L

LM3402, LM3402HV

SNVS450D – SEPTEMBER 2006 – REVISED FEBRUARY 2010

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DESIGN CONSIDERATIONS

SWITCHING FREQUENCY

Switching frequency is selected based on the tradeoffs between efficiency (better at low frequency), solution

size/cost (smaller at high frequency), and the range of output voltage that can be regulated (wider at lower

frequency.) Many applications place limits on switching frequency due to EMI sensitivity. The on-time of the

LM3402/02HV can be programmed for switching frequencies ranging from the 10’s of kHz to over 1 MHz. The

maximum switching frequency is limited only by the minimum on-time requirement.

LED RIPPLE CURRENT

Selection of the ripple current,

a standard voltage regulator. Where the output ripple in a voltage regulator is commonly ±1% to ±5% of the DC

output voltage, LED manufacturers generally recommend values for

LED ripple current allows the use of smaller inductors, smaller output capacitors, or no output capacitors at all.

The advantages of higher ripple current are reduction in the solution size and cost. Lower ripple current requires

more output inductance, higher switching frequency, or additional output capacitance. The advantages of lower

ripple current are a reduction in heating in the LED itself and greater range of the average LED current before

the current limit of the LED or the driving circuitry is reached.

BUCK CONVERTERS WITHOUT OUTPUT CAPACITORS

The buck converter is unique among non-isolated topologies because of the direct connection of the inductor to

the load during the entire switching cycle. By definition an inductor will control the rate of change of current that

flows through it, and this control over current ripple forms the basis for component selection in both voltage

regulators and current regulators. A current regulator such as the LED driver for which the LM3402/02HV was

designed focuses on the control of the current through the load, not the voltage across it. A constant current

regulator is free of load current transients, and has no need of output capacitance to supply the load and

maintain output voltage. Referring to the Typical Application circuit on the front page of this datasheet, the

inductor and LED can form a single series chain, sharing the same current. When no output capacitor is used,

the same equations that govern inductor ripple current,

controlled on-time converter such as LM3402/02HV the ripple current is described by the following expression:

(10)

A minimum ripple voltage of 25 mV is recommended at the CS pin to provide good signal-to-noise ratio (SNR).

The CS pin ripple voltage,

(11)

BUCK CONVERTERS WITH OUTPUT CAPACITORS

A capacitor placed in parallel with the LED or array of LEDs can be used to reduce the LED current ripple while

keeping the same average current through both the inductor and the LED array. This technique is demonstrated

in Design Example 1. With this topology the output inductance can be lowered, making the magnetics smaller

and less expensive. Alternatively, the circuit could be run at lower frequency but keep the same inductor value,

improving the efficiency and expanding the range of output voltage that can be regulated. Both the peak current

limit and the OVP/OCP comparator still monitor peak inductor current, placing a limit on how large

even if

input voltage tolerance is poor. Adding a capacitor that reduces

and its equivalent series resistance (ESR) are placed in parallel with the LED array. The entire inductor ripple

comparator.

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