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LTC3409A Datasheet(PDF) 13 Page - Linear Technology

Part No. LTC3409A
Description  600mA Low VIN Buck Regulator in 3mm 3mm DFN
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LTC3409A Datasheet(HTML) 13 Page - Linear Technology

 
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LTC3409A
13
3409af
Notethatathighersupplyvoltages,thejunctiontemperature
is lower due to reduced switch resistance (RDS(ON)).
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, VOUT immediately shifts by an amount
equal to (
ΔILOAD • ESR), where ESR is the effective series
resistance of COUT. ΔILOAD also begins to charge or dis-
charge COUT, which generates a feedback error signal. The
regulator loop then acts to return VOUT to its steady state
value. During this recovery time VOUT can be monitored
for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
A second, more severe transient is caused by switching
in loads with large (>1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in paral-
lel with COUT, causing a rapid drop in VOUT. No regulator
can deliver enough current to prevent this problem if the
load switch resistance is low and it is driven quickly. The
only solution is to limit the rise time of the switch drive
so that the load rise time is limited to approximately
(25 • CLOAD). Thus, a 10μF capacitor charging to 3.3V
would require a 250μs rise time, limiting the charging
current to about 130mA.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3409A. These items are also illustrated graphically
in the layout diagram of Figure 3. Check the following in
your layout.
1. Does the capacitor CIN connect to the power VIN
(Pins 3, 4) and GND (Exposed Pad) as close as pos-
sible? This capacitor provides the AC current to the
internal power MOSFETs and their drivers.
2. Are the COUT and L1 closely connected? The (–) plate of
COUT returns current to GND and the (–) plate of CIN.
3. The resistor divider, R1 and R2, must be connected
between the (+) plate of COUT and a ground sense line
terminated near GND (Exposed Pad). The feedback
signals VFB should be routed away from noisy compo-
nents and traces, such as the SW line (Pins 6), and its
trace should be minimized.
4. The SW trace should be kept as small as possible. Keep
sensitive components away from the SW pins. The input
capacitor CIN and the resistors R1 and R2 should be
routed away from the SW traces and the inductors.
5. A ground plane is preferred, but if not available, keep
the signal and power grounds segregated with small
signal components returning to the GND pin at one
point. They should not share the high current path of
CIN or COUT.
6. Flood all unused areas on all layers with copper. Flooding
with copper will reduce the temperature rise of power
components. These copper areas should be connected
to VIN or GND (preferably).
VIN
VIN
LTC3409A
RUN
VFB
SYNC
MODE
SW
L1
C1
CIN
R1
R2
VIN
SGND GND
COUT
3409A F03
VOUT
Figure 3
APPLICATIONS INFORMATION


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