LTC3603
14
3603fc
For more information www.linear.com/LTC3603
applications inForMation
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:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent
-
age of input power.
Although
all
dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses: V
IN
operating current and I
2
R losses.
The V
IN
operating current loss dominates the efficiency loss
at very low load currents whereas the I
2
R loss dominates
the efficiency loss at medium to high load currents.
1. The V
IN
operating current comprises three components:
The DC supply current as given in the electrical char-
acteristics, the internal MOSFET gate charge currents
and the internal topside MOSFET transition losses. The
MOSFET gate charge current results from switching the
gate capacitance of the internal power MOSFET switches.
The gates of these switches are driven from the INTV
CC
supply. Each time the gate is switched from high to
low to high again, a packet of charge, dQ, moves from
INTV
CC
to ground. The resulting dQ/dt is the current
out of INTV
CC
that is typically larger than the DC bias
current. In continuous mode, the gate charge current
can be approximated by I
GATECHG
= f(9.5nC). Since the
INTV
CC
voltage is generated from V
IN
by a linear regula-
tor, the current that is internally drawn from the INTV
CC
supply can be treated as V
IN
current for the purposes
of efficiency considerations.
Transition losses apply only to the internal topside
MOSFET and become more prominent at higher input
voltages. Transition losses can be estimated from:
Transition Loss = (1.7) V
IN
2
• I
O(MAX)
• (120pF) • f
2. I
2
R losses are calculated from the resistances of the
internal switches, R
SW
and external inductor R
L
. In
continuous mode, the average output current flow-
ing 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
DS(ON)
and the duty cycle
(DC) as follows:
R
SW
= (R
DS(ON)TOP
)(DC) + (R
DS(ON)BOT
)(1 – DC)
The R
DS(ON)
for both the top and bottom MOSFETs can
be obtained from the Typical Performance Characteristics
curves. Thus, to obtain I
2
R losses, simply add R
SW
to
R
L
and multiply the result by the square of the average
output current:
I
2
R Loss = I
O
2
(R
SW
+ R
L
)
Other losses, including C
IN
and C
OUT
ESR dissipative
losses and inductor core losses, generally account for
less than 2% of the total power loss.
Thermal Considerations
In most applications, the LTC3603 does not dissipate much
heat due to its high efficiency. But, in applications where the
LTC3603 is running at high ambient temperature 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.
TIME
Figure 5b. Ratiometric Tracking
V
X
V
OUT
OUTPUT VOLTAGE
TIME
3603 F05b,c
Figure 5c. Coincident Tracking
V
X
V
OUT
OUTPUT VOLTAGE
