LTC3546 Datasheet, PDF (21/28 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Confi gurable Step-Down DC/DC Regulator

LTC3546
APPLICATIONS INFORMATION
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 capability
does decrease due to the decreasing voltage across the
inductor. Applications that require large load step capabil-
ity 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 (>1μF) input capacitors.
The discharged input capacitors are effectively put in paral-
lel with C_OUT, causing a rapid drop in V_OUT. No regulator
can deliver enough current to prevent this problem, 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 incorporates 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% – (P1 + P2 + P3+…)
where P1, P2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of
the losses in LTC3546 circuits: 1) LTC3546 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 re-
sults 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 moves from
V_IN to ground. The resulting charge over the switching
period is a current out of V_IN that is typically much larger
than the DC bias current. 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 the external inductor,
R_L. In continuous mode, the average output current
flowing through inductor L 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 Character-
istics curves. Thus, to obtain I²R losses:
I²R losses = I_OUT2(R_SW + R_L)
Where R_L is the resistance of the inductor.
4. Other hidden losses such as copper trace and inter-
nal battery resistances can account for additional ef-
ficiency 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 frequency. Other losses including
diode conduction losses during dead-time and inductor
core losses generally account for less than 2% total
additional loss.
Thermal Considerations
The LTC3546 requires the backplane metal (Pin 29) to be
well soldered to the PC board. This gives the UFD pack-
age exceptional thermal properties, compared to similar
packages of this size, making it difficult in normal opera-
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