LM3421 datasheet(33/72 Pages) TI1

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D =

V -

D =

rD = N x rLED

VO = N x VLED

LM3421, LM3421-Q1, LM3423, LM3423-Q1

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SNVS574F – JULY 2008 – REVISED JULY 2015

Typical Applications (continued)

8.2.1.1 Design Requirements

Number of series LEDs: N

Single LED forward voltage: VLED

Single LED dynamic resistance: rLED

Nominal input voltage: VIN

Input voltage range: VIN-MAX, VIN-MIN

Switching frequency: fSW

Current sense voltage: VSNS

Average LED current: ILED

Inductor current ripple:

ΔiL-PP

LED current ripple:

ΔiLED-PP

Peak current limit: ILIM

Input voltage ripple:

ΔvIN-PP

Output OVLO characteristics: VTURN-OFF, VHYSO

Input UVLO characteristics: VTURN-ON, VHYS

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Operating Point

Given the number of series LEDs (N), the forward voltage (VLED) and dynamic resistance (rLED) for a single LED,

solve for the nominal output voltage (VO) and the nominal LED string dynamic resistance (rD):

(34)

(35)

Solve for the ideal nominal duty cycle (D):

Buck:

(36)

Boost:

(37)

Buck-Boost:

(38)

Using the same equations, find the minimum duty cycle (DMIN) using maximum input voltage (VIN-MAX) and the

maximum duty cycle (DMAX) using the minimum input voltage (VIN-MIN). Also, remember that D' = 1 - D.

8.2.1.2.2

Switching Frequency

Set the switching frequency (fSW) by assuming a CT value of 1 nF and solving for RT:

Buck (Constant Ripple vs. VIN)

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LM3421 Datasheet(PDF) 33 Page - Texas Instruments

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