LM3402HV datasheet(16/39 Pages) TI1

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CIN(MIN) =

'VIN(MAX)

IF x tON

iF =

ZC = ESR +

1 +

2S x fSW x CO

'iL

ESR

RSNS

'iC

'iF

'iL

LM3402, LM3402HV

SNVS450F – SEPTEMBER 2006 – REVISED OCTOBER 2015

www.ti.com

Application Information (continued)

Figure 20 shows the equivalent impedances presented to the inductor current ripple when an output capacitor,

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

ripple current flows through RSNS to provide the required 25 mV of ripple voltage for proper operation of the CS

comparator.

Figure 20. 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 data sheet, but it can be calculated as the

inverse slope of the LED’s VF vs. IF curve. 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/HV. EMI reduction becomes more important as the length of the connections

between the LED and the rest of the circuit increase.

8.1.5 Input Capacitors

Input capacitors at the VIN pin of the LM3402/HV 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/HV 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)

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