13

LTC3783

3783f

For applications where the RUN pin is only to be used as

a logic input, the user should be aware of the 7V Absolute

Maximum Rating for this pin! The RUN pin can be con-

nected to the input voltage through an external 1M resis-

tor, as shown in Figure 5c, for “always on” operation.

Soft-Start Capacitor Selection

For proper soft-start operation, the LTC3783 should have

the SS pin. The minimum soft-start capacitor size can be

estimated on the basis of output voltage, capacitor size

and load current. In addition, PWM operation reduces the

effective SS capacitor value by the dimming ratio.

C

g ratio

A C

V

R

mV

V

SS MIN

OUT

OUT

DS ON SENSE

()

()/

• dimmin

•

•

•

•

•.

>

µ

250

150

1 2

Application Circuits

A basic LTC3783 PWM-dimming LED application is shown

on the first page of this data sheet.

Operating Frequency and PWM Dimming Ratio

proper operation of a PWM dimming application depends

f

Nf

D

OSC

PWM

PWM

>

•

OPERATIO

Figure 5a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin

Figure 5b. On/Off Control Using External Logic

Figure 5c. External Pull-Up Resistor on

RUN Pin for “Always On” Operation

–

+

RUN

COMPARATOR

RUN

R2

R1

INPUT

SUPPLY

OPTIONAL

FILTER

CAPACITOR

+

–

GND

3783 F05a

BIAS AND

START-UP

CONTROL

1.248V

µPOWER

REFERENCE

6V

–

+

RUN

COMPARATOR

1.248V

3483 F05b

RUN

6V

EXTERNAL

LOGIC CONTROL

–

+

RUN

COMPARATOR

RUN

R2

1M

INPUT

SUPPLY

+

–

GND

1.248V

3483 F05c

6V