Datasheet

LTC3783
13
3783fb
OPERATION
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 resistor,
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
•R
DS(ON)/SENSE
150mV • 1.2V
assuming 50% ripple current, where R
DS(ON)/SENSE
represents 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
–
+
RUN
COMPARATOR
V
IN
RUN
R2
R1
INPUT
SUPPLY
OPTIONAL
FILTER
CAPACITOR
+
–
GND
3783 F05a
BIAS AND
START-UP
CONTROL
1.248V
µPOWER
REFERENCE
6V
Figure 5a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin
–
+
RUN
COMPARATOR
1.248V
3483 F05b
RUN
6V
EXTERNAL
LOGIC CONTROL
Figure 5b. On/Off Control Using External Logic
–
+
RUN
COMPARATOR
V
IN
RUN
R2
1M
INPUT
SUPPLY
+
–
GND
1.248V
3483 F05c
6V
Figure 5c. External Pull-Up Resistor on
RUN Pin for “Always On” Operation