LTC3548-1
10
35481fc
APPLICATIONS INFORMATION
can induce ringing at the V
IN
pin. At best, this ringing can
couple to the output and be mistaken as loop instability.
At worst, the ringing at the input can be large enough to
damage the part.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfi ll a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation and
the output capacitor size. Typically, 3-4 cycles are required
to respond to a load step, but only in the fi rst cycle does
the output drop linearly. The output droop, V
DROOP
, is
usually about 2-3 times the linear drop of the fi rst cycle.
Thus, a good place to start is with the output capacitor
size of approximately:
C
OUT
≈2.5
ΔI
OUT
f
O
•V
DROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance
to the supply is very low. A 10µF ceramic capacitor is
usually enough for these conditions.
Checking Transient Response
The regulator loop response can be checked by look-
ing at the load transient response. Switching regulators
take several cycles to respond to a step in load current.
When a load step occurs, V
OUT
immediately shifts by an
amount equal to ∆I
LOAD
• ESR, where ESR is the effective
series resistance of C
OUT
. ∆I
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 band-
width of the feedback loop, so the standard second-order
overshoot/DC ratio cannot be used to determine phase
margin. In addition, a feed-forward capacitor, C
FF
, is added
externally to improve the high frequency response. Capaci-
tor C
FF
provides phase lead by creating a high frequency
zero with R1, 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 re-
view of control loop theory, refer to Application Note 76.
In some applications, a more severe transient can be
caused by switching loads with large (>1µF) load input
capacitors. The discharged load input capacitors are ef-
fectively 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 LTC3548-1 circuits: 1) V
IN
quiescent 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.
Hot Swap is a trademark of Linear Technology Corporation.
