LTC3411 Datasheet, PDF (13/20 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3411
APPLICATIO S I FOR ATIO
The output voltage settling behavior is related to the
stability of the closed-loop system and will demonstrate
the actual overall supply performance. For a detailed
explanation of optimizing the compensation components,
including a review of control loop theory, refer to Linear
Technology Application Note 76.
Although a buck regulator is capable of providing the full
output current in dropout, it should be noted that as the
input voltage V_IN drops toward V_OUT, the load step capa-
bility does decrease due to the decreasing voltage across
the inductor. Applications that require large load step
capability near dropout should use a different topology
such as SEPIC, Zeta or single inductor, positive buck/
boost.
In some applications, a more severe transient can be
caused by switching in loads with large (>1uF) input
capacitors. The discharged input capacitors are effectively
put in parallel with C_OUT, causing a rapid drop in V_OUT. No
regulator can deliver enough current to prevent this prob-
lem, if the switch connecting the load has low resistance
and is driven quickly. The solution is to limit the turn-on
speed of the load switch driver. A hot swap controller is
designed specifically for this purpose and usually incorpo-
rates current limiting, short-circuit protection, and soft-
starting.
Efficiency Considerations
The percent 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. Percent efficiency can be
expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3411 circuits: 1) LTC3411 V_IN current,
2)␣ switching losses, 3) I²R losses, 4) other losses.
1) The V_IN current is the DC supply current given in the
electrical characteristics which excludes MOSFET driver
and control currents. V_IN current results in a small (<0.1%)
loss that increases with V_IN, even at no load.
2) The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current results
from switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high to
low again, a packet of charge dQ moves from V_IN to
ground. The resulting dQ/dt is a current out of V_IN that is
typically much larger than the DC bias current. In continu-
ous mode, I_GATECHG = f_O(QT + QB), where QT and QB are
the gate charges of the internal top and bottom MOSFET
switches. The gate charge losses are proportional to V_IN
and thus their effects will be more pronounced at higher
supply voltages.
3) I²R Losses are calculated from the DC resistances of the
internal switches, R_SW, and external inductor, RL. In
continuous mode, the average output current flowing
through inductor L but is “chopped” between the internal
top and bottom switches. 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:
I²R losses = I_OUT2(R_SW + RL)
4) Other “hidden” losses such as copper trace and internal
battery resistances can account for additional efficiency
degradations in portable systems. It is very important to
include these “system” level losses in the design of a
system. The internal battery and fuse resistance losses
can be minimized by making sure that C_IN has adequate
charge storage and very low ESR at the switching fre-
quency. Other losses including diode conduction losses
during dead-time and inductor core losses generally ac-
count for less than 2% total additional loss.
sn3411 3411fs
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