Datasheet

21
Atmel MSL3086/MSL3088 Datasheet
8-String 60mA LED Drivers with Integrated Boost Controller and Phase Shifted Dimming
Next determine the desired crossover frequency as 1/5th of the lower of the ESR zero f
ESRZ
, the right-half-plane zero f
RHPZ
or the
switching frequency fSW. The crossover frequency equation is:
where f
C
is the crossover frequency, R
TOP
is the top side voltage divider resistor (from the output voltage to FB), R
COMP
is the resistor of
the series RC compensation network. Rearranging the factors of this equation yields the solution for R
COMP
as:
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 1/5th of the crossover frequency, or:
Solving for C
COMP
:
Example:
As an example, set the maximum (un-optimized) output voltage to 39V, using voltage divider as follows:
R
TOP
= 49.9k
R
BOTTOM
= 3.40k
Let the load current be 800mA maximum, use 10µH inductor, a 20µF output capacitor, a 12V input voltage, a 0.25Ω R
CS
, and the
switching frequency is 625kHz.
Set the crossover frequency to 1/5th f
RHPZ
:
Next calculate the compensation resistor value to achieve the 15kHz crossover frequency, or
Then calculate the compensation capacitor, C
COMP
, to set the compensation zero to 1/5th of the crossover frequency, or 3kHz
When laying out the circuit board, place the voltage divider resistors and compensation resistor/capacitors as close to the MSL3086/88
as possible and minimize trace lengths connected to COMP and FB.
MSL3086/MSL3087/MSL3088 Datasheet
Page 22 of 26
© Atmel Inc., 2011. All rights reserved.
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:
L
R
V
V
f
LOAD
OUT
IN
RHPZ
 2
2
,
where R
LOAD
is the minimum equivalent load resistor, or
)( MAXOUT
OUT
LOAD
I
V
R
 .
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
f


 2
1
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
th
of the ESR zero frequency.
Next determine the desired crossover frequency as 1/5
th
of the lower of the ESR zero f
ESRZ
, the right-half-plane zero f
RHPZ
or the switching frequency f
SW
. The crossover frequency equation is:























OUTLOADCS
LOAD
TOP
COMP
C
CRR
R
R
R
f
 2
1
11
,
where f
C
is the crossover frequency, R
TOP
is the top side voltage divider resistor (from the output voltage to FB), R
COMP
is
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
th
of the crossover frequency, or:
COMPCOMP
C
COMPZ
CR
f
f
 2
1
5
.
MSL3086/MSL3087/MSL3088 Datasheet
Page 22 of 26
© Atmel Inc., 2011. All rights reserved.
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:
L
R
V
V
f
LOAD
OUT
IN
RHPZ
 2
2
,
where R
LOAD
is the minimum equivalent load resistor, or
)( MAXOUT
OUT
LOAD
I
V
R
 .
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
f


 2
1
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
th
of the ESR zero frequency.
Next determine the desired crossover frequency as 1/5
th
of the lower of the ESR zero f
ESRZ
, the right-half-plane zero f
RHPZ
or the switching frequency f
SW
. The crossover frequency equation is:























OUTLOADCS
LOAD
TOP
COMP
C
CRR
R
R
R
f
 2
1
11
,
where f
C
is the crossover frequency, R
TOP
is the top side voltage divider resistor (from the output voltage to FB), R
COMP
is
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
th
of the crossover frequency, or:
COMPCOMP
C
COMPZ
CR
f
f
 2
1
5
.
MSL3086/MSL3087/MSL3088 Datasheet
Page 23 of 26
© Atmel Inc., 2011. All rights reserved.
Solving for C
COMP
:
CCOMP
COMP
fR
C
 2
5
.
Example:
As an example, set the maximum (un-optimized) output voltage to 39V, using voltage divider as follows:
R
TOP
= 49.9k
R
BOTTOM
= 3.40k
Let the load current be 800mA maximum, use 10uH inductor, a 20F output capacitor, a 12V input voltage, a 12m R
CS
,
and the switching frequency is 625kHz.
 75.48
8.0
39
A
V
I
V
R
LOAD
OUT
LOAD

kHz
L
R
V
V
f
LOAD
OUT
IN
RHPZ
73
10102
75.48
39
12
2
6
2
2

































Set the crossover frequency to 1/5
th
f
RHPZ
:
kHz
f
f
RHPZ
C
6 .1 4
5

.
Next calculate the compensation resistor value to achieve the 15kHz crossover frequency, or







     kF kkC f RR R
OUTCCSTOPCOMP
9.2520152025.119.4921 1



Then calculate the compensation capacitor, C
COMP
, to set the compensation zero to 1/5
th
of the crossover frequency, or
3kHz
nF
kkfR
C
COMPZCOMP
COMP
1.2
3 2 52
1
2
1






.
When laying out the circuit board, place the voltage divider resistors and compensation resistor/capacitors as close to the
MSL3086/88 as possible and minimize trace lengths connected to COMP and FB.
LED Dimming Control
E
XTERNAL AND I
2
C CONTROL OF LED BRIGHTNESS
Control MSL3086/87 LED brightness using Pulse Width Modulation (PWM) with a PWM signal applied to the external
PWM input. The PWM dimming signals (outputs) take the frequency and duty cycle of the input signal but are staggered
in time so that they start at evenly spaced intervals relative to the PWM input signal. When one or more strings are
disabled by fault response, the stagger delays automatically re-calculate for the remaining enabled strings.
MSL3086/MSL3087/MSL3088 Datasheet
Page 23 of 26
© Atmel Inc., 2011. All rights reserved.
Solving for C
COMP
:
CCOMP
COMP
fR
C
 2
5
.
Example:
As an example, set the maximum (un-optimized) output voltage to 39V, using voltage divider as follows:
R
TOP
= 49.9k
R
BOTTOM
= 3.40k
Let the load current be 800mA maximum, use 10uH inductor, a 20F output capacitor, a 12V input voltage, a 12m R
CS
,
and the switching frequency is 625kHz.
 75.48
8.0
39
A
V
I
V
R
LOAD
OUT
LOAD

kHz
L
R
V
V
f
LOAD
OUT
IN
RHPZ
73
10102
75.48
39
12
2
6
2
2

































Set the crossover frequency to 1/5
th
f
RHPZ
:
kHz
f
f
RHPZ
C
6 .1 4
5

.
Next calculate the compensation resistor value to achieve the 15kHz crossover frequency, or







     kF kkC f RR R
OUTCCSTOPCOMP
9.2520152025.119.4921 1



Then calculate the compensation capacitor, C
COMP
, to set the compensation zero to 1/5
th
of the crossover frequency, or
3kHz
nF
kkfR
C
COMPZCOMP
COMP
1.2
3 2 52
1
2
1






.
When laying out the circuit board, place the voltage divider resistors and compensation resistor/capacitors as close to the
MSL3086/88 as possible and minimize trace lengths connected to COMP and FB.
LED Dimming Control
E
XTERNAL AND I
2
C CONTROL OF LED BRIGHTNESS
Control MSL3086/87 LED brightness using Pulse Width Modulation (PWM) with a PWM signal applied to the external
PWM input. The PWM dimming signals (outputs) take the frequency and duty cycle of the input signal but are staggered
in time so that they start at evenly spaced intervals relative to the PWM input signal. When one or more strings are
disabled by fault response, the stagger delays automatically re-calculate for the remaining enabled strings.
MSL3086/MSL3087/MSL3088 Datasheet
Page 23 of 26
© Atmel Inc., 2011. All rights reserved.
Solving for C
COMP
:
CCOMP
COMP
fR
C
 2
5
.
Example:
As an example, set the maximum (un-optimized) output voltage to 39V, using voltage divider as follows:
R
TOP
= 49.9k
R
BOTTOM
= 3.40k
Let the load current be 800mA maximum, use 10uH inductor, a 20F output capacitor, a 12V input voltage, a 12m R
CS
,
and the switching frequency is 625kHz.
 75.48
8.0
39
A
V
I
V
R
LOAD
OUT
LOAD

kHz
L
R
V
V
f
LOAD
OUT
IN
RHPZ
73
10102
75.48
39
12
2
6
2
2

































Set the crossover frequency to 1/5
th
f
RHPZ
:
kHz
f
f
RHPZ
C
6 .1 4
5

.
Next calculate the compensation resistor value to achieve the 15kHz crossover frequency, or

    
kF kk
C f RR R
OUTCCSTOPCOMP
9.2520152025.119.4921 1



Then calculate the compensation capacitor, C
COMP
, to set the compensation zero to 1/5
th
of the crossover frequency, or
3kHz
nF
kkfR
C
COMPZCOMP
COMP
1.2
3 2 52
1
2
1






.
When laying out the circuit board, place the voltage divider resistors and compensation resistor/capacitors as close to the
MSL3086/88 as possible and minimize trace lengths connected to COMP and FB.
LED Dimming Control
E
XTERNAL AND I
2
C CONTROL OF LED BRIGHTNESS
Control MSL3086/87 LED brightness using Pulse Width Modulation (PWM) with a PWM signal applied to the external
PWM input. The PWM dimming signals (outputs) take the frequency and duty cycle of the input signal but are staggered
in time so that they start at evenly spaced intervals relative to the PWM input signal. When one or more strings are
disabled by fault response, the stagger delays automatically re-calculate for the remaining enabled strings.
MSL3086/MSL3087/MSL3088 Datasheet
Page 23 of 26
© Atmel Inc., 2011. All rights reserved.
Solving for C
COMP
:
CCOMP
COMP
fR
C
 2
5
.
Example:
As an example, set the maximum (un-optimized) output voltage to 39V, using voltage divider as follows:
R
TOP
= 49.9k
R
BOTTOM
= 3.40k
Let the load current be 800mA maximum, use 10uH inductor, a 20F output capacitor, a 12V input voltage, a 12m R
CS
,
and the switching frequency is 625kHz.
 75.48
8.0
39
A
V
I
V
R
LOAD
OUT
LOAD

kHz
L
R
V
V
f
LOAD
OUT
IN
RHPZ
73
10102
75.48
39
12
2
6
2
2

































Set the crossover frequency to 1/5
th
f
RHPZ
:
kHz
f
f
RHPZ
C
6 .1 4
5

.
Next calculate the compensation resistor value to achieve the 15kHz crossover frequency, or







     kF kkC f RR R
OUTCCSTOPCOMP
9.2520152025.119.4921 1



Then calculate the compensation capacitor, C
COMP
, to set the compensation zero to 1/5
th
of the crossover frequency, or
3kHz
nF
kkfR
C
COMPZCOMP
COMP
1.2
3 2 52
1
2
1






.
When laying out the circuit board, place the voltage divider resistors and compensation resistor/capacitors as close to the
MSL3086/88 as possible and minimize trace lengths connected to COMP and FB.
LED Dimming Control
E
XTERNAL AND I
2
C CONTROL OF LED BRIGHTNESS
Control MSL3086/87 LED brightness using Pulse Width Modulation (PWM) with a PWM signal applied to the external
PWM input. The PWM dimming signals (outputs) take the frequency and duty cycle of the input signal but are staggered
in time so that they start at evenly spaced intervals relative to the PWM input signal. When one or more strings are
disabled by fault response, the stagger delays automatically re-calculate for the remaining enabled strings.