LTC4002 datasheet(20/24 Pages) LINER

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LTC3783

3783f

out of the IC ground pad in one direction (to bottom plate

of the INTVCC 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 CIN

of the LTC3783 and the actual source VIN should be

sufficient to prevent current-sharing problems.

Returning the Load to VIN: 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

VIN, hence providing a load voltage (VOUT – VIN) which can

be greater or less than the input voltage VIN. 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 VDS(MAX) equal to VIN + VLOAD.

The design of this circuit resembles that of the boost

converter above, and the procedure is much the same,

except VOUT is now (VIN + VLOAD), 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

ILIM pin, which adjusts the offset voltage to which the

loop will drive (VFBP – VFBN). 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 VOUT level while PWMIN is low.

OPERATIO

Figure 11. Single Inductor Buck-Boost Application with Analog Dimming and Low Frequency PWM Dimming

LTC3783

RUN

PWMIN

ITH

VREF

FBP

FBN

FREQ

SYNC

VIN

OV/FB

PWMOUT

ILIM

GATE

SENSE

INTVCC

GND

VIN

9V TO 26V

0.28

Ω

VOUT

LED STRING 1-4 EA

LUMILEDS LHXL-BW02

EACH LED IS 3V TO 4.2V

AT 350mA

µF, 50V

C5750X7R1H106M

CERAMIC

0V TO

1.23V

µF, 50V

×2

UMK432C106MM

µH

SUMIDA

CDRH8D28-100

GND

3783 F11

20k

PMEG6010

FAIRCHILD

FDN5630

40.2k

4.7

µF

100k

PWM

5V AT 0Hz TO 10Hz

4.7

µF

0.05

Ω

µF

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