LTC3411 Linear Technology, LTC3411 Datasheet - Page 11

• Manufacturer Part Number: LTC3411
• Description: 1.25A/ 4MHz/ Synchronous Step-Down DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Setting the Output Voltage
The LTC3411 develops a 0.8V reference voltage between
the feedback pin, V
Figure 5. The output voltage is set by a resistive divider
according to the following formula:
Keeping the current small (<5 A) 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
route the V
inductor or the SW line.
Shutdown and Soft-Start
The SHDN/R
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
also be used to increase the soft-start time. Soft start
reduces surge currents from V
V
Figure 3. SHDN/R
OUT
RUN
TH
pin is primarily for loop compensation, but it can
F
may also be used. Great care should be taken to
0 8
FB
(3a)
.
T
line away from noise sources, such as the
V
3411 F03a
SHDN/R
pin is a dual purpose pin that sets the
1
T
FB
R
U
RUN OR V
T
T
Pin Interfacing and External Soft-Start
, and the signal ground as shown in
R
R
2
1
R1
C1
U
IN
D1
(3c)
IN
I
TH
RUN
R
by gradually increasing
C
C
3411 F03c
C
W
SHDN/R
(3b)
R
T
T
SV
3411 F03b
U
IN
1M
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
The soft-start time can be increased by ramping the
voltage on I
the voltage on I
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
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
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.
TH
500mA/DIV
over 1024 clock cycles, as can be seen in Figure 4.
2V/DIV
2V/DIV
V
OUT
V
IN
I
L
V
V
R
TH
IN
OUT
L
= 1.4
= 3.3V
= 2.5V
during start-up as shown in Figure 3(c). As
TH
IN
Figure 4. Digital Soft-Start
ramps through its operating range the
enables Burst Mode operation, which
L
). Since the switching noise is con-
200 s/DIV
LTC3411
3411 F04.eps
sn3411 3411fs
11