LTC3407A-2 Linear Technology, LTC3407A-2 Datasheet - Page 11

• Manufacturer Part Number: LTC3407A-2
• Description: Dual Synchronous 800mA2.25MHz Step-Down DC/DC Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
this recovery time, V
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
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
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
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:
Hot Swap is a registered trademark of Linear Technology Corporation.
t
SS
=
R C In
SS SS
⎛
⎝ ⎜
U
OUT
V
OUT
IN
IN
V
IN
, causing a rapid drop in V
−
enables regulator 1. In order to
can be monitored for overshoot
−
1 6 .
U
1
⎞
⎠ ⎟
( )
s
W
TM
controller is
U
OUT
. No
For approximately a 1ms ramp time, use R
C
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:
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
switching losses, 3) I
1) The V
Electrical Characteristics which excludes MOSFET driver
and control currents. V
loss that increases with V
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
ground. The resulting dQ/dt is a current out of V
typically much larger than the DC bias current. In continu-
ous mode, I
gate charges of the internal top and bottom MOSFET
switches. The gate charge losses are proportional to V
and thus their effects will be more pronounced at higher
supply voltages.
3) I
internal switches, R
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
SS
%Efficiency = 100% - (L1 + L2 + L3 + ...)
2
= 680pF at V
R losses are calculated from the DC resistances of the
IN
current is the DC supply current given in the
GATECHG
IN
= 3.3V.
= f
SW
2
IN
O
R losses, 4) other losses.
(Q
, and external inductor, R
current results in a small (<0.1%)
IN
T
+ Q
, even at no load.
B
), where Q
LTC3407A-2
www.DataSheet4U.com
IN
quiescent current, 2)
SS
T
and Q
= 4.7MΩ and
IN
B
11
are the
that is
IN
L
3407a2f
. In
to
IN