LP8556 datasheet(14/63 Pages) TI1

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Time

VBOOST

Driver

headroom

VBOOST

LP8556

SNVS871I – JULY 2012 – REVISED MARCH 2016

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8.3.1.3 Output Voltage Control

The LP8556 device supports two modes of controlling the boost output voltage: Adaptive Boost Voltage Control

(see Adaptive Control) and Manual Boost Output Control (see Manual Control).

8.3.1.3.1

Adaptive Control

LP8556 supports a mode of output voltage control called Adaptive Boost Control mode. In this mode, the voltage

at the LED pins is periodically monitored by the control loop and adaptively adjusted to the optimum value based

on the comparator thresholds set using LED DRIVER_HEADROOM, LED_COMP_HYST, BOOST_STEP_UP,

BOOST_STEP_DOWN fields in the EPROM. Settings under LED DRIVER_HEADROOM along with

LED_COMP_HYST fields determine optimum boost voltage for a given condition. Boost voltage is raised if the

voltage measured at any of the LED strings falls below the threshold setting determined with LED

DRIVER_HEADROOM field. Likewise, boost voltage is lowered if the voltage measured at any of the LED strings

is above the combined setting determined under LED DRIVER_HEADROOM and LED_COMP_HYST fields.

LED_COMP_HYST field serves to fine tune the headroom voltage for a given peak LED current. The boost

voltage up/down step size can be controlled with the BOOST_STEP_UP and BOOST_STEP_DN fields.

The initial boost voltage is configured with the VBOOST field. This field also sets the minimum boost voltage.

The VBOOST_MAX field sets the maximum boost voltage. When an LED pin is open, the monitored voltage

never has enough headroom, and the adaptive mode control loop keeps raising the boost voltage. The

VBOOST_MAX field allows the boost voltage to be limited to stay under the voltage rating of the external

components.

NOTE

Only LED strings that are enabled are monitored and PS_MODE field determines which

LED strings are enabled.

The adaptive mode is selected using ADAPTIVE bit set to 1 (CFGA EPROM Register) and is the recommended

mode of boost control.

Figure 5. Boost Adaptive Control Principle

8.3.1.3.2

Manual Control

User can control the boost output voltage with the VBOOST EPROM field when adaptive mode is not used.

Equation 1 shows the relationship between the boost output voltage and the VBOOST field.

VBOOST = VBOOST_MIN + 0.42 × VBOOST[dec]

(1)

The expression is only valid when the calculated values are between the minimum boost output voltage and the

maximum boost output voltage. The minimum boost output voltage is set with the VBOOST_RANGE field. The

maximum boost output voltage is set with the VBOOST_MAX EPROM field.

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