LTC3412A Datasheet, PDF (13/20 Pages) Linear Technology – 3A, 4MHz, Monolithic Synchronous Step-Down Regulator

LTC3412A
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:
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 quiescent current and I²R losses.
The V_IN quiescent 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. 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.
1. The V_IN quiescent current is due to two components:
the DC bias current as given in the electrical characteris-
tics 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_IN to ground. The resulting dQ/dt is the current out
of V_IN that is typically larger than the DC bias current. In
continuous mode, I_GATECHG = f(QT + QB) where QT and
QB are the gate charges of the internal top and bottom
switches. Both the DC bias and gate charge losses are
proportional to V_IN; thus, their effects will be more pro-
nounced at higher supply voltages.
2. I²R losses are calculated from the resistances of the
internal switches, R_SW, and external inductor R_L. In con-
tinuous 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_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. To obtain I²R losses, simply add R_SW to R_L and
multiply the result by the square of the average output
current.
Other losses including C_IN and C_OUT ESR dissipative losses
and inductor core losses generally account for less than
2% of the total loss.
Thermal Considerations
In most applications, the LTC3412A does not dissipate
much heat due to its high efficiency.
However, in applications where the LTC3412A 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.
To avoid the LTC3412A from exceeding the maximum junc-
tion 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 temperature rise is
given by:
t_r = (P_D)(θ_JA)
where P_D is the power dissipated by the regulator and θ_JA
is the thermal resistance from the junction of the die to
the ambient temperature. For the 16-lead exposed TSSOP
package, the θ_JA is 38°C/W. For the 16-lead QFN package
the θ_JA is 34°C/W.
The junction temperature, T_J, is given by:
T_J = T_A + t_r
where T_A is the ambient temperature.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (R_DS(ON)).
To maximize the thermal performance of the LTC3412A,
the exposed pad should be soldered to a ground plane.
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current.
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