SSL5021BTS datasheet(8/20 Pages) NXP

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SSL5021BTS

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Product data sheet

Rev. 2 — 11 March 2015

8 of 20

NXP Semiconductors

SSL5021BTS

Compact low-ripple buck LED driver IC

The sense circuit is activated after the leading-edge blanking time (tleb). Because the LED

current is half the peak current by design, the sense circuit automatically provides

protection for the maximum LED current during operation. A propagation delay exists

between the overcurrent detection and the actual switch switch-off. Due to this delay, the

actual peak current is slightly higher than the OCP level set by the resistor connected in

series with the ISNS pin.

8.6.3 OverTemperature Protection (OTP)

The converter stops switching when the internal OTP function is triggered at the IC

junction temperature Tpl(IC). The safe-restart protection is triggered and the IC resumes

switching when the IC temperature drops to below Trst(IC).

8.6.4 Cycle-by-cycle maximum on-time protection

Measuring the inductor current IL using sense resistor R4 regulates the on-time. The

on-time is limited to a fixed value (ton(max)). It protects the system and the IC when the

ISNS pin is shorted or when the system works at very low mains voltage.

8.6.5 Output OverVoltage Protection (OVP)

An accurate output OVP is implemented by measuring the voltage at the DEMOVP pin

during the secondary stroke. The resistive divider connected between the LEDP node and

the DEMOVP pin sets the maximum LED voltage.

An internal counter prevents false OVP detection because of noise on the DEMOVP pin.

After three continuous cycles with a DEMOVP pin voltage above the OVP level, the OVP

protection is triggered.

The over voltage protection triggers a restart sequence: A discharge current (ICC(dch)) is

enabled and discharges the voltage on the VCC pin to below Vrst(latch). When Vrst(latch) is

reached, the system restarts.

8.6.6 Output Short Protection (OSP)

The converter operates in Discontinuous Conduction Mode (DCM). A new cycle is only

started after the previous cycle has ended. The end of the cycle is detected by measuring

the voltage on the DEMOVP pin. When the DEMOVP pin voltage drops to below the

demagnetization level (Vth(comp)DEMOVP) and a valley is detected, a new cycle starts.

When output is shorted, the demagnetization is not finalized within the 40

s. The

converter still regulates the adjusted output current and the on-time is reduced to a safe

value by this feedback. The reduced on-time in combination with very long

demagnetization time prevents that the converter is damaged or excessive dissipation

occurs.

A blanking time (tsup(xfmr_ring)) is implemented at the start of the secondary stroke to

prevent false demagnetization detection.

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