Texas Instruments LM3434 Evaluation Board LM3434SQ-20AEV/NOPB LM3434SQ-20AEV/NOPB 데이터 시트
제품 코드
LM3434SQ-20AEV/NOPB
R
ON
=
TIME
ON
C
ON
(0.3/(|V
EE
|-V
LED
))
TIME
ON
=
V
LED
f|V
EE
|
f =
(MHz)
A
I
LED
0.3V < V
ADJ
< (The greater of 1.5V or (V
IN
- 1.9V))
V
SENSE
= (V
ADJ
- V
CGND
)/16.66
R
SENSE
= 60mV/I
LED
SNVS619B – MARCH 2010 – REVISED MAY 2013
FIXED LED CURRENT
The ADJ pin sets V
SENSE
. Tie ADJ to V
IN
to use a fixed 60mV internal reference for V
SENSE
. Select R
SENSE
to fix
the LED current based on the following equation:
(2)
ADJUSTABLE LED CURRENT
When tied to an external voltage the ADJ pin sets V
SENSE
based on the following equation:
(3)
When the reference on ADJ is adjustable, V
SENSE
and I
LED
can be adjusted within the linear range of the ADJ pin.
This range has the following limitations:
(4)
When V
ADJ
is less than this linear range the V
SENSE
is specified by design to be less than or equal to
0.3V/16.667. When V
ADJ
is greater than this linear range and less than V
IN
- 1V, V
SENSE
is specified by design to
be less than or equal to V
ADJ
/16.667. If V
ADJ
is greater than V
IN
- 1V, V
SENSE
switches to 60mV.
INPUT CAPACITOR SELECTION
A low ESR ceramic capacitor is needed to bypass the MOSFETs. This capacitor is connected between the drain
of the synchronous FET (CGND) and the source of the main switch (V
of the synchronous FET (CGND) and the source of the main switch (V
EE
). This capacitor prevents large voltage
transients from appearing at the V
EE
pin of the LM3434. Use a 22µF value minimum with X5R or X7R dielectric.
In addition to the FET bypass capacitors, additional bypass capacitors should be placed near the V
EE
and V
IN
pins and should be returned to CGND.
The input capacitor must also be sized to handle the dimming frequency input ripple when the DIM function is
used. This ripple may be as high as 85% of the nominal DC input current (at 50% duty cycle). When dimming
this input capacitor should be selected to handle the input ripple current.
used. This ripple may be as high as 85% of the nominal DC input current (at 50% duty cycle). When dimming
this input capacitor should be selected to handle the input ripple current.
RECOMMENDED OPERATING FREQUENCY AND ON TIME "TIME
ON
" CALCULATION
Although the switching frequency can be set over a wide range, the following equation describes the
recommended frequency selection given inexpensive magnetic materials available today:
recommended frequency selection given inexpensive magnetic materials available today:
(5)
In the above equation A=1.2 for powdered iron core inductors and A=0.9 or less for ferrite core inductors. This
difference takes into account the fact that ferrite cores generally become more lossy at higher frequencies. Given
the switching frequency f calculated above, TIME
difference takes into account the fact that ferrite cores generally become more lossy at higher frequencies. Given
the switching frequency f calculated above, TIME
ON
can be calculated. If V
LED
is the forward voltage drop of the
LED that is being driven, TIME
ON
can be calculated with the following equation:
(6)
TIMING COMPONENTS (R
ON
and C
ON
)
Using the calculated value for TIME
ON
, the timing components R
ON
and C
ON
can be selected. C
ON
should be
large enough to dominate the parasitic capacitance of the T
ON
pin. A good C
ON
value for most applications is
1nF. Based on calculated TIME
ON
, C
ON
, and the nominal V
EE
and V
LED
voltages, R
ON
can be calculated based on
the following equation:
(7)
10
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