L99LD20 datasheet(6/61 Pages) STMICROELECTRONICS

6 / 61 page

Introduction

L99LD20

6/61

DS11366 Rev 3

Introduction

The L99LD20 is a monolithic driver IC, which controls the current of two independent high

power LED strings, whose forward current and voltage can reach up to 1.5 A (average) and

up to 50 V respectively.

This device has been designed with dedicated functions, in order to fulfill the stringent

requirements of automotive front lighting applications.

The device offers a high level of flexibility, without any change of the external components,

thanks to its programmability through the ST SPI interface. This feature support generic

platform approaches, which require a software configurability of several parameters. This

robust interface, offers a detailed diagnostic of the device itself, as well as of the controlled

LED strings.

As the device potentially controls safety critical functions such as low beams and turn

indicators, built-in features are integrated in order to support a high level of functional safety.

The L99LD20 features a timeout watchdog, a monitoring of the watchdog counter, a limp

home function and a direct input. The ST SPI protocol takes into account FMEA case.

The device consists of two independent integrated buck converters, whose input voltage is

compatible with VBUCKIN. The integrated buck converters are based on constant off-time

architecture (for a given LED output voltage) and control the peak current and the peak-to-

peak current ripple of their respective inductors. Operating in continuous conduction mode,

the average of each LED string’s current, which is connected to the output of each buck

converter, is tightly controlled. This architecture, which consists of two independent buck

stages, allows the control of a wide range of LED strings, whose forward voltage is

independent from the battery voltage.

With the aim of ensuring a wide operating inductor current range, the Buck mosfets can be

set in low or high RDS_ON modes, so that two different inductor peak current (ILx_PEAK)

ranges [0.179 A ÷ 0.849 A] or [0.362 A ÷ 1.695 A] can be selected.

The average LED current is controlled by setting the inductor's peak current and peak-to-

peak current ripple. Sensing of the peak current is integrated, not requiring any external

shunt resistance, which saves cost and reduces the power dissipation.

Buck n-channel mosfet RDS_ON value depends on the operative conditions as junction

temperature, Input voltage and LED string current. For example, at VBuckin = 45 V,

Iled = 700 mA, Tj = 25 °C the maximum RDS_ON is 400 mΩ (low RDS_ON mode).

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