L99LD21 datasheet(18/73 Pages) STMICROELECTRONICS

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Buck converters

L99LD21

18/73

DS11130 Rev 5

Buck converters

3.1

General description

The L99LD21 features two independent buck converters with integrated switching mosfets

with forward peak current as high as specified maximum ILx_PEAK (where x indicates Buckx

peak current) 1.695 A. They are optimized to deliver a constant current to LED strings.

The RDS_ON of the n-channel mosfets can be set programming the appropriate bit in the

control register (see bits <3:2> on Table 14: CR#1: Control Register 1): high RDS_ON mode

(only one half power stage enabled) or low RDS_ON mode (both half power stages enabled).

This feature allows having two different inductor peak current ranges, 0.179 A ÷ 0.849 A or

0.362 A ÷ 1.695 A, respectively for high RDS_ON and low RDS_ON mode, so achieving the

highest of current sense accuracy in the whole current range.

The buck converters are based on constant off-time architecture, which regulates the peak

current in each inductor. The monitoring of the inductor peak current is done through

integrated senseFETs. This results in a lossless high side current sensing, which does not

require any external shunt resistor, and improves the system efficiency.

This architecture provides an inherent cycle-by cycle current limitation and a fast transient

response, without any compensation of the control loop.

The average LED current in each LED string is configurable by the SPI, through

configuration of the inductor peak current and peak-to-peak current.

The dimming of the LED strings can be realized through the direct input pin (DIN) or through

the internal 10-bit PWM dimming generator.

3.2

Bootstrap circuit

The L99LD21 has built-in high side n-channel switching mosfets, which are driven by gate

drivers. Each gate driver uses a bootstrap circuit, consisting of an integrated diode and an

external capacitor between the LX1S and CBOOT1 pins, respectively between the LX2S

and CBOOT2 pins.

The buck converters impose a minimum off-time (TOFF_MIN) to ensure that the bootstrap

capacitor recharges every cycle to a voltage which avoids the switching mosfet to operate in

linear mode. TOFF_MIN restricts the maximum duty cycle of the buck converters for a given

switching frequency. This effect is more pronounced at high switching frequencies and limits

the maximum ratio between the buck input voltage (VBOOST) and the LED strings’ forward

voltage. One way to overcome this limitation is reducing switching frequency, by selecting

high constant VLED xTOFF and/or increase the inductance value.

3.3

Peak and average current setting

In buck converters, the inductor is directly connected to the load during the complete

switching cycle (see Figure 7: Peak current control principle). The average inductor current

is equal to the average LED string current. Operating in continuous conduction mode (i.e.

the inductor current never decays to zero during the off-phase), if the inductor current is

tightly controlled, the LED current will be regulated as well.

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