LM3402 datasheet(16/38 Pages) TI

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IIN(rms) = IF x D(1 - D)

CIN(MIN) =

'VIN(MAX)

IF x tON

iF =

ZC = ESR +

1 +

2S x fSW x CO

'iL

ESR

RSNS

'iC

'iF

'iL

LM3402, LM3402HV

SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013

www.ti.com

Figure 22. LED and CO Ripple Current

To calculate the respective ripple currents the LED array is represented as a dynamic resistance, rD. LED

dynamic resistance is not always specified on the manufacturer’s datasheet, but it can be calculated as the

inverse slope of the LED’s VF 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 rD of one

device multiplied by n. Inductor ripple current is still calculated with the expression from Buck Regulators without

Output Capacitors. The following equations can then be used to estimate

ΔiF when using a parallel capacitor:

(12)

The calculation for ZC assumes that the shape of the inductor ripple current is approximately sinusoidal.

Small values of CO that do not significantly reduce ΔiF 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.

INPUT CAPACITORS

Input capacitors at the VIN pin of the LM3402/02HV are selected using requirements for minimum capacitance

and rms ripple current. The input capacitors supply pulses of current approximately equal to IF while the power

MOSFET is on, and are charged up by the input voltage while the power MOSFET is off. Switching converters

such as the LM3402/02HV have a negative input impedance due to the decrease in input current as input

voltage increases. This inverse proportionality of input current to input voltage can cause oscillations (sometimes

called ‘power supply interaction’) if the magnitude of the negative input impedance is greater the the input filter

impedance. Minimum capacitance can be selected by comparing the input impedance to the converter’s negative

resistance; however this requires accurate calculation of the input voltage source inductance and resistance,

quantities which can be difficult to determine. An alternative method to select the minimum input capacitance,

CIN(MIN), is to select the maximum voltage ripple which can be tolerated. This value,ΔvIN(MAX), is equal to the

change in voltage across CIN during the converter on-time, when CIN supplies the load current. CIN(MIN) can be

selected with the following:

(13)

A good starting point for selection of CIN is to use an input voltage ripple of 5% to 10% of VIN. A minimum input

capacitance of 2x the CIN(MIN) value is recommended for all LM3402/02HV circuits. To determine the rms current

rating, the following formula can be used:

(14)

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