LM3421 datasheet(14/24 Pages) NSC

14 / 24 page

SEPIC application. When it is placed into a Buck converter a

current is set charging the RCT pin set up by the PNP tran-

sistor and resistor network (see Figure 13) the Off Time

OFF is controlled to be:

This promotes a constant ripple converter were the ripple cur-

rent magnitude is a function of the input voltage. There is no

output capacitor and the Dimming control MOSFET is shunt-

ing the current away from the LEDs. As the converter is

always in continuous conduction mode the duty factor is set

by the input and output voltages. This fact allows us to give

an equation for selecting the frequency setting components

for the Buck converter. To select a timing resistor use this

equation:

In the above equation R

T is in kΩ, CT is in nF, and f is in MHz.

One could also select the timing resistor by setting their de-

sired ripple current using the following equation:

For this equation R

T is in kΩ, CT is in nF, LCHOKE is in µH, and

RIPPLE is in A.

The above describes a buck converter with constant ripple

regardless of V

LED but that varies with VIN. The LM3421/

LM3423 can also be set up in a buck configuration where the

ripple current varies with V

LED but remains constant over

varying V

IN. See Figure 14 for an example of how to imple-

ment constant ripple vs. V

IN.

INDUCTOR SELECTION

The inductor should be selected such that the switching reg-

ulator maintains continuous inductor current conduction over

the input and output operating voltage and current ranges.

The minimum inductor value is shown in the following equa-

tion for the non-Buck topologies:

In the above equation K should be a value between 3 and 5

depending on the most important application requirements. A

lower value of K results in a smaller, lower cost inductor but

also in higher ripple and lower efficiency. A higher value of K

results in a larger, more costly inductor but will have lower

ripple and higher efficiency.

For the Buck topology the inductor value is selected for a de-

sired ripple current as shown in the previous section.

COMPENSATION

The controllers’ error amplifier is a high output impedance,

transconductance amplifier for easy, single-pin compensa-

tion. This controller is a current mode controller and the

control loop feedback is monitoring the average output (LED)

current. As such it would be expected that the compensation

network could comprise a single capacitor to ground on the

COMP pin. However, a two pole system results when an out-

put capacitor is used to reduce the ripple current in the LEDs.

Two pole systems can become unstable because the total

phase shift approaches 180 degrees at unity gain crossover.

A zero in the control compensation is needed; this takes the

form of the resistor in series with the compensation capacitor.

The value of this resistor should be designed to provide the

same RC time constant with the compensation capacitor as

the output capacitor has with the dynamic impedance of the

LED string. If additional phase margin is desired, make the

compensation time constant slower than the output time con-

stant (larger value of resistor).

FAST PWM DIMMING CAPABILITY

These devices provide fast PWM LED dimming, thus enabling

constant LED current for optimal color temperature. The

DDRV pin is meant to drive the gate of an external dimming

MOSFET. This drive will follow the PWM signal applied at the

nDIM pin. The active low nDIM pin can be driven with a PWM

signal up to 50kHz; the brightness of the LEDs can be varied

by modulating the duty cycle of this signal. LED brightness is

approximately proportional to the PWM signal duty cycle, so

30% duty cycle equals approximately 30% LED brightness.

This function can be ignored if PWM dimming is not required

by using nDIM solely as a V

IN UVLO input or by tying it directly

to V

CC or VIN (if less than 60VDC).

If high side dimming is implemented with a PMOS instead of

an NMOS, the polarity of the dimming MOSFET driver must

be reversed. The LM3423’s DPOL pin is used to set the po-

larity of the DIM driver output, DDRV. Tying DPOL to ground

causes the DDRV pin to be pulled up to V

CC during dim op-

eration, and should be used when driving a PMOS dimming

MOSFET. Note that when high side dimming, the high side

PMOS gate protection zener’s breakdown voltage should be

selected to be roughly equal to the V

CC output voltage of ap-

proximately 7V. See Figure 16 for further information. Tying

DPOL to V

CC or leaving it open causes the DDRV pin to be

low during dim operation and should be used when driving an

NMOS dimming MOSFET.

A minimum on-time must be maintained in order for PWM

dimming to operate in the linear region of its transfer function

(see the graphs Averege LED Current vs. PWM DIM Duty

Cycle and 30kHz PWM Dimming (5% Duty Cycle ON)). Be-

cause the controller is disabled during dimming, the PWM

pulse must be long enough such that the energy intercepted

from the input is greater than or equal to the energy being put

into the LEDs. For a boost and buck-boost regulator, the fol-

lowing condition must be maintained:

In the previous equation, t

PULSE is the length of the PWM pulse

is seconds, I

LED is the average current in the LEDs in am-

peres, V

LED is the LED stack voltage in volts which is also

often referred to as V

OUT or VBOOST, L in the inductance in

henries, and V

IN is the input voltage in volts.

BUCK HIGH SPEED DIMMING

These devices are able to implement a constant ripple buck

converter. In this mode the PWM control of LED dimming is

performed by shunting the current away from the LEDs and

through a MOSFET. Please refer to Figure 13 for the circuit

details.

LM3421 датащи(PDF) 14 Page - National Semiconductor (TI)

LM3421 датащи(HTML) 14 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

датащи скачать

ссылки URL