LM3409 Datasheet, PDF(11/30 Page) National Semiconductor (TI) – PFET Buck Controller for High Power LED Drives

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LM3409 Datasheet, PDF (11/30 Pages) National Semiconductor (TI) – PFET Buck Controller for High Power LED Drives

◁

AVERAGE LED CURRENT

For a buck converter, the average LED current is simply the

average inductor current.

INDUCTOR CURRENT RIPPLE

Because the LM3409/09HV swaps the polarity of the differ-

ential current sense comparator every cycle, a minimum in-

ductor current ripple (ÎiL-PP) is necessary to maintain

accurate ILED regulation. Referring to Figure 6, the first tON is

terminated at the higher of the two polarity-swapped thresh-

olds (corresponding to IL-MAXH). During the following tOFF, iL

decreases until the second tON begins. If tOFF is too short, then

as the second tON begins, iL will still be above the lower peak

current threshold (corresponding to IL-MAXL) and a minimum

tON pulse will follow. This will result in degraded ILED regula-

tion. The minimum inductor current ripple (ÎiL-PP-MIN) should

adhere to the following equation in order to ensure accurate

ILED regulation:

30085611

FIGURE 5. Sense Voltage vSNS(t)

Using the COFT architecture, the peak transistor current (IT-

MAX) is sensed as shown in Figure 5, which is equal to the

peak inductor current (IL-MAX) given by the following equation:

Because IL-MAX is set using peak current control and ÎiL-PP is

set using the controlled off-timer, IL and correspondingly

ILED can be calculated as follows:

The threshold voltage VCST seen by the high-side sense com-

parator is affected by the comparatorâs input offset voltage,

which causes an error in the calculation of IL-MAX and ulti-

mately ILED. To mitigate this problem, the polarity of the com-

parator inputs is swapped every cycle, which causes the

actual IL-MAX to alternate between two peak values (IL-MAXH

and IL-MAXL), equidistant from the theoretical IL-MAX as shown

in Figure 6. ILED remains accurate through this averaging.

SWITCHING FREQUENCY

The switching frequency is dependent upon the actual oper-

ating point (VIN and VO). VO will remain relatively constant for

a given application, therefore the switching frequency will vary

with VIN (frequency increases as VIN increases). The target

switching frequency (fSW) at the nominal operating point is

selected based on the tradeoffs between efficiency (better at

low frequency) and solution size/cost (smaller at high fre-

quency). The off-time of the LM3409/09HV can be pro-

grammed for switching frequencies up to 5 MHz (theoretical

limit imposed by minimum tON). In practice, switching fre-

quencies higher than 1MHz may be difficult to obtain due to

gate drive limitations, high input voltage, and thermal consid-

erations.

At CCM operating points, fSW is defined as:

At DCM operating points, fSW is defined as:

30085612

FIGURE 6. Inductor Current iL(t) Showing IL-MAX Offset

In the CCM equation, it is apparent that the efficiency (Î·) fac-

tors into the switching frequency calculation. Efficiency is hard

to estimate and, since switching frequency varies with input

voltage, accuracy in setting the nominal switching frequency

is not critical. Therefore, a general rule of thumb for the

LM3409/09HV is to assume an efficiency between 85% and

100%. When approximating efficiency to target a nominal

switching frequency, the following condition must be met:

www.national.com

▷