VBUCK

8.33 ms

30°

tX

t

150°

180°

0°

VVF-CAP =

2

VAC(MAX)

#stages

IL-PK(UNDIM) =

750 mV

R3

IAVE(UNDIM) = IL2-PK(UNDIM) -

'iL

2

IL2-PK = IAVE +

'iL

2

LM3444

SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015

www.ti.com

Application Information (continued)

Knowing the desired average LED current, IAVE, and the nominal inductor current ripple, ΔiL, the peak current for

an application running in continuous conduction mode (CCM) is defined in Equation 23.

(23)

The LED current would then be calculated using Equation 24.

(24)

This is important to calculate because this peak current multiplied by the sense resistor R3 determines when the

internal comparator is tripped. The internal comparator turns the control MOSFET off once the peak sensed

voltage reaches 750 mV.

(25)

Current Limit: The trip voltage on the PWM comparator is 750 mV. However, if there is a short circuit or an

excessive load on the output, higher than normal switch currents cause a voltage greater than 1.27 V on the

ISNS pin which trip the I-LIM comparator. The I-LIM comparator resets the RS latch, turning off Q2. It also

inhibits the Start Pulse Generator and the COFF comparator by holding the COFF pin low. A delay circuit

prevents the start of another cycle for 180 µs.

8.1.6 Valley Fill Capacitors

Determining voltage rating and capacitance value of the valley-fill capacitors:

The maximum voltage seen by the valley-fill capacitors is calculated by Equation 26.

(26)

This is, of course, if the capacitors chosen have identical capacitance values and split the line voltage equally.

Often a 20% difference in capacitance could be observed between like capacitors. Therefore a voltage rating

margin of 25% to 50% should be considered.

8.1.7 Determining the Capacitance Value of the Valley-Fill Capacitors

The valley-fill capacitors must be sized to supply energy to the buck converter (VBUCK) when the input line is less

than its peak divided by the number of stages used in the valley fill (tX). The capacitance value must be

calculated for the maximum LED current.

Figure 17. Two Stage Valley-Fill VBUCK Voltage

18

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