RL1632R-R100-F datasheet(9/22 Pages) ONSEMI

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NCV78514

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DETAILED OPERATING DESCRIPTION

BAT ELECTRICAL DESCRIPTION

For a proper operation and a safe start−up sequence, an

under−voltage comparator senses the BAT pin. Its own wide

hysteresis protects against slow battery rising and falling

behavior

An overvoltage comparator turns off the DC−to−DC

controller in case of a too high or inappropriate battery

voltage is applied. The IC starts again when the input voltage

retrieves to normal level, below the OVLO_operating minus

hysteresis.

POWER TREE

The single channel LED driver is supplied from the BAT

pin.

BAT can be supplied from the car battery, for instance by

a SmartFET. Two local supplies are generated from BAT:

VDD and VDRV.

VDD supplies most of the internal circuitry and is

decoupled with the CVDD capacitor. VDRV supplies the

driver of the external low side Boost switch and is decoupled

with the CVDRV capacitor.

No external components or system (except the respective

decoupling capacitors and potential TCS pull−up resistor)

are allowed to be connected to VDD and VDRV pins.

DC−DC BUCK – BOOST CONVERTER OPERATION

The NCV78514 is powered from an automotive battery

and operates with a fixed switching frequency

SWITCH_FREQ, DIM mode (duty cycle modulation).

It is designed to supply a constant current between

ILED_MAX and ILED_MIN into a single LED string,

programmed by a pull−down resistor connected on ISET

pin.

The synchronous Buck integrates the high side and the

low side switch. Whereas the asynchronous boost utilizes an

external low side N−MOSFET and a Schottky diode.

The constant LED current is achieved by sensing peak

current through integrated high side switch.

The sensed peak current is compared to voltage over the

RSENSE resistor, which is in series with LED string.

The output voltage varies with number of LEDs in series

and programmed output current.

When the input voltage is well above the differential

output voltage between LED+ and LED − (Voltage across

the LEDs), the DC−DC converter is in buck mode (BAT >

VLED).

Consequently, the LSSG pin used to drive the low side

switch in boost mode, is then deactivated (retained to GND).

Upon the buck operation, integrated high side and low

side switches, transfer the energy from the input to the

inductor. Their resistances are represented by the

P_HSS_RON and P_LSS_RON parameters. The high side

switch is driven from bootstrap capacitor Cboot (HSS gate

voltage = BAT + VDD − Vf).

During the ton time, HSS is turned on, and LSS is made

non−conductive. The peak current is tracked during this

phase. The conductivity of integrated switches is inverted

during the toff phase. Behaving like a synchronous buck

converter until the maximum duty cycle is reached

(DC_Buck_Max).

In case of the BAT is close to the output voltage, the IC

enters in the 4−phases cycle, called the Buck−Boost mode.

The LSSG is then re−activated and the 3 switches (both

integrated switches and external N−MOSFET) will be

alternatively controlled, where the sequence depends on

BAT voltage in reference to VLED.

During boost phase, LSS is open and HSS is closed.

Allowing to continuously track the Inductor IPEAK current.

The ton boost phase (inductor current increasing phase) is

when external N−MOSFET is on, and no current crosses the

schottky diode (VLED > schottky anode voltage).

The external N−MOSFET is made non−conductive

during the inductor discharge phase (toff).

The inductor is protected by an IPEAK protection. The

current is measured internally, through the high side switch.

The IPEAK is set at P_IPEAK. The maximum output power

will be limited in case of IPEAK event. The output power

limitation due to IPEAK protection mainly occurs in Boost

mode and in case of Battery cranking.

In case of IPEAK event, no error is reported to DIM pin.

SPREAD SPECTRUM FREQUENCY MODULATION

Spread spectrum is a technique using frequency

modulation to achieve lower peak electromagnetic

interference (EMI).

It is an elegant and complementary solution with filtering

and shielding techniques to improve EMC performance.

In order to “spread” the peak to broader band, the internal

oscillator frequency is modulated, decreasing the peak

amplitude at the center frequency and at the frequency’s

harmonics. This results in lower system EMI compared to

the typical narrow band signal produced by oscillators and

most clock generators.

The adopted spread spectrum technic results in double

peak triangle modulation, on FSSMB frequency range, and

FSSMB frequency modulation.

DIM PIN − DIMMING DESCRIPTION

The average current through the LEDs can be reduced in

a dimming fashion, with an external signal applied on DIM

pin.

The input signal frequency at DIM can be freely chosen

in the DIM_FREQ frequency range while the output

frequency is constant at FREQ_OUT.

The input signal is validated by the device before actual

dimming is applied on LED string (two periods).

An input frequency outside the DIM_FREQ_IN range is

considered as no DIM available (or disappearance) thus the

LED string is maintained off (or switched off).

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