Datasheet

LTC3561
10
3561f
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
problem, if the switch connecting the load has low resis-
tance 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 incor-
porates 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
APPLICATIO S I FOR ATIO
WUU
U
PV
IN
LTC3561
SW
SV
IN
V
FB
I
TH
SHDN/R
T
L1
D1
OPTIONAL
PGND
V
IN
2.63V
TO 5.5V
SGND PGND
C
F
R
T
R
C
R1
R2
3561 F05
C
C
C
ITH
C5
V
OUT
C
IN
+
+
C6
PGND
PGND
PGND PGND
C
OUT
R6
C8
Figure 4. LTC3561 General Schematic
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 LTC3561 circuits: 1) LTC3561 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 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
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) I
2
R 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