Texas Instruments LM3448 Evaluation Board LM3448-EDSNEV/NOPB LM3448-EDSNEV/NOPB Datenbogen

Produktcode

LM3448-EDSNEV/NOPB

LED

BUCK

OFF

LED

BUCK

K u

OFF

1 D

,and f

OFF

LED

OFF

BUCK

OFF

LED

C11 1.276V

u ¨

LED

BUCK

LM3448

SNOSB51C – SEPTEMBER 2011 – REVISED MAY 2013

www.ti.com

With efficiency considered:

(5)

For simplicity, choose efficiency between 75% and 85%.

CALCULATING OFF-TIME

The “Off-Time” of the LM3448 is set by the user and remains fairly constant as long as the voltage of the LED
stack remains constant. Calculating the off-time is the first step in determining the switching frequency (f

) of

the converter, which is integral in determining some external component values.

PNP transistor Q3, resistor R4, and the LED string voltage define a charging current into capacitor C11. A
constant current into a capacitor creates a linear charging characteristic.

(6)

Resistor R4, capacitor C11 and the current through resistor R4 (i

COLL

), which is approximately equal to V

LED

/R4,

are all fixed. Therefore, dv is fixed and linear, and dt (i.e. t

OFF

) can now be calculated.

(7)

Common equations for determining duty cycle and switching frequency in any buck converter:

(8)

(9)

(10)

Therefore:

(11)

With efficiency of the buck converter in mind:

(12)

Substitute equations and rearrange:

(13)

Off-time and switching frequency can now be calculated using the equations above.

SETTING THE SWITCHING FREQUENCY

Selecting the switching frequency for nominal operating conditions is based on tradeoffs between efficiency
(better at low frequency) and solution size/cost (smaller at high frequency). The input voltage to the buck
converter (V

BUCK

) changes with both line variations and over the course of each half-cycle of the input line

voltage. The voltage across the LED string will, however, remain constant and therefore the off-time remains
constant.

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