LTC3783 Datasheet, PDF (20/24 Pages) Linear Technology – PWM LED Driver and Boost, Flyback and SEPIC Controller

LTC3783
U
OPERATIO
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 configu-
ration allows for complete overlap of input and output
voltages, with the disadvantages that only the load cur-
rent, 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.
Similar to the boost converter, which can be dimmed via
the digital PWMIN input or the analog FBP pin, the buck-
boost can be dimmed via the PWMIN pin or the analog
I_LIM pin, which adjusts the offset voltage to which the
loop will drive (V_FBP – V_FBN). In the case of the buck-
boost, however, the dimming ratio cannot be as high as
in the boost converter, since there is no load switch to
preserve the V_OUT level while PWMIN is low.
PWM
5V AT 0Hz TO 10Hz
1µF
4.7µF
1M
100k
20k
LTC3783
RUN
V_IN
PWMIN OV/FB
I_TH PWMOUT
SS
I_LIM
V_REF GATE
FBP SENSE
FBN INTV_CC
FREQ GND
SYNC
10µF, 50V
10µH
×2
SUMIDA
UMK432C106MM CDRH8D28-100
PMEG6010
0V TO
1.23V
40.2k
FAIRCHILD
FDN5630
4.7µF
0.05Ω
1k
V_IN
R_L ^{9V TO 26V}
0.28Ω
LED STRING 1-4 EA
LUMILEDS LHXL-BW02
EACH LED IS 3V TO 4.2V
AT 350mA
V_OUT
10µF, 50V
C5750X7R1H106M
CERAMIC
GND
3783 F11
Figure 11. Single Inductor Buck-Boost Application with Analog Dimming and Low Frequency PWM Dimming
3783f
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