LM3434QX/NOPB National Semiconductor, LM3434QX/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM3434QX/NOPB
• Description: IC LED DRIVER DIMM 24-LLP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3434QX/NOPB

• Constant Current: Yes
• Topology: PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Internal Driver: No
• Type - Primary: Automotive
• Type - Secondary: High Brightness LED (HBLED)
• Frequency: 1MHz
• Voltage - Supply: 3 V ~ 5.8 V
• Mounting Type: Surface Mount
• Package / Case: 24-LLP
• Operating Temperature: -40°C ~ 125°C
• Internal Switch(s): No
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Current - Output / Channel: -
• Efficiency: -
• Other names: LM3434QX

RECOMMENDED OPERATING FREQUENCY AND ON
TIME "TIME
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:
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 fre-
quency f calculated above, TIME
V
en, TIME
TIMING COMPONENTS (R
Using the calculated value for TIME
nents R
enough to dominate the parasitic capacitance of the T
A good C
culated TIME
R
INDUCTOR SELECTION
The most critical inductor parameters are inductance, current
rating, and DC resistance. To calculate the inductance, use
the desired peak to peak LED ripple current (I
and C
inductor value is calculated using the following equation:
For all V
and is only dependent on the passive external compo-
nents R
The I
an important parameter affecting the efficiency. Lower DC re-
sistance inductors are larger. A good tradeoff point between
the efficiency and the core size is letting the inductor I
equal 1% to 2% of the output power. The inductor should have
a current rating greater than the peak current for the applica-
tion. The peak current is I
POWER FET SELECTION
FETs should be chosen so that the I
1% of the total output power. Analysis shows best efficiency
LED
ON
can be calculated based on the following equation:
is the forward voltage drop of the LED that is being driv-
2
ON
R loss caused by the DC resistance of the inductor is
ON
ON
. A reasonable value for I
ON
ON
LED
, C
and C
can be calculated with the following equation:
value for most applications is 1nF. Based on cal-
ON
ON
and V
ON
" CALCULATION
, C
, and L.
ON
ON
EE
, and the nominal V
can be selected. C
voltages, I
LED
ON
plus 1/2 I
and C
RIPPLE
RIPPLE
ON
2
ON
R
ON
RIPPLE
can be calculated. If
DSON
EE
)
, the timing compo-
ON
is 10% of I
remains constant
and V
should be large
.
loss is less than
RIPPLE
LED
voltages,
LED
2
), R
ON
R loss
. The
pin.
ON
,
9
with around 8mΩ of R
application. All of the switching loss is in the main switch FET.
An additional important parameter for the synchronous FET
is reverse recovery charge (Q
the transient voltages seen by the IC. A low Q
be used.
DIM FET SELECTION
Choose a DIM FET with the lowest R
cieny and low input current draw during the DIM cycle. The
output voltage during DIM will determine the switching fre-
quency. A lower output voltage results in a lower switching
frequency. If the lower frequency during DIM must be bound,
choose a FET with a higher R
quency higher during the DIM cycle.
Placement of the Parallel Dimming FET
When using a FET in parallel with the LED for PWM dimming
special consideration must be used for the location of the
FET. The ideal placement of the FET is directly next to the
LED. Any distance between this FET and the LED results in
line inductance. Fast current changes through this inductance
can induce large voltage spikes due to v = Ldi/dt. These can
be mitigated by either reducing the distance between the FET
and the LED and/or slowing the PWM edges, and therefore
the dt, by using some gate resistance on the FET. In cases
where the dimming FET is not placed close to the LED and/
or very fast switching edges are desired the induced voltages
can become great enough to damage the dimming FET and/
or the LM3434 HS pin. This can also result in a large spike of
current into the LED when the FET is turned off. In these cas-
es a snubber should be placed across the dimming FET to
protect the device(s).
BOOTSTRAP CAPACITORS
The LM3434 uses two bootstrap capacitors and a bypass ca-
pacitor on V
external FETs. A 2.2µF ceramic capacitor or larger is recom-
mended between the V
mended between the HS and BST pins. A 0.1µF is
recommended between BST2 and CGND.
SOFT-START CAPACITOR
The LM3434 integrates circuitry that, when used in conjunc-
tion with the SS pin, will slow the current ramp on start-up.
The SS pin is used to tailor the soft-start for a specific appli-
cation. A capacitor value of 0.1µF on the SS pin will yield a
12mS soft start time. For most applications soft start is not
needed.
ENABLE OPERATION
The EN pin of the LM3434 is designed so that it may be con-
trolled using a 1.6V or higher logic signal. If the enable func-
tion is not used, the EN pin may be tied to V
This pin is pulled to V
sistor.
PWM DIM OPERATION
The DIM pin of the LM3434 is designed so that it may be con-
trolled using a 1.6V or higher logic signal. The PWM frequen-
cy easily accomodates more than 40kHz dimming and can be
much faster if needed. If the PWM DIM pin is not used, tie it
to CGND or leave it open. The DIM pin is tied to CGND inter-
nally through a 100k pull down resistor.
CC
to generate the voltages needed to drive the
DSON
IN
CC
internally through a 100k pull up re-
and LS pins. A 0.47µF is recom-
and 15nC of gate charge for a 6A
RR
DSON
). High Q
to force the switching fre-
DSON
RR
for maximum effi-
adversely affects
IN
RR
or left open.
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FET should