Datasheet

20
Atmel MSL3080 Datasheet
8 String 60mA LED Driver with Integrated Boost Controller
9.7 Setting the Boost Regulator Output Voltage
Select the voltage divider resistors (R
TOP
and R
BOTTOM
in Figure 8.1 on page 14) to set the boost regulator output voltage by rst
determining V
OUT(MIN)
and V
OUT(MAX)
, the required minimum and maximum LED string anode power supply (boost regulator) voltage,
using:
and
where V
f(MIN)
and V
f(MAX)
are the LED’s minimum and maximum forward voltage drops at the full-scale current set by R
ILED
(page 17).
For example, if the LED minimum forward voltage is V
f(MIN)
= 3.5V and maximum is V
f(MAX)
= 3.8V, using 10 LEDs in a string, the total
minimum and maximum voltage drop across a string is 35V and 38V. Adding allowance of 0.5V for the current regulator headroom brings
V
OUT(MIN)
to 35.5V and V
OUT(MAX)
to 38.5. Next determine R
TOP
using:
Where 350 x 10
-6
is the maximum EO output current.
Then determine R
BOTTOM
using:
where 2.5V is the internal reference voltage.
9.8 Choosing the Input and output Capacitors
The input and output capacitors carry the high frequency current due to the boost regulator switching. The input capacitor prevents this high
frequency current from travelling back to the input voltage source, reducing conducted and radiated noise. The output capacitor prevents high
frequency current to the load, in this case the LEDs, and also prevents conducted and radiated noise. The output capacitors also have a large
effect on the boost regulator loop stability and transient response, and so are critical to optimal boost regulator operation.
Use ceramic input and output capacitors that keep their rated capacitance values at the expected operating voltages. The Typical
Application Circuit on page 10 shows recommended values for and 10 LEDs and 60mA per string. Use a bulk electrolytic capacitor where
power enters the circuit board.
9.9 Choosing the Inductor
The boost regulator inductor takes the current from the input source and directs that current to the load. Using the proper inductor is critical
to proper boost regulator operation. Choose an inductor with sufcient inductance to keep the inductor ripple current within limits, and with
sufcient current handling capability for steady-state and transient conditions.
The boost regulator switching causes ripple current through the inductor. The current rises during the on-time and falls during the off time.
The slope of the inductor current is a function of the voltage across the inductor, and so the total change in current, ΔI
L
, is the current slope
multiplied by the time in that phase (on time, t
ON
, or off time, t
OFF
). In steady-state, where the load current, input voltage, and output voltage
are all constant, the inductor current does not change over one cycle, and so the amount the current rises during the on time is the same as
the amount the current drops during the off time. Calculate the duty cycle (equal to the on-time divided by the switching period) using:
where V
OUT
is the output voltage and V
IN
is the input voltage. Calculate the on-time in seconds using:
where f
SW
is the boost regulator switching frequency. Calculate the inductor ripple current using:
where L is the inductance value in Henrys. Choose a value for L that produces a ripple current in the range of 25% to 50% of the steady
state DC inductor current. The steady state DC inductor current is equal to the input current. Estimate the steady-state DC input current
using:
where I
LOAD
is the sum of all strings steady-state LED currents with all LEDs on simultaneously, V
OUT
is the maximum (un-optimized)
boost regulator output voltage, and V
IN
is the minimum boost regulator input voltage.
MSL3050/MSL3060/MSL3080 Datasheet
Preliminary
MSL3050/60/80 preliminary datasheet revision 0.1 Page 20 of 24
© Atmel Inc., 2011. All rights reserved.



5.0#
)()(
 ofLEDsVV
MINfMINOUT
, and



5.0#
)()(
 ofLEDsVV
MAXfMAXOUT
,
where V
f(MIN)
and V
f(MAX)
are the LED’s minimum and maximum forward voltage drops at the full-scale current set by R
ILED
(page 16). For example, if the LED minimum forward voltage is V
f(MIN)
= 3.5V and maximum is V
f(MAX)
= 3.8V, using 10
LEDs in a string, the total minimum and maximum voltage drop across a string is 35V and 38V. Adding allowance of 0.5V
for the current regulator headroom brings V
OUT(MIN)
to 35.5V and V
OUT(MAX)
to 38.5. Next determine R
TOP
using:




6
)()(
10365
MINOUTMAXOUT
TOP
VV
R
,
Where 3.65x10
-6
is the maximum EO output current.
Then determine R
BOTTOM
using:



5.2
5.2
)(MAXOUT
TOPBOTTOM
V
RR
,
where 2.5V is the internal reference voltage.
Other Boost Regulator Components
Use the component values shown in the Typical Applications Circuits beginning on page 10. When custom boost regulator
design is required use the guidelines presented in the following sections.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
The input and output capacitors carry the high frequency current due to the boost regulator switching. The input capacitor
prevents this high frequency current from travelling back to the input voltage source, reducing conducted and radiated
noise. The output capacitor prevents high frequency current to the load, in this case the LEDs, and also prevents
conducted and radiated noise. The output capacitors also have a large effect on the boost regulator loop stability and
transient response, and so are critical to optimal boost regulator operation.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
Use ceramic input and output capacitors that keep their rated capacitance values at the expected operating voltages. The
Typical Application Circuit on page 10 shows recommended values for and 10 LEDs and 60mA per string. Use a bulk
electrolytic capacitor where power enters the circuit board.
CHOOSING THE INDUCTOR
The boost regulator inductor takes the current from the input source and directs that current to the load. Using the proper
inductor is critical to proper boost regulator operation. Choose an inductor with sufficient inductance to keep the inductor
ripple current within limits, and with sufficient current handling capability for steady-state and transient conditions.
The boost regulator switching causes ripple current through the inductor. The current rises during the on-time and falls
during the off time. The slope of the inductor current is a function of the voltage across the inductor, and so the total
change in current, ∆I
L
, is the current slope multiplied by the time in that phase (on time, t
ON
, or off time, t
OFF
). In steady-
state, where the load current, input voltage, and output voltage are all constant, the inductor current does not change over
one cycle, and so the amount the current rises during the on time is the same as the amount the current drops during the
off time. Calculate the duty cycle (equal to the on-time divided by the switching period) using:



5.0#
) () (
 ofLEDsVV
MAXfMAXOUT
,
where V
f(MIN)
and V
f(MAX)
are the LED’s minimum and maximum forward voltage drops at the full-scale current set by R
ILED
(page 14). For example, if the LED minimum forward voltage is V
f(MIN)
= 3.5V and maximum is V
f(MAX)
= 3.8V, using 10
LEDs in a string, the total minimum and maximum voltage drop across a string is 35V and 38V. Adding allowance of 0.5V
for the current regulator headroom brings V
OUT(MIN)
to 35.5V and V
OUT(MAX)
to 38.5. Next determine R
TOP
using:




6
)()(
10365
MINOUTMAXOUT
TOP
VV
R
.
Then determine R
BOTTOM
using:



5.2
5.2
)( MAXOUT
TOPBOTTOM
V
RR
.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
The input and output capacitors carry the high frequency current due to the boost regulator switching. The input capacitor
prevents this high frequency current from travelling back to the input voltage source, reducing conducted and radiated
noise. The output capacitor prevents high frequency current to the load, in this case the LEDs, and also prevents
conducted and radiated noise. The output capacitors also have a large effect on the boost regulator loop stability and
transient response, and so are critical to optimal boost regulator operation.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
Use ceramic input and output capacitors that keep their rated capacitance values at the expected operating voltages. The
Typical Application Circuit on page 10 shows recommended values for and 10 LEDs and 120mA per string. Use a bulk
electrolytic capacitor where power enters the circuit board.
CHOOSING THE INDUCTOR
The boost regulator inductor takes the current from the input source and directs that current to the load. Using the proper
inductor is critical to proper boost regulator operation. Choose an inductor with sufficient inductance to keep the inductor
ripple current within limits, and with sufficient current handling capability for steady-state and transient conditions.
The boost regulator switching causes ripple current through the inductor. The current rises during the on-time and falls
during the off time. The slope of the inductor current is a function of the voltage across the inductor, and so the total
change in current, ∆I
L
, is the current slope multiplied by the time in that phase (on time, t
ON
, or off time, t
OFF
). In steady-
state, where the load current, input voltage, and output voltage are all constant, the inductor current does not change over
one cycle, and so the amount the current rises during the on time is the same as the amount the current drops during the
off time. Calculate the duty cycle (equal to the on-time divided by the switching period) using:
IN
INOUT
V
VV
D


,
where V
OUT
is the output voltage and V
IN
is the input voltage.
Calculate the on-time in seconds using:
SWIN
INOUT
SW
ON
fV
VV
f
D
t



,
Page 18 of 22
© Atmel Inc., 2011. All rights reserved.



5.0#
) () (
 ofLEDsVV
MAXfMAXOUT
,
where V
f(MIN)
and V
f(MAX)
are the LED’s minimum and maximum forward voltage drops at the full-scale current set by R
ILED
(page 14). For example, if the LED minimum forward voltage is V
f(MIN)
= 3.5V and maximum is V
f(MAX)
= 3.8V, using 10
LEDs in a string, the total minimum and maximum voltage drop across a string is 35V and 38V. Adding allowance of 0.5V
for the current regulator headroom brings V
OUT(MIN)
to 35.5V and V
OUT(MAX)
to 38.5. Next determine R
TOP
using:




6
)()(
10365
MINOUTMAXOUT
TOP
VV
R
.
Then determine R
BOTTOM
using:



5.2
5.2
)( MAXOUT
TOPBOTTOM
V
RR
.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
The input and output capacitors carry the high frequency current due to the boost regulator switching. The input capacitor
prevents this high frequency current from travelling back to the input voltage source, reducing conducted and radiated
noise. The output capacitor prevents high frequency current to the load, in this case the LEDs, and also prevents
conducted and radiated noise. The output capacitors also have a large effect on the boost regulator loop stability and
transient response, and so are critical to optimal boost regulator operation.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
Use ceramic input and output capacitors that keep their rated capacitance values at the expected operating voltages. The
Typical Application Circuit on page 10 shows recommended values for and 10 LEDs and 120mA per string. Use a bulk
electrolytic capacitor where power enters the circuit board.
CHOOSING THE INDUCTOR
The boost regulator inductor takes the current from the input source and directs that current to the load. Using the proper
inductor is critical to proper boost regulator operation. Choose an inductor with sufficient inductance to keep the inductor
ripple current within limits, and with sufficient current handling capability for steady-state and transient conditions.
The boost regulator switching causes ripple current through the inductor. The current rises during the on-time and falls
during the off time. The slope of the inductor current is a function of the voltage across the inductor, and so the total
change in current, ∆I
L
, is the current slope multiplied by the time in that phase (on time, t
ON
, or off time, t
OFF
). In steady-
state, where the load current, input voltage, and output voltage are all constant, the inductor current does not change over
one cycle, and so the amount the current rises during the on time is the same as the amount the current drops during the
off time. Calculate the duty cycle (equal to the on-time divided by the switching period) using:
IN
INOUT
V
VV
D


,
where V
OUT
is the output voltage and V
IN
is the input voltage.
Calculate the on-time in seconds using:
SWIN
INOUT
SW
ON
fV
VV
f
D
t



,
Page 18 of 22
© Atmel Inc., 2011. All rights reserved.



5.0#
) () (
 ofLEDsVV
MAXfMAXOUT
,
where V
f(MIN)
and V
f(MAX)
are the LED’s minimum and maximum forward voltage drops at the full-scale current set by R
ILED
(page 14). For example, if the LED minimum forward voltage is V
f(MIN)
= 3.5V and maximum is V
f(MAX)
= 3.8V, using 10
LEDs in a string, the total minimum and maximum voltage drop across a string is 35V and 38V. Adding allowance of 0.5V
for the current regulator headroom brings V
OUT(MIN)
to 35.5V and V
OUT(MAX)
to 38.5. Next determine R
TOP
using:




6
)()(
10365
MINOUTMAXOUT
TOP
VV
R
.
Then determine R
BOTTOM
using:



5.2
5.2
)( MAXOUT
TOPBOTTOM
V
RR
.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
The input and output capacitors carry the high frequency current due to the boost regulator switching. The input capacitor
prevents this high frequency current from travelling back to the input voltage source, reducing conducted and radiated
noise. The output capacitor prevents high frequency current to the load, in this case the LEDs, and also prevents
conducted and radiated noise. The output capacitors also have a large effect on the boost regulator loop stability and
transient response, and so are critical to optimal boost regulator operation.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
Use ceramic input and output capacitors that keep their rated capacitance values at the expected operating voltages. The
Typical Application Circuit on page 10 shows recommended values for and 10 LEDs and 120mA per string. Use a bulk
electrolytic capacitor where power enters the circuit board.
CHOOSING THE INDUCTOR
The boost regulator inductor takes the current from the input source and directs that current to the load. Using the proper
inductor is critical to proper boost regulator operation. Choose an inductor with sufficient inductance to keep the inductor
ripple current within limits, and with sufficient current handling capability for steady-state and transient conditions.
The boost regulator switching causes ripple current through the inductor. The current rises during the on-time and falls
during the off time. The slope of the inductor current is a function of the voltage across the inductor, and so the total
change in current, ∆I
L
, is the current slope multiplied by the time in that phase (on time, t
ON
, or off time, t
OFF
). In steady-
state, where the load current, input voltage, and output voltage are all constant, the inductor current does not change over
one cycle, and so the amount the current rises during the on time is the same as the amount the current drops during the
off time. Calculate the duty cycle (equal to the on-time divided by the switching period) using:
IN
INOUT
V
VV
D


,
where V
OUT
is the output voltage and V
IN
is the input voltage.
Calculate the on-time in seconds using:
SWIN
INOUT
SW
ON
fV
VV
f
D
t



,
Page 18 of 22
© Atmel Inc., 2011. All rights reserved.
MSL3050/MSL3060/MSL3080 Datasheet
Preliminary
MSL3050/60/80 preliminary datasheet revision 0.1 Page 21 of 24
© Atmel Inc., 2011. All rights reserved.
IN
INOUT
V
VV
D


,
where V
OUT
is the output voltage and V
IN
is the input voltage.
Calculate the on-time in seconds using:
SWIN
INOUT
SW
ON
fV
VV
f
D
t



,
where f
SW
is the boost regulator switching frequency.
Calculate the inductor ripple current using:

LfV
VVV
L
tV
I
SWIN
INOUTINONIN
L





,
where L is the inductance value in Henrys. Choose a value for L that produces a ripple current in the range of 25% to 50%
of the steady state DC inductor current. The steady state DC inductor current is equal to the input current. Estimate the
steady-state DC input current using:
IN
OUT
LOADIN
V
V
II
,
where I
LOAD
is the sum of all strings steady-state LED currents with all LEDs on simultaneously, V
OUT
is the maximum (un-
optimized) boost regulator output voltage, V
IN
is the minimum boost regulator input voltage, and n is the boost regulator
efficiency.
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.
SETTING THE EXTERNAL MOSFET 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 TBDmV minimum. Choose the current sense resistor value to set
the current limit using:

)(MAXL
CS
I
TBD
R
, (TBD = the minimum CS Current Limit Threshold Voltage from the Electrical Characteristics Table)
where I
L(MAX)
is the maximum inductor current. When R
CS
is not equal to a standard 1% resistor value use the next lower
value.
where f
SW
is the boost regulator switching frequency.
Calculate the inductor ripple current using:
L fV
VV V
L
tV
I
SWOUT
INOUTINONIN
L
,
where L is the inductance value in Henrys. Choose a value for L that produces a ripple current in the range of 25% to 50%
of the steady state DC inductor current. The steady state DC inductor current is equal to the input current. Estimate the
steady-state DC input current using:







IN
OUT
LOADIN
V
V
II
,
where I
LOAD
is the sum of all strings steady-state LED currents with all LEDs on simultaneously, V
OUT
is the maximum (un-
optimized) boost regulator output voltage, and V
IN
is the minimum boost regulator input voltage.
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.
SETTING THE EXTERNAL MOSFET 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 TBDmV minimum. Choose the current sense resistor value to set
the current limit using:

)(
1.0
MAXL
CS
I
R
,
where I
L(MAX)
is the maximum inductor current.
CHOOSING THE SWITCHING MOSFET
The MSL3040/41 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 rectifier recovery artefacts. Make sure that the MOSFET package can withstand the worst-case power
dissipation while maintaining die temperature within the MOSFET ratings.
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.
Page 19 of 22
© Atmel Inc., 2011. All rights reserved.
MSL3050/MSL3060/MSL3080 Datasheet
Preliminary
MSL3050/60/80 preliminary datasheet revision 0.1 Page 20 of 24
© Atmel Inc., 2011. All rights reserved.



5.0#
)()(
 ofLEDsVV
MINfMINOUT
, and



5.0#
)()(
 ofLEDsVV
MAXfMAXOUT
,
where V
f(MIN)
and V
f(MAX)
are the LED’s minimum and maximum forward voltage drops at the full-scale current set by R
ILED
(page 16). For example, if the LED minimum forward voltage is V
f(MIN)
= 3.5V and maximum is V
f(MAX)
= 3.8V, using 10
LEDs in a string, the total minimum and maximum voltage drop across a string is 35V and 38V. Adding allowance of 0.5V
for the current regulator headroom brings V
OUT(MIN)
to 35.5V and V
OUT(MAX)
to 38.5. Next determine R
TOP
using:




6
)()(
10365
MINOUTMAXOUT
TOP
VV
R
,
Where 3.65x10
-6
is the maximum EO output current.
Then determine R
BOTTOM
using:



5.2
5.2
)(MAXOUT
TOPBOTTOM
V
RR
,
where 2.5V is the internal reference voltage.
Other Boost Regulator Components
Use the component values shown in the Typical Applications Circuits beginning on page 10. When custom boost regulator
design is required use the guidelines presented in the following sections.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
The input and output capacitors carry the high frequency current due to the boost regulator switching. The input capacitor
prevents this high frequency current from travelling back to the input voltage source, reducing conducted and radiated
noise. The output capacitor prevents high frequency current to the load, in this case the LEDs, and also prevents
conducted and radiated noise. The output capacitors also have a large effect on the boost regulator loop stability and
transient response, and so are critical to optimal boost regulator operation.
CHOOSING THE INPUT AND OUTPUT CAPACITORS
Use ceramic input and output capacitors that keep their rated capacitance values at the expected operating voltages. The
Typical Application Circuit on page 10 shows recommended values for and 10 LEDs and 60mA per string. Use a bulk
electrolytic capacitor where power enters the circuit board.
CHOOSING THE INDUCTOR
The boost regulator inductor takes the current from the input source and directs that current to the load. Using the proper
inductor is critical to proper boost regulator operation. Choose an inductor with sufficient inductance to keep the inductor
ripple current within limits, and with sufficient current handling capability for steady-state and transient conditions.
The boost regulator switching causes ripple current through the inductor. The current rises during the on-time and falls
during the off time. The slope of the inductor current is a function of the voltage across the inductor, and so the total
change in current, ∆I
L
, is the current slope multiplied by the time in that phase (on time, t
ON
, or off time, t
OFF
). In steady-
state, where the load current, input voltage, and output voltage are all constant, the inductor current does not change over
one cycle, and so the amount the current rises during the on time is the same as the amount the current drops during the
off time. Calculate the duty cycle (equal to the on-time divided by the switching period) using: