LTC3783 Datasheet, PDF (13/24 Pages) Linear Technology – PWM LED Driver and Boost, Flyback and SEPIC Controller

U
OPERATIO
+
R2
INPUT
SUPPLY OPTIONAL
FILTER
R1
CAPACITOR
–
V_IN
RUN
+
6V
–
RUN
COMPARATOR
BIAS AND
START-UP
CONTROL
1.248V
µPOWER
REFERENCE
GND
3783 F05a
Figure 5a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin
LTC3783
RUN
EXTERNAL
LOGIC CONTROL
6V
1.248V
RUN
COMPARATOR
+
–
3483 F05b
+
R2
1M
INPUT
SUPPLY
–
V_IN
RUN
+
6V
–
GND 1.248V
RUN
COMPARATOR
3483 F05c
Figure 5b. On/Off Control Using External Logic
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
a sufficiently large soft-start capacitor, C_SS, attached to
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_SS(MIN)
>
2
•
dimming
ratio
•
50µA • C_OUT • V_OUT
150mV • 1.2V
•
R_DS(ON)/SENSE
Figure 5c. External Pull-Up Resistor on
RUN Pin for “Always On” Operation
assuming 50% ripple current, where R_DS(ON)/SENSE repre-
sents either the R_DS(ON) of the switching MOSFET or
R_SENSE, whichever is used on the SENSE pin. Dimming
ratio is described by 1/D_PWM as shown in Figure 6.
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
The minimum operating frequency, f_OSC, required for
proper operation of a PWM dimming application depends
on the minimum PWM frequency, f_PWM, the dimming ratio
1/D_PWM, and N, the number of f_OSC cycles per PWM cycle:
f_OSC
>
N • f_PWM
D_PWM
3783f
13