Datasheet

LTC3411
11
sn3411 3411fs
Setting the Output Voltage
The LTC3411 develops a 0.8V reference voltage between
the feedback pin, V
FB
, and the signal ground as shown in
Figure 5. The output voltage is set by a resistive divider
according to the following formula:
VV
R
R
OUT
≈+






08 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 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.
Shutdown and Soft-Start
The SHDN/R
T
pin is a dual purpose pin that sets the
oscillator frequency and provides a means to shut down
the LTC3411. This pin can be interfaced with control logic
in several ways, as shown in Figure 3(a) and Figure 3(b).
The I
TH
pin is primarily for loop compensation, but it can
also be used to increase the soft-start time. Soft start
reduces surge currents from V
IN
by gradually increasing
the internal peak inductor current. Power supply sequenc-
ing can also be accomplished using this pin. The LTC3411
has an internal digital soft-start which steps up a clamp on
I
TH
over 1024 clock cycles, as can be seen in Figure 4.
The soft-start time can be increased by ramping the
voltage on I
TH
during start-up as shown in Figure 3(c). As
the voltage on I
TH
ramps through its operating range the
internal peak current limit is also ramped at a proportional
linear rate.
Mode Selection and Frequency Synchronization
The SYNC/MODE 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 pro-
vides the lowest output voltage and current ripple at the
cost of low current efficiency. Applying a voltage within 1V
of the supplies, results in forced continuous mode, which
creates a fixed output ripple and is capable of sinking some
current (about 1/2∆I
L
). Since the switching noise is con-
stant in this mode, it is also the easiest to filter out. In many
cases, the output voltage can be simply connected to the
SYNC/MODE pin, giving the forced continuous mode,
except at startup.
APPLICATIO S I FOR ATIO
WUU
U
Figure 3. SHDN/R
T
Pin Interfacing and External Soft-Start
3411 F03a
RUN
R
T
SHDN/R
T
3411 F03b
RUN
R
T
SHDN/R
T
1M
SV
IN
3411 F03c
RUN OR V
IN
I
TH
C1 C
C
D1
R
C
R1
(3c)
(3b)(3a)
Figure 4. Digital Soft-Start
V
IN
2V/DIV
V
OUT
2V/DIV
I
L
500mA/DIV
V
IN
= 3.3V 200µs/DIV 3411 F04.eps
V
OUT
= 2.5V
R
L
= 1.4Ω