LX1994

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 5

High Efficiency LED Driver

Copyright

© 2003

Rev. 1.0a, 2004-08-10

TM

®

TH EORY OF O PERAT ION

Basic PFM operation

The LX1994 dual mode PFM modulator is implemented in

two switching modes: the hysteretic and Continuous

Switching Mode (CSM).

In hysteretic switching mode, the basic PFM modulator

logic/timing block uses a Fixed Peak Current/ Fixed Off

Time where the switch turns on and allows the inductor

current to ramp to a finite peak level then shuts off for a

fixed duration of time. The basic modulation cycle repeats

as long as the converter output voltage is less than the

maximum regulation level. When the maximum regulation

level is reached, the switch remains off until the output

voltage capacitor discharges to a level less than the

minimum regulation level. The input signals to the switch

logic block are the burst on/off control signal and the peak

current detection signals. For low and negligible switch

conduction losses the designer may set the peak current

comparator at 20mV corresponding to 200mA of output

current.

In Continuous Switching Mode (CSM), the level to the

peak current comparator is variable. This current level is

developed by integrating the output of the feedback

comparator which functions as a high gain bandwidth

limited error amplifier. This current is clamped to the peak

switch current limit of 600mA. The integrated capacitor is

attached at the CMP pin when the burst on/off control line

is forced to the “ON” state.

The conversion from hysteretic to CSM mode is performed

when the burst length exceeds more than 16 switching

cycles counting by an internal 16 bits shift register. The

internal register is clocked by the switch transitions during

each burst period. When the switching cycles exceed 16

cycles, the converter automatically switches over to CSM

mode. CSM mode switching is latched by a J/K flip-flop.

The conversion from CSM mode to hysteretic mode is

performed when the error amplifier output falls below

10mV (corresponding to 100mA peak current) as

determined by a comparator. This resets the J/K flip-flop

and converts back to hysteric mode.

The LX1994 is a highly efficient PFM boost converter, its

design is based on dual mode PFM for driving a series of

white or color LEDs. The advantage of PFM switching is

to minimize system efficiency losses in both heavy and

light load operations. The LX1994 does not require an

external oscillator due to PFM dual modes switching.

In light load operation, the converter minimizes switching

losses by delivering more energy than necessary during

switching burst period than the inactivity coast period.

In heavy load condition, the converter uses the

Continuous Switching Current Mode (CSM) regulation

scheme. This minimized peak switching current and

thereby minimizes the conduction losses.

Losses

There are two types of losses in PFM regulator design: the

switching loss, and conduction loss; that contribute to

system inefficiency.

Switching loss: Energy switching losses are associated

with a NFET’s switch changing state (from on to off or

vice versa) as a simultaneous high level of voltage and

current are at the NFET’s switch during the transition.

This switching loss is proportional to the switching

frequency.

Conduction loss: the loss due to current flow in the series

resistance of the switch, inductor, and current sense

resistor. Conduction loss is proportional to the square of

the switch current.

Output Current Selection

The LED output current is regulated by adjusting of the

FB pin voltage. If the FB pin voltage equals the BRT pin

voltage, the LED current is the result of the FB pin

voltage divided by the selected current sense resistor.

For example: in a 100% duty cycle design, FB pin voltage

is 300mV, the current sense resistor is 15Ω. The LED

current equals:

300mV

= 20mA

15Ω