

• Manufacturer Part Number: ltc3405a-1.375
• Description: 1.375v, 1.5mhz, 300ma Synchronous Step-down Regulators In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
often useful to analyze individual losses to determine what
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC3405A-1.375 circuits: V
and I
the efficiency loss at very low load currents whereas the
I
load currents. In a typical efficiency plot, the efficiency
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
2
R loss dominates the efficiency loss at medium to high
Efficiency = 100% – (L1 + L2 + L3 + ...)
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
2
R losses. The V
IN
0.0001
0.001
0.01
quiescent current is due to two components:
0.1
1
0.1
Figure 2. Power Lost vs Load Current
V
IN
= 3.6V
U
1
IN
LOAD CURRENT (mA)
quiescent current loss dominates
U
10
IN
IN
that is typically larger than
to ground. The resulting
W
100
IN
3405A1375 F02
quiescent current
1000
U
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 LTC3405A-1.375 does not
dissipate much heat due to its high efficiency. But, in
applications where they run at high ambient temperature
with low supply voltage, 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 keep the LTC3405A-1.375 from exceeding the maxi-
mum 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:
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.
T
2
R
L
R losses are calculated from the resistances of the
R
T
and multiply the result by the square of the average
= (P
SW
+ Q
DS(ON)
D
= (R
B
)(θ
) where Q
JA
DS(ON)TOP
for both the top and bottom MOSFETs can
)
T
and Q
SW
)(DC) + (R
, and external inductor R
IN
LTC3405A-1.375
2
R losses, simply add R
B
and C
are the gate charges of the
DS(ON)
DS(ON)BOT
OUT
and the duty cycle
ESR dissipative
IN
)(1 – DC)
GATECHG
and thus
3405a1375f
SW
L
. In
9
to
=