LTC3603 Datasheet, PDF (14/22 Pages) Linear Technology – 2.5A, 15V Monolithic Synchronous Step-Down Regulator

LTC3603
APPLICATIONS INFORMATION
V_X
V_OUT
TIME
Figure 5b. Ratiometric Tracking
V_X
V_OUT
TIME
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Figure 5c. Coincident Tracking
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 I²R losses.
The V_IN operating current loss dominates the efficiency loss
at very low load currents whereas the I²R 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
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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_IN2 • I_O(MAX) • (120pF) • f
2. I²R 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 I²R losses, simply add R_SW to
R_L and multiply the result by the square of the average
output current:
I²R Loss = I_O2(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.
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