LM3530 datasheet(35/50 Pages) TI1

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where

I =

MAX

OUT_

OUT

x K

PEAK

OUT

)

(

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OUT

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(

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LM3530

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SNVS606L – JUNE 2009 – REVISED DECEMBER 2014

9.2.2 Detailed Design Procedure

9.2.2.1 LED Current Setting/Maximum LED Current

The maximum LED current is restricted by the following factors: the maximum duty cycle that the boost converter

can achieve, the peak current limitations, and the maximum output voltage.

9.2.2.2 Maximum Duty Cycle

The LM3530 can achieve up to typically 94% maximum duty cycle. Two factors can cause the duty cycle to

increase: an increase in the difference between VOUT and VIN and a decrease in efficiency. This is shown by

Equation 12:

(12)

For a 9-LED configuration VOUT = (3.6 V x 9LED + VHR) = 33 V operating with η = 70% from a 3-V battery, the

duty cycle requirement would be around 93.6%. Lower efficiency or larger VOUT to VIN differentials can push the

duty cycle requirement beyond 94%.

9.2.2.3 Peak Current Limit

The LM3530 boost converter has a peak current limit for the internal power switch of 839 mA typical (739 mA

minimum). When the peak switch current reaches the current limit, the duty cycle is terminated resulting in a limit

on the maximum output current and thus the maximum output power the LM3530 can deliver. Calculate the

maximum LED current as a function of VIN, VOUT, L, efficiency (η) and IPEAK as:

where

•

ƒSW = 500 kHz

•

η and IPEAK can be found in the Efficiency and IPEAK curves in the Specifications and Application Curves.

(13)

9.2.2.4 Output Voltage Limitations

The LM3530 has a maximum output voltage of 41 V typical (40 V minimum) for the LM3530-40 version and 24 V

typical (23.6 V minimum) for the LM3530-25 version. When the output voltage rises above this threshold (VOVP)

the overvoltage protection feature is activated and the duty cycle is terminated. Switching will cease until VOUT

drops below the hysteresis level (typically 1 V below VOVP). For larger numbers of series connected LEDs the

output voltage can reach the OVP threshold at larger LED currents and colder ambient temperatures. Typically

white LEDs have a –3mV/°C temperature coefficient.

9.2.2.5 Output Capacitor Selection

The LM3530’s output capacitor has two functions: filtering of the boost converters switching ripple, and to ensure

feedback loop stability. As a filter, the output capacitor supplies the LED current during the boost converters on

time and absorbs inductor energy during the switch off time. This causes a sag in the output voltage during the

on time and a rise in the output voltage during the off time. Because of this, the output capacitor must be sized

large enough to filter the inductor current ripple that could cause the output voltage ripple to become excessive.

As a feedback loop component, the output capacitor must be at least 1 µF and have low ESR otherwise the

LM3530 boost converter can become unstable. This requires the use of ceramic output capacitors. Table 14 lists

part numbers and voltage ratings for different output capacitors that can be used with the LM3530.

Table 14. Recommended Input/Output Capacitors

MANUFACTURER

PART NUMBER

VALUE (µF)

SIZE

RATING (V)

DESCRIPTION

Murata

GRM21BR71H105KA12

0805

COUT

Murata

GRM188B31A225KE33

2.2

0805

CIN

TDK

C1608X5R0J225

2.2

0603

6.3

CIN

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