Datasheet

ManualsBrandsMICROCHIP TECHNOLOGY ManualsPMICsPMIC - LED driver DC-DC converter VQFN 24 Surface-mount

Atmel MSL3086/MSL3088 Datasheet

8-String 60mA LED Drivers with Integrated Boost Controller and Phase Shifted Dimming

Inductors have two types of maximum current ratings, RMS current and saturation current. Make sure that the peak inductor current is

less than the saturation current rating. Note that during load current transients, which occur whenever the LEDs are turned on or off (due

to PWM dimming), the inductor current may overshoot its steady state value. How much it overshoots depends on the boost regulator

loop dynamics. If unsure of the loop dynamics, a typical value to use for the overshoot is 50% of the steady-state current. Add half of the

inductor ripple current to this value to determine the peak inductor current. With inductor ripple current in the 25% to 50% range, estimate

the inductor RMS current as 115% of the DC steady state inductor current.

9.10 Setting the Current Limit

The current sense resistor, connected from the switching MOSFET source to GND, sets the boost regulator current limit. The cycle-by-

cycle current limit turns-off the boost regulator switching MOSFET when the current sense input detects instantaneous current above the

current limit threshold. This causes the current to drop until the end of the switching cycle. The current limit threshold is 100mV typical,

and 75mV minimum. Choose the current sense resistor value to set the current limit using:

where I

L(MAX)

is the maximum inductor current.

9.11 Choosing the Switching MOSFET

The MSL3086/88 use an external logic level MOSFET to implement the boost converter. Choose a MOSFET designed to pass twice at

least the peak inductor current, and that has the lowest possible R

DSon

while maintaining minimal gate charge for fast switching speed.

Make sure that the MOSFET drain-source voltage rating is above the maximum un-optimized boost output voltage, with some extra

margin for voltage overshoot due to excess circuit board stray inductance and output rectier recovery artIfacts. Make sure that the

MOSFET package can withstand the worst-case power dissipation while maintaining die temperature within the MOSFET ratings.

9.12 Choosing the Output Rectier

The output rectier passes the inductor current to the output capacitor and load during the switching off-time. Due to the high boost

regulator switching frequency use a Schottky rectier. Use a Schottky diode that has a current rating at least as high as that of the

external MOSFET, and a voltage rating higher than the maximum boost regulator output voltage. Schottky rectiers have very low on

voltage and fast switching speed, however at high voltage and temperatures Schottky leakage current can be signicant. Make sure

that the rectier power dissipation is within the rectier specications. Place the MOSFET and rectier close together and as close to the

output capacitor(s) as possible to reduce circuit board radiated emissions.

9.13 Loop Compensation

Use a series RC network from COMP to FB to compensate the MSL3086/88 regulation loop (Figure 8.1 on page 13). The regulation loop

dynamics are sensitive to output capacitor and inductor values. To begin, determine the right-half-plane zero frequency:

where RLOAD is the minimum equivalent load resistor, or

The output capacitance and type of capacitor affect the regulation loop and method of compensation. In the case of ceramic capacitors

the zero caused by the equivalent series resistance (ESR) is at such a high frequency that it is not of consequence. In the case of

electrolytic or tantalum capacitors the ESR is signicant, so must be considered when compensating the regulation loop. Determine the

ESR zero frequency by the equation:

where C

OUT

is the value of the output capacitor, and ESR is the Equivalent Series Resistance of the output capacitor. Assure that the loop

crossover frequency is at least 1/5th of the ESR zero frequency.

MSL3086/MSL3087/MSL3088 Datasheet

Page 22 of 26

CHOOSING THE OUTPUT RECTIFIER

The output rectifier passes the inductor current to the output capacitor and load during the switching off-time. Due to the

high boost regulator switching frequency use a Schottky rectifier. Use a Schottky diode that has a current rating at least as

high as that of the external MOSFET, and a voltage rating higher than the maximum boost regulator output voltage.

Schottky rectifiers have very low on voltage and fast switching speed, however at high voltage and temperatures Schottky

leakage current can be significant. Make sure that the rectifier power dissipation is within the rectifier specifications.

Place the MOSFET and rectifier close together and as close to the output capacitor(s) as possible to reduce circuit board

radiated emissions.

LOOP COMPENSATION

Use a series RC network from COMP to FB to compensate the MSL3086/88 regulation loop (Figure 3 on page 13). The

regulation loop dynamics are sensitive to output capacitor and inductor values. To begin, determine the right-half-plane

zero frequency:

LOAD

OUT

RHPZ



where R

LOAD

is the minimum equivalent load resistor, or

)( MAXOUT

OUT

LOAD

 .

The output capacitance and type of capacitor affect the regulation loop and method of compensation. In the case of

ceramic capacitors the zero caused by the equivalent series resistance (ESR) is at such a high frequency that it is not of

consequence. In the case of electrolytic or tantalum capacitors the ESR is significant, so must be considered when

compensating the regulation loop. Determine the ESR zero frequency by the equation:

OUT

ESRZ

CESR







where C

OUT

is the value of the output capacitor, and ESR is the Equivalent Series Resistance of the output capacitor.

Assure that the loop crossover frequency is at least 1/5

of the ESR zero frequency.

Next determine the desired crossover frequency as 1/5

of the lower of the ESR zero f

ESRZ

, the right-half-plane zero f

RHPZ

or the switching frequency f

. The crossover frequency equation is:













































OUTLOADCS

LOAD

TOP

COMP

CRR



where f

is the crossover frequency, R

TOP

is the top side voltage divider resistor (from the output voltage to FB), R

COMP

the resistor of the series RC compensation network. Rearranging the factors of this equation yields the solution for R

COMP

as:

OUTCCSTOPCOMP

C f RR R     



21 1 .

These equations are accurate if the compensation zero (formed by the compensation resistor R

COMP

and the

compensation capacitor C

COMP

) happens at a lower frequency than crossover. Therefore the next step is to choose the

compensation capacitor such that the compensation zero is 1/5

of the crossover frequency, or:

COMPCOMP

COMPZ



MSL3086/MSL3087/MSL3088 Datasheet

Page 22 of 26

CHOOSING THE OUTPUT RECTIFIER

The output rectifier passes the inductor current to the output capacitor and load during the switching off-time. Due to the

high boost regulator switching frequency use a Schottky rectifier. Use a Schottky diode that has a current rating at least as

high as that of the external MOSFET, and a voltage rating higher than the maximum boost regulator output voltage.

Schottky rectifiers have very low on voltage and fast switching speed, however at high voltage and temperatures Schottky

leakage current can be significant. Make sure that the rectifier power dissipation is within the rectifier specifications.

Place the MOSFET and rectifier close together and as close to the output capacitor(s) as possible to reduce circuit board

radiated emissions.

LOOP COMPENSATION

Use a series RC network from COMP to FB to compensate the MSL3086/88 regulation loop (Figure 3 on page 13). The

regulation loop dynamics are sensitive to output capacitor and inductor values. To begin, determine the right-half-plane

zero frequency:

LOAD

OUT

RHPZ



where R

LOAD

is the minimum equivalent load resistor, or

)(

MAXOUT

OUT

LOAD



The output capacitance and type of capacitor affect the regulation loop and method of compensation. In the case of

ceramic capacitors the zero caused by the equivalent series resistance (ESR) is at such a high frequency that it is not of

consequence. In the case of electrolytic or tantalum capacitors the ESR is significant, so must be considered when

compensating the regulation loop. Determine the ESR zero frequency by the equation:

OUT

ESRZ

CESR







where C

OUT

is the value of the output capacitor, and ESR is the Equivalent Series Resistance of the output capacitor.

Assure that the loop crossover frequency is at least 1/5

of the ESR zero frequency.

Next determine the desired crossover frequency as 1/5

of the lower of the ESR zero f

ESRZ

, the right-half-plane zero f

RHPZ

or the switching frequency f

. The crossover frequency equation is:













































OUTLOADCS

LOAD

TOP

COMP

CRR



where f

is the crossover frequency, R

TOP

is the top side voltage divider resistor (from the output voltage to FB), R

COMP

the resistor of the series RC compensation network. Rearranging the factors of this equation yields the solution for R

COMP

as:

OUTCCSTOPCOMP

C f RR R     



21 1 .

These equations are accurate if the compensation zero (formed by the compensation resistor R

COMP

and the

compensation capacitor C

COMP

) happens at a lower frequency than crossover. Therefore the next step is to choose the

compensation capacitor such that the compensation zero is 1/5

of the crossover frequency, or:

COMPCOMP

COMPZ

