Datasheet

LTC3415
20
3415fa
Switching regulators take several cycles to respond to a
step in load current. When a load step occurs, V
OUT
im-
mediately shifts by an amount equal to ΔI
LOAD
• ESR, where
ESR is the effective series resistance of C
OUT
. ΔI
LOAD
also
begins to charge or discharge C
OUT
generating a feedback
error signal used by the regulator to return V
OUT
to its
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 the 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 feed forward capacitor C
F
can be added to improve the high frequency response, as
shown in Figure 9. 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 (>10μF) input
capacitors. The discharged input capacitors are effec-
tively 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 specifi cally for this purpose and
usually incorporates current limiting, short-circuit protec-
tion, and soft-starting.
Effi ciency Considerations
The percent effi ciency 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 effi ciency and which change would
produce the most improvement. Percent effi ciency can
be expressed as:
% Effi ciency = 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 LTC3415 circuits: 1) LTC3415 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
(<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 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 continuous mode, I
GATECHG
= f (QT + QB),
where QT and QB are the gate charges of the internal
top and bottom MOSFET switches and f is the operat-
ing frequency. The gate charge losses are proportional
to V
IN
and thus their effects will be more pronounced
at higher supply voltages and higher switching
frequencies.
Hot Swap is a trademark of Linear Technology Corporation.
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