LM3424 Datasheet, PDF(24/50 Page) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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LM3424 Datasheet, PDF (24/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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Design Guide

Refer to Basic Topology Schematics section.

SPECIFICATIONS

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: ÎiL-PP

LED current ripple: ÎiLED-PP

Peak current limit: ILIM

Input voltage ripple: ÎvIN-PP

Output OVLO characteristics: VTURN-OFF, VHYSO

Input UVLO characteristics: VTURN-ON, VHYS

Thermal foldback characteristics: TBK, TEND

Total start-up time: tTSU

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):

Solve for the ideal nominal duty cycle (D):

Buck

3. AVERAGE LED CURRENT

For all topologies, set the average LED current (ILED) knowing

the desired current sense voltage (VSNS) and solving for

RSNS:

If the calculated RSNS is too far from a desired standard value,

then VSNS will have to be adjusted to obtain a standard value.

Setup the suggested signal current of 100 ÂµA by assuming

RCSH = 12.4 kâ¦ and solving for RHSP:

If the calculated RHSP is too far from a desired standard value,

then RCSH can be adjusted to obtain a standard value.

4. THERMAL FOLDBACK

For all topologies, set the thermal foldback breakpoint (TBK)

by finding corresponding RNTC-BK from manufacturer's

datasheet and solving for RBIAS:

The easiest approach is to set RREF1 = RREF2, therefore set-

ting RBIAS = RNTC-BK will properly set TBK. Remember, capac-

itance is recommended at the TSENSE and TREF pins, so

ensure CREF > CNTC to prevent start-up in foldback.

Then set the thermal foldback endpoint (TEND) by finding the

corresponding RNTC-END from manufacturer's datasheet and

solving for RGAIN:

Boost

Buck-boost

5. INDUCTOR RIPPLE CURRENT

Set the nominal inductor ripple current (ÎiL-PP) by solving for

the appropriate inductor (L1):

Buck

Using the same equations, find the minimum duty cycle

(DMIN) using maximum input voltage (VIN-MAX) and the maxi-

mum duty cycle (DMAX) using the minimum input voltage (VIN-

MIN). Also, remember that D' = 1 - D.

2. SWITCHING FREQUENCY

Set the switching frequency (fSW) by solving for RT:

Boost and Buck-boost

To set the worst case inductor ripple current, use VIN-MAX and

DMIN when solving for L1.

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