Datasheet

LTC3411A
15
3411afc
For more information www.linear.com/LTC3411A
steady-state value. During this recovery time, V
OUT
can
be monitored for overshoot or ringing that would indicate
a stability problem.
The initial output voltage step may not be within the
bandwidth of the feedback loop, so the standard second
order overshoot/DC ratio cannot be used to determine
phase margin. The gain of the loop increases with R and
the bandwidth of the loop increases with decreasing C.
If R is increased by the same factor that C is decreased,
the zero frequency will be kept the same, thereby keeping
the phase the same in the most critical frequency range
of the feedback loop. In addition, a feedforward capacitor
C
F
can be added to improve the high frequency response,
as shown in Figure 5. Capacitor C
F
provides phase lead by
creating a high frequency zero with R2 which improves
the phase margin.
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 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 cur-
rent 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. Per
cent 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, four main sources usually account for most of the
losses in LTC3411A circuits: 1) V
IN
current, 2) switching
losses, 3) I
2
R 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.
applicaTions inForMaTion
Figure 6. Power Loss vs Load Currrent
LOAD CURRENT (mA)
V
OUT
= 1.2V
V
OUT
= 1.5V
V
OUT
= 1.8V
0.0001
POWER LOSS (W)
0.001
1
0.1 1 10 100 1000 10000
3411A F06
0.01
0.1
V
IN
= 3.6V
f
O
= 1MHz
V
OUT
= 1.2V - 1.8V