Datasheet

I
LED
= V
SENSE
/R
SENSE
LM3434
www.ti.com SNVS619B –MARCH 2010–REVISED MAY 2013
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
programmed 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
DSON
.
The LM3434 system has been evaluated to be a very accurate, high compliance current source. This is manifest
in its high output impedance and accurate current control. The current is measured to vary less than 6mA out of
6A when transitioning from LED OFF (output shorted) to LED ON (output ~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
CC
undervoltage lockout
(UVLO) comparator protects the power devices during power supply startup and shutdown to prevent operation
at voltages less than the minimum operating input voltage. The V
CC
pin is short circuit protected to V
EE
. The
LM3434 also features a shutdown mode 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
IN
or 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 current
rating equal to that of the maximum LED current driven.
DESIGN PROCEDURE
This section presents guidelines for selecting external components.
SETTING LED CURRENT CONTROL
LM3434 uses average current mode control to regulate the current delivered to the LED (I
LED
). An external
current sense resistor (R
SENSE
) in series with the LED is used to convert I
LED
into a voltage that is sensed by the
LM3434 at the CSP and CSN pins. CSP and CSN are the inputs to an error amplifier with a programmed input
offset voltage (V
SENSE
). V
SENSE
is used to regulate I
LED
based on the following equation:
(1)
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