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LTC3407 Datasheet(PDF) 10 Page - Linear Technology

No. de Pieza. LTC3407
Descripción  Dual Synchronous, 600mA, 1.5MHz Step-Down DC/DC Regulator
Descarga  16 Pages
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Fabricante  LINER [Linear Technology]
Página de inicio  http://www.linear.com
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LTC3407 Datasheet(HTML) 10 Page - Linear Technology

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LTC3407
10
sn3407 3407fs
Hot Swap is registered trademark of Linear Technology Corporation.
VV
R
R
OUT =+
06
1
2
1
.
Keeping the current small (<5µA) in these resistors maxi-
mizes efficiency, but making them too small may allow
stray capacitance to cause noise problems and reduce the
phase margin of the error amp loop.
To improve the frequency response, a feed-forward ca-
pacitor CF may also be used. Great care should be taken to
route the VFB line away from noise sources, such as the
inductor or the SW line.
Power-On Reset
The POR pin is an open-drain output which pulls low when
either regulator is out of regulation. When both output
voltages are within ±8.5% of regulation, a timer is started
which releases POR after 218 clock cycles (about 175ms).
This delay can be significantly longer in Burst Mode
operation with low load currents, since the clock cycles
only occur during a burst and there could be milliseconds
of time between bursts. This can be bypassed by tying the
POR output to the MODE/SYNC input, to force pulse
skipping mode during a reset. In addition, if the output
voltage faults during Burst Mode sleep, POR could have a
slight delay for an undervoltage output condition and may
not respond to an overvoltage output. This can be avoided
by using pulse skipping mode instead. When either chan-
nel is shut down, the POR output is pulled low, since one
or both of the channels are not in regulation.
Mode Selection & Frequency Synchronization
The MODE/SYNC pin is a multipurpose pin which provides
mode selection and frequency synchronization. Connect-
ing this pin to VIN enables Burst Mode operation, which
provides the best low current efficiency at the cost of a
higher output voltage ripple. When this pin is connected to
ground, pulse skipping operation is selected which pro-
vides the lowest output ripple, at the cost of low current
efficiency.
The LTC3407 can also be synchronized to an external
1.5MHz clock signal by the MODE/SYNC pin. During
synchronization, the mode is set to pulse skipping and the
top switch turn-on is synchronized to the rising edge of the
external clock.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to ∆ILOAD • ESR, where ESR is the effective series
resistance of COUT. ∆ILOAD also begins to charge or
discharge COUT generating a feedback error signal used by
the regulator to return VOUT to its steady-state value.
During this recovery time, VOUT can be monitored for
overshoot or ringing that would indicate a stability prob-
lem.
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, CF,
can be added to improve the high frequency response, as
shown in Figure 2. Capacitor CF provides phase lead by
creating a high frequency zero with R2 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 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 COUT, causing a rapid drop in VOUT. 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
APPLICATIO S I FOR ATIO


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