LM3434SQX National Semiconductor Corporation, LM3434SQX Datasheet - Page 8

• Manufacturer Part Number: LM3434SQX
• Description: Lm3434 Common Anode Capable High Brightness Led Driver With High Frequency Dimming
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation
CURRENT REGULATOR OPERATION
The LM3434 is a controller for a Continuous Conduction Buck
Converter. Because of its buck topology and operation in the
continuous mode, the output current is very well controlled. It
only varies within a switching frequency cycle by the inductor
ripple current. This ripple current is normally set at 10% of the
DC current. Setting the ripple current lower than 10% is a
useful tradeoff of inductor size for less LED light output ripple.
Additional circuitry can be added to achieve any LED light
ripple desired.
The LED current is set by the voltage across a sense resistor.
This sense voltage is nominally 60mV but can be pro-
grammed higher or lower by an external control voltage.
The running frequency of the converter is programmed by an
external RC network in conjunction with the LED's forward
voltage. The frequency is nominally set around 200kHz to
500khz. Fast PWM control is available by shorting the output
of the current source by a MOSFET in parallel with the LED.
During the LED OFF time the running frequency is determined
by the RC network and the parasitic resistance of the output
circuit including the DIM FET R
The LM3434 system has been evaluated to be a very accu-
rate, high compliance current source. This is manifest in its
high output impedance and accurate current control. The cur-
rent is measured to vary less than 6mA out of 6A when
transitioning from LED OFF (output shorted) to LED ON (out-
put ~6V).
PROTECTION
The LM3434 has dedicated protection circuitry running during
normal operation. The thermal shutdown circuitry turns off all
power devices when the die temperature reaches excessive
levels. The V
protects the power devices during power supply startup and
shutdown to prevent operation at voltages less than the min-
imum operating input voltage. The V
protected to V
which decreases the supply current to approximately 35µA.
The ADJ, EN, and DIM pins are capable of sustaining up to
+/-2mA. If the voltages on these pins will exceed either V
CGND by necessity or by a potential fault, an external resistor
is recommended for protection. Size this resistor to limit pin
current to under 2mA. A 10k resistor should be sufficient. This
resistor may be used in any application for added protection
without any impact on function or performance.
Output Open Circuit
The LM3434 can be powered up with an open circuit without
sustaining any damage to the circuit. During normal operation
the circuit will also tolerate an open circuit condition without
sustaining damage provided a schottky diode is placed within
the circuit to provide a current path to disharge the energy
stored in the inductor. The anode of the schottky should be
connected to the negative supply voltage while the cathode
should be connected to the point where the inductor and
sense resistor intersect. This diode should have a surge cur-
rent rating equal to that of the maximum LED current driven.
EE
CC
. The LM3434 also features a shutdown mode
undervoltage lockout (UVLO) comparator
DSON
.
CC
pin is short circuit
IN
or
8
DESIGN PROCEDURE
This section presents guidelines for selecting external com-
ponents.
SETTING LED CURRENT CONTROL
LM3434 uses average current mode control to regulate the
current delivered to the LED (I
resistor (R
I
and CSN pins. CSP and CSN are the inputs to an error am-
plifier with a programmed input offset voltage (V
V
tion:
FIXED LED CURRENT
The ADJ pin sets V
internal reference for V
current based on the following equation:
ADJUSTABLE LED CURRENT
When tied to an external voltage the ADJ pin sets V
based on the following equation:
When the reference on ADJ is adjustable, V
can be adjusted within the linear range of the ADJ pin. This
range has the following limitations:
When V
anteed by design to be less than or equal to 0.3V/16.667.
When V
V
equal to V
V
INPUT CAPACITOR SELECTION
A low ESR ceramic capacitor is needed to bypass the MOS-
FETs. This capacitor is connected between the drain of the
synchronous FET (CGND) and the source of the main switch
(V
appearing at the V
minimum with X5R or X7R dielectric. In addition to the FET
bypass capacitors, additional bypass capacitors should be
placed near the V
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.
LED
SENSE
IN
SENSE
EE
- 1V, V
). This capacitor prevents large voltage transients from
into a voltage that is sensed by the LM3434 at the CSP
is used to regulate I
switches to 60mV.
ADJ
ADJ
SENSE
SENSE
ADJ
is less than this linear range the V
is greater than this linear range and less than
/16.667. If V
) in series with the LED is used to convert
is guaranteed by design to be less than or
EE
SENSE
EE
and V
pin of the LM3434. Use a 22µF value
SENSE
. Tie ADJ to V
IN
LED
ADJ
. Select R
pins and should be returned to
LED
based on the following equa-
). An external current sense
is greater than V
IN
SENSE
to use a fixed 60mV
SENSE
to fix the LED
SENSE
IN
and I
is guar-
SENSE
SENSE
- 1V,
LED
).