LTC3411_15 Datasheet, PDF (17/24 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3411
APPLICATIONS INFORMATION
The output voltage can now be programmed by choosing
the values of R1 and R2. To maintain high efficiency, the
current in these resistors should be kept small. Choosing
2μA with the 0.8V feedback voltage makes R1~400k. A
close standard 1% resistor is 412k and R2 is then 887k.
The compensation should be optimized for these compo-
nents by examining the load step response but a good place
to start for the LTC3411 is with a 13kΩ and 1000pF filter.
The output capacitor may need to be increased depending
on the actual undershoot during a load step.
The PGOOD pin is a common drain output and requires
a pull-up resistor. A 100k resistor is used for adequate
speed.
Figure 1 shows the complete schematic for this design
example.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3411. These items are also illustrated graphically
in the layout diagram of Figure 6. Check the following in
your layout:
1. Does the capacitor C_IN connect to the power V_IN (Pin 6)
and power GND (Pin 5) as close as possible? This capacitor
provides the AC current to the internal power MOSFETs
and their drivers.
2. Are the C_OUT and L1 closely connected? The (–) plate of
C_OUT returns current to PGND and the (–) plate of C_IN.
3. The resistor divider, R1 and R2, must be connected
between the (+) plate of C_OUT and a ground line terminated
near SGND (Pin 3). The feedback signal V_FB should be
routed away from noisy components and traces, such as
the SW line (Pin 4), and its trace should be minimized.
4. Keep sensitive components away from the SW pin. The
input capacitor C_IN, the compensation capacitor C_C and
C_ITH and all the resistors R1, R2, R_T, and R_C should be
routed away from the SW trace and the inductor L1.
5. A ground plane is preferred, but if not available, keep
the signal and power grounds segregated with small signal
components returning to the SGND pin at one point which
is then connected to the PGND pin.
6. Flood all unused areas on all layers with copper. Flood-
ing with copper will reduce the temperature rise of power
components. These copper areas should be connected to
one of the input supplies: PV_IN, PGND, SV_IN or SGND.
C_IN
V_IN
R5
PGOOD
C4
R2 R1 R3
PV_IN
PGND
L1
SV_IN
SW
LTC3411 SGND
PGOOD
V_IN
V_FB
I_TH
SYNC/MODE
SHDN/R_T ^{PS BM}
R_T
C3
BOLD LINES INDICATE HIGH CURRENT PATHS
C_OUT
V_OUT
3411 F06
Figure 6. LTC3411 Layout Diagram (See Board Layout Checklist)
3411fb
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