LM3435

SNVS724C – JUNE 2011 – REVISED MAY 2013

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The flyback converter of LM3435 employs a proprietary Projected On-Time (POT) control scheme to determine

the on-time of the main N-channel MOSFET with respect to the input and output voltages together with an

circuit operations. During the on-time period, the inductor connecting to the flyback converter is charged up and

the output capacitor is discharged to supply power to the LED. A cycle-by-cycle current limit of typical 6A is

imposed on the main N-channel MOSFET for protection. After the on-time period, the main N-channel MOSFET

is turned off and the synchronous P-channel MOSFET is turned on in order to discharge the inductor. The off

state will last until VOUT is dropped below a reference voltage. Such reference voltage is derived from the

required LED current to be regulated at a particular LED channel. The flyback converter under POT control can

maintain a fairly constant switching frequency that depends mainly on value of the resistor connected across

approximated by the following relationship:

(1)

In addition, POT control requires no external compensation and achieves fast transient response of the output

voltage changes that perfectly matches the requirements of a sequential RGB LED driver. The POT flyback

converter only operates at Continuous Conduction Mode. Dead-time between main MOSFET and synchronous

MOSFET switching is adaptively controlled by a minimum non-overlap timer to prevent current shoot through.

Initial VOUT will be regulated at around 3.2V to 3.5V above VIN before any control signals being turned on.

Three small capacitors connected to CR, CG and CB pins are charged by an internal current source and act as

soft-start capacitors of the flyback converter during start-up. Once initial voltage of VOUT is settled, the

capacitors will be used as a memory element to store the VOUT information for each channel respectively. This

information will be used for VOUT regulation of respective LED channel during channel switching. In between the

is available after the correct VOUT for the color is stabilized. This control scheme ensures the minimal voltage

headroom for different color LED and hence best conversion efficiency can be achieved.

HIGH CURRENT REGULATORS

The LM3435 contains three internal current regulators powered by the output of the synchronous Flyback

Converter, VOUT. Three low side power MOSFETs are included. These current regulators control the current

supplied to the LED channels individually and maintain accurate current regulation by internal feedback and

control mechanism. The regulation is achieved by a Gm-C circuit comparing the sensing voltage of the internal

passing N-channel MOSFET and an internal LED current reference voltage generated from the external

reference current setting resistor, RIREFx connect to IREFG, IREFB or IREFR pin, of the corresponding LED

channel. The nominal maximum LED current is governed by the equation in below:

(2)

The LED current setting can be in the range of 0.5A up to 2A maximum. The nominal maximum of the device is

1.5A and for applications need higher than 1.5A LED current, VIN and thermal constrains must be complied. The

actual LED current can be adjusted on-the-fly by the internal ten bits register for individual channel. The content

on-the-fly during normal operation. The resolution is 1 out of 1024 part of the LED current setting. The user can

program the registers in the range of 1(001H) to 1023(3FFH) for each channel independently, provided the

converter is not entered the Discontinuous Conduction Mode. Whenever the converter operation entered the

Discontinuous Conduction Mode, the regulation will be deteriorated. A value of “0” may cause false fault

detection, so it must be avoided.

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