LM3404MASLASHNOPB datasheet(12/39 Pages) TI1

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TSW

300 ns

VO(MIN) = VIN x

TSW

TSW - 300 ns

VO(MAX) = VIN x

TSW = 1/fSW

nMAX =

VF(MAX)

VO(max) - 200 mV

DMAX =

tON

tON + tOFF-MIN

VO(max) = DMAX x VIN

IL-MIN =

0.2

RSNS

VO x tSNS

LM3404, LM3404HV

SNVS465F – OCTOBER 2006 – REVISED MAY 2013

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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 the following

expression:

(3)

In this equation 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

(4)

Detailed information for the calculation of

ΔiL is given in the Design Considerations section.

MAXIMUM OUTPUT VOLTAGE

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

output voltage, VO(MAX), determined by the following equations:

(5)

The maximum number of LEDs, nMAX, that can be placed in a single series string is governed by VO(MAX) and the

maximum forward voltage of the LEDs used, VF(MAX), using the expression:

(6)

At low switching frequency the maximum duty cycle and output voltage are higher, allowing the LM3404/04HV to

regulate output voltages that are nearly equal to input voltage. The following equation relates switching frequency

to maximum output voltage, and is also shown graphically in the Typical Performance Characteristics section:

(7)

MINIMUM OUTPUT VOLTAGE

The minimum recommended on-time for the LM3404/04HV is 300 ns. This lower limit for tON determines the

minimum duty cycle and output voltage that can be regulated based on input voltage and switching frequency.

The relationship is determined by the following equation, shown on the same graphs as maximum output voltage

in the Typical Performance Characteristics section:

(8)

HIGH VOLTAGE BIAS REGULATOR

The LM3404/04HV contains an internal linear regulator with a 7V output, connected between the VIN and the

VCC pins. The VCC pin should be bypassed to the GND pin with a 0.1 µF ceramic capacitor connected as close

as possible to the pins of the IC. VCC tracks VIN until VIN reaches 8.8V (typical) and then regulates at 7V as

VIN increases. Operation begins when VCC crosses 5.25V.

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