LTC3407A-2 Datasheet, PDF (11/16 Pages) Linear Technology – Dual Synchronous 800mA,2.25MHz Step-Down DC/DC Regulator

LTC3407A-2
APPLICATIO S I FOR ATIO
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
bandwidth 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 can be
added to improve the high frequency response, as shown
in Figure 1. Capacitors C1 and C2 provide phase lead by
creating high frequency zeros with R2 and R4 respec-
tively, which improve 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 review of control loop theory, refer to Applica-
tion Note 76.
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.
Soft-Start
The RUN/SS pins provide a means to separately run or
shut down the two regulators. In addition, they can option-
ally be used to externally control the rate at which each
regulator starts up and shuts down. Pulling the RUN/SS1
pin below 1V shuts down regulator 1 on the LTC3407A-2.
Forcing this pin to V_IN enables regulator 1. In order to
control the rate at which each regulator turns on and off,
connect a resistor and capacitor to the RUN/SS pins as
shown in Figure 1. The soft-start duration can be calcu-
lated by using the following formula:
t_SS
=
⎛
R_SSC_SSIn_⎝⎜
V_IN − 1
V_IN − 1.6
⎞
⎠⎟
(s)
Hot Swap is a registered trademark of Linear Technology Corporation.
For approximately a 1ms ramp time, use R_SS = 4.7MΩ and
C_SS = 680pF at V_IN = 3.3V.
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 percentage
of input power.
Although all dissipative elements in the circuit produce
losses, 4 main sources usually account for most of the
losses in LTC3407A-2 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.
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 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 continu-
ous mode, I_GATECHG = f_O(Q_T + Q_B), where Q_T and Q_B 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) I²R 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 flows through
inductor L, but is “chopped” between the internal top and
bottom switches. Thus, the series resistance looking into
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