LM3402 datasheet(13/22 Pages) NSC

13 / 22 page

Design Considerations

SWITCHING FREQUENCY

Switching frequency is selected based on the tradeoffs be-

tween 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 ap-

plications place limits on switching frequency due to EMI

sensitivity. The on-time of the LM3402/02HV can be pro-

grammed 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,

∆i

F, through the LED array is

analogous to the selection of output ripple voltage in 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

∆i

F ranging from ±5% to ±20% of IF. Higher 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 com-

ponent selection in both voltage regulators and current regu-

lators. 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 Appli-

cation 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 equa-

tions that govern inductor ripple current,

∆i

L, also apply to the

LED ripple current,

∆i

F. For a controlled on-time converter

such as LM3402/02HV the ripple current is described by the

following expression:

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,

∆V

SNS, is described by the following:

∆V

SNS =

∆i

F xRSNS

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 Ex-

ample 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

∆i

L can be even if

∆i

F is made very small. A parallel

output capacitor is also useful in applications where the

inductor or input voltage tolerance is poor. Adding a capaci-

tor that reduces

∆i

F to well below the target provides head-

room for changes in inductance or V

IN that might otherwise

push the peak LED ripple current too high.

Figure 4 shows the equivalent impedances presented to the

inductor current ripple when an output capacitor, C

O, and its

equivalent series resistance (ESR) are placed in parallel with

the LED array. The entire inductor ripple current flows

through R

SNS to provide the required 25 mV of ripple voltage

for proper operation of the CS comparator.

To calculate the respective ripple currents the LED array is

represented as a dynamic resistance, r

D. LED dynamic re-

sistance is not always specified on the manufacturer’s

datasheet, but it can be calculated as the inverse slope of

the LED’s V

F vs. IF curve. Note that dividing VF by IF will give

an incorrect value that is 5x to 10x too high. Total dynamic

resistance for a string of n LEDs connected in series can be

calculated as the r

D of one device multiplied by n. Inductor

ripple current is still calculated with the expression from Buck

Regulators without Output Capacitors. The following equa-

tions can then be used to estimate

∆i

F when using a parallel

capacitor:

The calculation for Z

C assumes that the shape of the induc-

tor ripple current is approximately sinusoidal.

Small values of C

O that do not significantly reduce

∆i

F can

also be used to control EMI generated by the switching

action of the LM3402/02HV. EMI reduction becomes more

important as the length of the connections between the LED

and the rest of the circuit increase.

20192115

FIGURE 4. LED and C

O Ripple Current

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