LTC3548A Datasheet, PDF (12/20 Pages) Linear Technology – Dual Synchronous 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3548A
APPLICATIONS INFORMATION
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 2¹⁶ clock cycles (about 29ms
in pulse-skipping mode). 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 channel is shut down, the POR output
is pulled low, since one or both of the channels are not
in regulation.
Mode Selection and Frequency Synchronization
The MODE/SYNC pin is a multipurpose pin which provides
mode selection and frequency synchronization. Connect-
ing this pin to V_IN 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
provides the lowest output ripple, at the cost of low cur-
rent efficiency.
The LTC3548A can also be synchronized to another
LTC3548A by the MODE/SYNC pin. During synchroniza-
tion, 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, V_OUT immediately shifts by an amount
equal to ΔI_LOAD • ESR, where ESR is the effective series
resistance of C_OUT. ΔI_LOAD also begins to charge or dis-
charge C_OUT generating a feedback error signal used by the
12
regulator to return V_OUT to its steady-state value. During
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 feedforward 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 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 LTC3548A.
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)
Hot Swap is a registered trademark of Linear Technology Corporation.
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