LTC3548-1 Datasheet, PDF (10/16 Pages) Linear Technology – Dual Synchronous, Fixed Output 2.25MHz Step-Down DC/DC Regulator

LTC3548-1
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 fulfill 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 first cycle does
the output drop linearly. The output droop, V_DROOP, is
usually about 2-3 times the linear drop of the first cycle.
Thus, a good place to start is with the output capacitor
size of approximately:
C_OUT
≈
2.5
f_O
ΔI_OUT
• 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 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 band-
width of the feedback loop, so the standard second-order
overshoot/DC ratio cannot be used to determine phase
10
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 specifically for this purpose and
usually incorporates current limiting, short-circuit protec-
tion, 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 usually account for most of
the losses in LTC3548-1 circuits: 1)V_IN quiescent current,
2) switching losses, 3) I²R 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.
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