LTC3417 Datasheet, PDF (14/20 Pages) Linear Technology – Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator

LTC3417
APPLICATIO S I FOR ATIO
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 R_ITH and
the bandwidth of the loop increases with decreasing C_ITH.
If R_ITH is increased by the same factor that C_ITH is de-
creased, 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, feedforward ca-
pacitors, C1 and C2, can be added to improve the high
frequency response, as shown in Figure 4. Capacitor C1
provides phase lead by creating a high frequency zero with
R1 which improves the phase margin for the 1.4A SW1
channel. Capacitor C2 provides phase lead by creating a
high frequency zero with R3 which improves the phase
margin for the 800mA SW2 channel.
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 (>1µF) 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 prob-
lem, 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^TM controller is
designed specifically for this purpose and usually incorpo-
rates 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
produce the most improvement. Percent efficiency can be
expressed as:
% Efficiency = 100% – (P1+ P2 + P3 +…)
where P1, P2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources account for most of the losses
in LTC3417 circuits: 1) LTC3417 I_S current, 2) switching
losses, 3) I²R losses, 4) other losses.
1) The I_S current is the DC supply current given in the
electrical characteristics which excludes MOSFET driver
and control currents. I_S 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 results
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 moves from V_IN to ground.
The resulting charge over the switching period is a current
out of V_IN that is typically much larger than the DC bias
current. The gate charge losses are proportional to V_IN and
thus their effects will be more pronounced at higher
supply voltages.
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