Datasheet

LTC3833
25
3833f
APPLICATIONS INFORMATION
The gain of the loop increases with R
ITH
and the bandwidth
of the loop increases with decreasing C
ITH1
. If R
ITH
is
increased by the same factor that C
ITH1
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
FF
, can
be added to improve the high frequency response, as shown
in Figure 1. Capacitor C
FF
provides phase lead by creating
a high frequency zero with R
FB2
which improves the phase
margin. The output voltage settling behavior is related to
the stability of the closed-loop system and will demonstrate
overall performance of the step-down regulator.
In some applications, a more severe transient can be caused
by switching in loads with large (>10μF) input capacitors.
If the switch connecting the load has low resistance and
is driven quickly, then 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. 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. 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. Although all dissipative elements in
the circuit produce losses, four main sources account for
most of the losses:
1. I
2
R losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause
the efficiency to drop at high output currents. In
continuous mode the average output current flows
though the inductor L, but is chopped between the
top and bottom MOSFETs. If the two MOSFETs have
approximately the same R
DS(ON)
, then the resistance
of one MOSFET can simply by summed with the re-
sistances of L and the board traces to obtain the DC
I
2
R loss. For example, if R
DS(ON)
= 0.01Ω and R
L
=
0.005Ω, the loss will range from 15mW to 1.5W as
the output current varies from 1A to 10A.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the
input voltage, load current, driver strength and MOSFET
capacitance, among other factors. The loss is significant
at input voltages above 20V.
3. INTV
CC
current. This 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 INTV
CC
to ground. The resulting dQ/dt is a current
out of INTV
CC
that is typically much larger than the
controller I
Q
current. In continuous mode, I
GATECHG
=
f • (Q
g(TOP)
+ Q
g(BOT)
), where Q
g(TOP)
and Q
g(BOT)
are the
gate charges of the topside and bottom side MOSFETs,
respectively.
Supplying INTV
CC
power through EXTV
CC
could save
several points of efficiency, especially for high V
IN
appli-
cations. Connecting EXTV
CC
to an output-derived source
will scale the V
IN
current required for the driver and
controller circuits by a factor of Duty Cycle/Efficiency.
For example, in a 20V to 5V application, 10mA of INTV
CC
current results in approximately 2.5mA of V
IN
current.
This reduces the mid-current loss from 10% or more
(if the driver was powered directly from V
IN
) to only a
few percent.
4. C
IN
loss. The input capacitor has the difficult job of
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
2
R loss
and sufficient capacitance to prevent the RMS current
from causing additional upstream losses in cabling,
fuses or batteries.
Other losses, which include the C
OUT
ESR loss, bottom
MOSFET reverse-recovery loss and inductor core loss
generally account for less than 2% additional loss.