LTC3548 Datasheet, PDF (10/16 Pages) Linear Technology – Dual Synchronous, 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3548
APPLICATIO S I FOR ATIO
V_OUT
=
⎛
0.6V_⎝⎜1+
R2^⎞
R1⎠⎟
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 C_F may also be used. Great care should be taken to
route the V_FB 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 above –8.5% of regulation, a timer is started
which releases POR after 2¹⁸ clock cycles (about 117ms).
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. 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. Connecting this pin to ground
selects pulse skipping mode, which provides the lowest
output ripple, at the cost of low current efficiency.
The LTC3548 can also be synchronized to an external
2.25MHz 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.
10
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 ∆I_LOAD • ESR, where ESR is the effective series
resistance of C_OUT. ∆I_LOAD also begins to charge or
discharge C_OUT, generating a feedback error signal used
by the 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 feed-forward capacitor, C_F,
can be added to improve the high frequency response, as
shown in Figure 2. Capacitor C_F 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 loads with large (>1µF) load input
capacitors. The discharged load input capacitors are ef-
fectively put in parallel 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^TM
controller is designed specifically for this purpose and
usually incorporates current limiting, short-circuit protec-
tion, 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
Hot Swap is a trademark of Linear Technology Corporation.
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