L99LD21 datasheet(8/73 Pages) STMICROELECTRONICS

8 / 73 page

Introduction

L99LD21

8/73

DS11130 Rev 5

Introduction

The L99LD21 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 50V 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 L99LD21 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 a boost controller, which controls the PWM of an external n-channel

mosfet and provides a stabilized voltage (VBOOST). The input of the boost stage must be

connected to the battery voltage through a reverse polarity protection.

The boost controllers of two L99LD21 can be combined to form a dual-phase, interleaved

boost controller. Special care has been taken for the current balancing between the different

phases and for the switching activity of the boost mosfets with 180° phase shift.

The output of the boost controller supplies the input of the two independent integrated buck

converters, or any other external buck converters, whose input voltage is compatible with

VBOOST. 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 cascaded boost and buck stages (see Figure 2), 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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