MSL3086-IUR Atmel, MSL3086-IUR Datasheet - Page 20

• Manufacturer Part Number: MSL3086-IUR
• Description: LED Lighting Drivers 8-Str LED Driver Internal Boost Cntr
• Manufacturer: Atmel

Specifications of MSL3086-IUR

• Rohs: yes
• Input Voltage: 5 V
• Operating Frequency: 200 Hz
• Maximum Supply Current: 18 mA
• Output Current: 350 uA
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: VQFN-24
• Minimum Operating Temperature: - 40 C
• Power Dissipation: 1850 mW

9.10 Setting the Current Limit
9.11 Choosing the Switching MOSFET
9.12 Choosing the Output Rectifier
9.13 Loop Compensation
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.
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
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
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 artIfacts. Make sure that the
MOSFET package can withstand the worst-case power dissipation while maintaining die temperature within the MOSFET ratings.
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.
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 significant, so must be considered when compensating the regulation loop. Determine the
ESR zero frequency by the equation:
where C
crossover frequency is at least 1/5th of the ESR zero frequency.
C
The output rectifier passes the inductor current to the output capacitor and load during the switching off-time. Due to th
high boost regulator switching frequency use a Schottky rectifier. Use a Schottky diode that has a current rating at leas
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 Scho
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 boa
radiated emissions.
L
Use a series RC network from COMP to FB to compensate the MSL3086/88 regulation loop (Figure 3 on page 13). Th
regulation loop dynamics are sensitive to output capacitor and inductor values. To begin, determine the right-half-plan
zero frequency:
where 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
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:
where C
Assure that the loop crossover frequency is at least 1/5
Next determine the desired crossover frequency as 1/5
or the switching frequency f
where f
the resistor of the series RC compensation network. Rearranging the factors of this equation yields the solution for R
R
f
f
f
C
The output rectifier passes the inductor current to the output capacitor and load during the switching off-time. Due to t
high boost regulator switching frequency use a Schottky rectifier. Use a Schottky diode that has a current rating at lea
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 Sch
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 bo
radiated emissions.
L
Use a series RC network from COMP to FB to compensate the MSL3086/88 regulation loop (Figure 3 on page 13). T
regulation loop dynamics are sensitive to output capacitor and inductor values. To begin, determine the right-half-pla
zero frequency:
where 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 no
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:
where C
Assure that the loop crossover frequency is at least 1/5
Next determine the desired crossover frequency as 1/5
or the switching frequency f
R
OOP
C
HOOSING THE
RHPZ
f
ESRZ
f
f
LOAD
OOP
HOOSING THE
RHPZ
ESRZ
C
LOAD


L(MAX)
OUT




C
C




R

C
LOAD
OUT

R
R
OMPENSATION
is the crossover frequency, R
2
COMP
LOAD
is the value of the output capacitor, and ESR is the Equivalent Series Resistance of the output capacitor. Assure that the loop
OUT
R
I
OMPENSATION
TOP
2
is the maximum inductor current.

V
COMP
I
OUT
TOP
V
V

V
OUT
OUT
is the value of the output capacitor, and ESR is the Equivalent Series Resistance of the output capacitor.
V

IN
is the minimum equivalent load resistor, or
V
is the value of the output capacitor, and ESR is the Equivalent Series Resistance of the output capacitor.
OUT
OUT

is the minimum equivalent load resistor, or
IN
(MAX



OUT
ESR
(MAX



ESR


O
2
1



2
O
1
)
11



UTPUT
)

11
.
R
UTPUT

.
C
R

LOAD
R
C

LOAD
OUT
2
R
R
LOAD
OUT
2

R
LOAD
CS

L
CS
R
L
SW



R
SW
ECTIFIER




. The crossover frequency equation is:
ECTIFIER
,

. The crossover frequency equation is:



,



2
2

MSL3086/MSL3087/MSL3088 Datashee

MSL3086/MSL3087/MSL3088 Datashe


TOP
R
R
LOAD
LOAD
is the top side voltage divider resistor (from the output voltage to FB), R
1
1


C
C
OUT
8-String 60mA LED Drivers with Integrated Boost Controller and Phase Shifted Dimming
OUT
DSon






,
th
th
,
th
th
while maintaining minimal gate charge for fast switching speed.
of the lower of the ESR zero f
of the ESR zero frequency.
of the lower of the ESR zero f
of the ESR zero frequency.
Atmel MSL3086/MSL3088 Datasheet
ESRZ
ESRZ
, the right-half-plane zero f
, the right-half-plane zero f
20
COMP
CO
RH