LTC3407A Datasheet, PDF (11/16 Pages) Linear Technology – Dual Synchronous 600mA, 1.5MHz Step-Down DC/DC Regulator

LTC3407A
APPLICATIONS INFORMATION
During this recovery time, V_OUT can be monitored for over-
shoot 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 respectively,
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 re-
view of control loop theory, refer to Application 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 paral-
lel 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 cur-
rent 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 optionally 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. 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 calculated by using the
following formula:
t_SS
=
R_SSC_SSIn


V_IN
1
V_IN
1.6


(s)
For approximately a 1ms ramp time, use R_SS = 4.7MΩ
and C_SS = 680pF at V_IN = 3.3V.
Hot Swap is a registered trademark of Linear Technology Corporation.
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, 4 main sources usually account for most of the
losses in LTC3407A 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 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 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 continuous 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 the SW pin is a function of both top and
bottom MOSFET R_DS(ON) and the duty cycle (D) as
follows:
R_SW = (R_DS(ON)TOP)(D) + (R_DS(ON)BOT)(1 – D)
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