L99LD01 datasheet(17/69 Pages) STMICROELECTRONICS

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L99LD01

Functional description

reduced (maximum 50 % of the nominal LED current), not allowing the LED temperature to

increase over the programmed limit.

Thermal limit intervention is reported by properly setting a bit inside the internal status

2.2.2

Slope compensation

Slope compensation is needed to ensure the stability of the control loop with all possible

values of duty cycle

(0 < D < 1)

especially for duty cycle greater than 0.5. The recommended slope SADD of the additional

ramp is proportional to the inductor current slope during the turn off phase, that is:

where SADD is the additional slope introduced by the circuit,

is the off-time inductor slope and

SL is also given by the formula:

Being GLA the gain of the linear amplifier (see Chapter 5: Electrical characteristics for GLA

parameter values) and RSHUNT is the resistor across pin ISP and ISN (see Chapter 7:

Application circuits).

The simplified internal circuit structure for the slope compensation is shown in Figure 8.

The additional slope is obtained from the internal oscillator ramp voltage. A fraction of the

oscillator voltage ramp is added to the output voltage of the sensing amplifier, which is

proportional to the sense resistor voltage drop, and therefore, to the current flowing through

power mosfet M1.

The added ramp voltage is

where

-----------

ADD

α S

⋅

--------

OFF

0.5

α 1

LA RSHUN T

OU T

–

()

⋅⋅

--------------------------------------------------------------------------------

ADD

SLOPE

⋅

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