LTC3565_15 Datasheet, PDF (15/22 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3565
APPLICATIONS INFORMATION
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 parallel
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% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
1
V_IN = 3.6V
f_O = 1MHz
0.1
0.01
0.001
0.0001
0.1
V_OUT = 1.2V
V_OUT = 1.5V
V_OUT^VO=U1T.2=V1-.81.V8V
1
10 100 1000 10000
LOAD CURRENT (mA)
3565 F05
Figure 3. Power Loss vs Load Currrent
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of
the losses in LTC3565 circuits: 1) LTC3565 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, 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 look-
ing 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 + R_L)
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 frequency. Other
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