LTC3547
U
W U U
APPLICATIO S I FOR ATIO
Setting the Output Voltage
a load step occurs, V
immediately shifts by an amount
OUT
equal to ΔI
• ESR, where ESR is the effective series
LOAD
The LTC3547 regulates the V
and V
pins to 0.6V
FB1
FB2
resistance of C . ΔI
also begins to charge or dis-
OUT
LOAD
during regulation. Thus, the output voltage is set by a
chargeC generatingafeedbackerrorsignalusedbythe
OUT
resistive divider according to the following formula:
regulator to return V
this recovery time, V
to its steady-state value. During
can be monitored for overshoot
OUT
OUT
R2
R1
⎛
⎞
VOUT = 0.6V 1+
⎜
⎝
⎟
⎠
or ringing that would indicate a stability problem.
(4)
The initial output voltage step may not be within the band-
width of the feedback loop, so the standard second-order
overshoot/DCratiocannotbeusedtodeterminethephase
Keeping the current small (<5µA) in these resistors maxi-
mizes efficiency, but making it too small may allow stray
capacitance to cause noise problems or reduce the phase
margin of the error amp loop.
margin. In addition, feedback capacitors (C and C )
F1
F2
can be added to improve the high frequency response, as
To improve the frequency response of the main control
shown in Figure 1. Capacitor C provides phase lead by
F
loop, a feedback capacitor (C ) may also be used. Great
F
creating a high frequency zero with R2 which improves
care should be taken to route the V line away from noise
FB
the phase margin.
sources, such as the inductor or the SW line.
Theoutputvoltagesettlingbehaviorisrelatedtothestability
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.
Fixed output versions of the LTC3547 (e.g. LTC3547-1)
include an internal resistive divider, eliminating the need
for external resistors. The resistor divider is chosen such
that the V input current is 3µA. For these versions the
FB
V pin should be connected directly to V . Table 2 lists
FB
OUT
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
the fixed output voltages available for the LTC35476-1.
Table 2. Fixed Output Voltage Versions
PART NUMBER
LTC3547
V
V
OUT2
OUT1
put in parallel with C , causing a rapid drop in V
.
OUT
OUT
Adjustable
1.8V
Adjustable
1.2V
No regulator can deliver enough current to prevent this
problemiftheswitchconnectingtheloadhaslowresistance
and is driven quickly. The solution is to limit the turn-on
LTC3547-1
Checking Transient Response
™
speed of the load switch driver. A Hot Swap controller
is designed specifically for this purpose and usually in-
corporates current limiting, short-circuit protection, and
soft-starting.
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
Hot Swap is a trademark of Linear Technology Corporation.
3547fa
10
Linear Systems [ Linear Systems ]
