LM3402 datasheet(11/39 Pages) TI1

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DMAX =

tON

tON + tOFF-MIN

VO(max) = DMAX x VIN

IL-MIN =

0.2

RSNS

VO x tSNS

fSW =

1.34 x 10

-10 x RON

VO = n x VF + 200 mV

LM3402/02HV

VSNS

RSNS

VREF

One-shot

Comparator

LED 1

LED n

LM3402, LM3402HV

www.ti.com

SNVS450F – SEPTEMBER 2006 – REVISED OCTOBER 2015

Feature Description (continued)

At the conclusion of tON the power MOSFET turns off for a minimum OFF-time, tOFF-MIN, of 300 ns. Once tOFF-MIN

is complete the CS comparator compares VSNS and VREF again, waiting to begin the next cycle.

Figure 17. Comparator and One-Shot

The LM3402/HV regulators should be operated in continuous conduction mode (CCM), where inductor current

stays positive throughout the switching cycle. During steady-state operation in the CCM, the converter maintains

a constant switching frequency, which can be selected using Equation 2.

(2)

VF = forward voltage of each LED, n = number of LEDs in series

(3)

7.3.2 Average LED Current Accuracy

The COT architecture regulates the valley of

ΔVSNS, the AC portion of VSNS. To determine the average LED

current (which is also the average inductor current) the valley inductor current is calculated using Equation 4.

(4)

In Equation 4 tSNS represents the propagation delay of the CS comparator, and is approximately 220 ns. The

average inductor/LED current is equal to IL-MIN plus one-half of the inductor current ripple, ΔiL:

IF = IL = IL-MIN + ΔiL / 2

(5)

Detailed information for the calculation of

ΔiL is given in the Application and Implementation section.

7.3.3 Maximum Output Voltage

The 300 ns minimum off-time limits on the maximum duty cycle of the converter, DMAX, and in turn ,the maximum

output voltage VO(MAX) is determined by Equation 6:

(6)

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