LM3533 Datasheet, PDF(46/53 Page) Texas Instruments – Complete Lighting Power Solution for Smartphone Handsets

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LM3533 Datasheet, PDF (46/53 Pages) Texas Instruments – Complete Lighting Power Solution for Smartphone Handsets

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LM3533

Applications Information

BOOST CONVERTER MAXIMUM OUTPUT POWER (BOOST)

The LM3533's maximum output power is governed by two factors: the peak current limit (ICL = 880mA min), and the maximum

output voltage (VOVP). When the application causes either of these limits to be reached it is possible that the proper current regulation

and matching between LED current strings will not be met.

PEAK CURRENT LIMITED

In the case of a peak current limited situation, when the peak of the inductor current hits the LM3533's current limit, the NFET switch

turns off for the remainder of the switching period. If this happens each switching cycle the LM3533 will regulate the peak of the

inductor current instead of the headroom across the current sinks. This can result in the dropout of the boost output connected

current sinks, and the LED current dropping below its programmed level.

The peak current in a boost converter is dependent on the value of the inductor, total LED current in the boost (IOUT), the boost

output voltage (VOUT) (which is the highest voltage LED string + 0.4V regulated headroom voltage), the input voltage (VIN), the

switching frequency, and the efficiency (Output Power/Input Power). Additionally, the peak current is different depending on whether

the inductor current is continuous during the entire switching period (CCM), or discontinuous (DCM) where it goes to 0 before the

switching period ends. For Continuous Conduction Mode the peak inductor current is given by:

(1)

For Discontinuous Conduction Mode the peak inductor current is given by:

(2)

To determine which mode the circuit is operating in (CCM or DCM) it is necessary to perform a calculation to test whether the

inductor current ripple is less than the anticipated input current (IIN). If ÎIL is less than IIN then the device will be operating in CCM.

If ÎIL is greater than IIN then the device is operating in DCM.

Typically at currents high enough to reach the LM3533's peak current limit, the device will be operating in CCM. When choosing

the switching frequency and the inductor value, equations (1) and (2) should be used to ensure that IPEAK stays below ICL_MIN (see

Electrical Characteristics (Note 2, Note 7)).

OUTPUT VOLTAGE LIMITED

In the case of a output voltage limited situation, when the boost output voltage hits the LM3533's OVP threshold, the NFET turns

off and stays off until the output voltage falls below the hysteresis level (typically 1V below the OVP threshold). This results in the

boost converter regulating the output voltage to the programmed OVP threshold (16V, 24V, 32V, or 40V), causing the current sinks

to go into dropout. The default OVP threshold is set at 16V. For LED strings higher than typically 4 series LEDs, the OVP will have

to be programmed higher after power-up or after a HWEN reset.

MAXIMUM OUTPUT POWER (CHARGE PUMP)

The maximum output power available from the LM3533's charge pump is determined by the maximum output voltage available

from the charge pump. In 1X gain the charge pump operates in Pass Mode so that the voltage at CPOUT tracks VIN (less the drop

across the charge pumps pass switch). In this case the maximum output power is given as:

where RCP is the resistance from IN to CPOUT and ILVLED_TOTAL is the maximum programmed current in the LVLED strings.

In 2X gain the voltage at CPOUT (VCPOUT_2X) is regulated to typically 4.4V. In this case the maximum output power is given by:

Both equations assume there is sufficient headroom at the top side of the low-voltage current sinks to ensure the LED current

remains in regulation (VHR_LV) in the electrical table.

LAYOUT GUIDELINES AND COMPONENT SELECTION (BOOST)

The LM3533 inductive boost converter sees a high switched voltage (up to 40V) at the SW pin, and a step current (up to 1A) through

the Schottky diode and output capacitor each switching cycle. The high switching voltage can create interference into nearby nodes

due to electric field coupling (I = CdV/dt). The large step current through the diode and the output capacitor can cause a large

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