Datasheet

LTC3783
20
3783fb
OPERATION
5. Check the stress on the power MOSFET by measuring its
drain-to-source voltage directly across the device terminals
(reference the ground of a single scope probe directly to the
source pad on the PC board). Beware of inductive ringing
which can exceed the maximum specified voltage rating of
the MOSFET. If this ringing cannot be avoided and exceeds the
maximum rating of the device, either choose a higher voltage
device or specify an avalanche-rated power MOSFET.
6. Place the small-signal components away from high
frequency switching nodes. All of the small-signal com-
ponents should be placed on one side of the IC and all
of the power components should be placed on the other.
This also allows the use of a pseudo-Kelvin connection for
the signal ground, where high di/dt gate driver currents
flow out of the IC ground pad in one direction (to bottom
plate of the INTV
CC
decoupling capacitor) and small-signal
currents flow in the other direction.
7. If a sense resistor is used in the source of the power
MOSFET, minimize the capacitance between the SENSE
pin trace and any high frequency switching nodes. The
LTC3783 contains an internal leading-edge blanking time
of approximately 160ns, which should be adequate for
most applications.
8. For optimum load regulation and true remote sensing,
the top of the output resistor should connect indepen-
dently to the top of the output capacitor (Kelvin connec-
tion), staying away from any high dV/dt traces. Place the
divider resistors near the LTC3783 in order to keep the
high impedance FBN node short.
9. For applications with multiple switching power convert-
ers connected to the same input supply, make sure that
the input filter capacitor for the LTC3783 is not shared
with any other converters. AC input current from another
converter could cause substantial input voltage ripple, and
this could interfere with the operation of the LTC3783. A
few inches of PC trace or wire (L ~ 100nH) between the
C
IN
of the LTC3783 and the actual source V
IN
should be
sufficient to prevent current-sharing problems.
Returning the Load to V
IN
: A Single Inductor
Buck-Boost Application
As shown in Figure 11, due to its available high side current
sensing mode, the LTC3783 is also well-suited to a boost
converter in which the load current is returned to V
IN
,
hence providing a load voltage (V
OUT
– V
IN
) which can be
greater or less than the input voltage V
IN
. This configuration
allows for complete overlap of input and output voltages,
with the disadvantages that only the load current, and not
the load voltage, can be tightly regulated. The switch must
be rated for a V
DS(MAX)
equal to V
IN
+ V
LOAD
.
The design of this circuit resembles that of the boost
converter above, and the procedure is much the same,
except V
OUT
is now (V
IN
+ V
LOAD
), and the duty cycles
and voltages must be adjusted accordingly.
LTC3783
RUN
PWMIN
I
TH
SS
V
REF
FBP
FBN
FREQ
SYNC
V
IN
OV/FB
PWMOUT
I
LIM
GATE
SENSE
INTV
CC
GND
V
IN
9V TO 26V
R
L
0.28Ω
V
OUT
LED STRING 1-4 EA
LUMILEDS LHXL-BW02
EACH LED IS 3V TO 4.2V
AT 350mA
10µF, 50V
C5750X7R1H106M
CERAMIC
0V TO
1.23V
10µF, 50V
×2
UMK432C106MM
10µH
SUMIDA
CDRH8D28-100
GND
3783 F11
1M
20k
PMEG6010
FAIRCHILD
FDN5630
1k
40.2k
4.7µF
100k
PWM
5V AT 0Hz TO 10Hz
4.7µF
0.05Ω
1µF
Figure 11. Single Inductor Buck-Boost Application with Analog Dimming and Low Frequency PWM Dimming