LTC3547B
10
3547bfb
APPLICATIO S I FOR ATIO
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Setting the Output Voltage
The LTC3547B regulates the V
FB1
and V
FB2
pins to 0.6V
during regulation. Thus, the output voltage is set by a
resistive divider according to the following formula:
V
OUT
= 0.6V 1+
R2
R
1
(4)
Keeping the current small (<5μA) in these resistors maxi-
mizes effi ciency, but making it too small may allow stray
capacitance to cause noise problems or reduce the phase
margin of the error amp loop.
To improve the frequency response of the main control
loop, a feedback capacitor (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.
Fixed output versions of the LTC3547B (e.g. LTC3547B-1)
include an internal resistive divider, eliminating the need
for external resistors. The resistor divider is chosen such
that the V
FB
input current is 3μA. For these versions, the
V
FB
pin should be connected directly to V
OUT
. Table 2 lists
the fi xed output voltages available for the LTC3547B.
Table 2. Fixed Output Voltage Versions
Part Number V
OUT1
V
OUT2
LTC3547B Adjustable Adjustable
LTC3547B-1 1.8V 1.2V
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
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 band-
width of the feedback loop, so the standard second-order
overshoot/DC ratio cannot be used to determine the phase
margin. In addition, feedback capacitors (C
F1
and C
F2
)
can be added to improve the high frequency response, as
shown in Figure 1. 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 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 ca-
pacitors. The discharged input capacitors are effectively
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
™
controller
is designed specifi cally for this purpose and usually in-
corporates current limiting, short-circuit protection, and
soft-starting.
Hot Swap is a trademark of Linear Technology Corporation.