LTC3603 Linear Dimensions Semiconductor, LTC3603 Datasheet - Page 13

• Manufacturer Part Number: LTC3603
• Description: Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Dimensions Semiconductor

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Effi ciency Considerations
The effi ciency 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 effi ciency and which change would produce
the most improvement. Effi ciency can be expressed as:
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
APPLICATIONS INFORMATION
Effi ciency = 100% – (L1 + L2 + L3 + ...)
Figure 5a. Using the TRACK/SS Pin to Track V
IN
operating current and I
V
(5b) Ratiometric Tracking
X
(5c) Coincident Tracking
R
R
TB
TA
TRACK/SS
LTC3603
TIME
TIME
3603 F05a
V
FB
2
R losses.
V
OUT
3603 F05b,c
R
R
B
A
V
V
V
V
X
OUT
X
OUT
X
The V
at very low load currents whereas the I
the effi ciency loss at medium to high load currents.
1. The V
2. I
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
supply. Each time the gate is switched from high to
low to high again, a packet of charge dQ moves from
INTV
out of INTV
current. In continuous mode, the gate charge current
can be approximated by I
INTV
tor, the current that is internally drawn from the INTV
supply can be treated as V
of effi ciency considerations.
Transition losses apply only to the internal topside
MOSFET and become more prominent at higher input
voltages. Transition losses can be estimated from:
The R
be obtained from the Typical Performance Characteristics
curves. Thus, to obtain I
R
output current:
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:
Other losses, including C
losses and inductor core losses, generally account for
less than 2% of the total power loss.
2
L
Transition Loss = (1.7) V
R losses are calculated from the resistances of the
R
I
IN
2
and multiply the result by the square of the average
SW
R Loss = I
CC
CC
operating current loss dominates the effi ciency loss
IN
DS(ON)
= (R
operating current comprises three components:
voltage is generated from V
to ground. The resulting dQ/dt is the current
CC
DS(ON)TOP
for both the top and bottom MOSFETs can
O
that is typically larger than the DC bias
2
(R
SW
SW
)(DC) + (R
+ R
2
and external inductor R
GATECHG
R losses, simply add R
IN
L
IN
IN
)
DS(ON)
and C
2
current for the purposes
• I
DS(ON)BOT
O(MAX)
= f(9.5nC). Since the
OUT
IN
and the duty cycle
2
by a linear regula-
R loss dominates
LTC3603
ESR dissipative
• (120pF) • f
)(1 – DC)
13
SW
L
. In
3603f
CC
CC
to