LM3402 datasheet(3/18 Pages) NSC

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30061606

FIGURE 5. I

SENSE Current Waveform

Using figures 3 and 5 and the equations of a line, calculate

LED-MIN.

Where

F = ILED-Average

The delta of the inductor current is given by:

There is a 220 ns delay (t

D) from the time that the current

sense comparator trips to the time at which the control MOS-

FET actually turns on. We can solve for i

TARGET knowing there

is a delay.

Δi

D is the magnitude of current beyond the target current and

equal to:

Therefore:

The point at which you want the current sense comparator to

give the signal to turn on the FET equals:

TARGET x RSNS = 0.20V

Therefore:

Finally R

SNS can be calculated.

Standard On-Time Set Calculation

The control MOSFET on-time is variable, and is set with an

external resistor R

ON (R2 from Figure1). On-time is governed

by the following equation:

Where

k = 1.34 x 10-10

At the conclusion of t

ON the control MOSFET turns off for a

minimum OFF time (t

OFF-MIN) of 300 ns, and once tOFF-MIN is

complete the CS comparator compares V

SNS and VREF again,

waiting to begin the next cycle.

The LM3402 / 04 have minimum ON and OFF time limitations.

The minimum on time (t

ON) is 300 ns, and the minimum al-

lowed off time (t

OFF) is 300 ns.

Designing for the highest switching frequency possible

means that you will need to know when minimum ON and OFF

times are observed.

Minimum OFF time will be seen when the input voltage is at

its lowest allowed voltage, and the output voltage is at its

maximum voltage (greatest number of series LEDs).

The opposite condition needs to be considered when design-

ing for minimum ON time. Minimum ON time is the point at

which the input voltage is at its maximum allowed voltage, and

the output voltage is at its lowest value.

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