Datasheet

13
Atmel MSL3086/MSL3088 Datasheet
8-String 60mA LED Drivers with Integrated Boost Controller and Phase Shifted Dimming
8.3 Boost Regulator Overview
The MSL3086/88 boost regulator boosts the input voltage up to the regulated output voltage. For design details see the section “Boost
Regulator” beginning on page 17, the following text presents an overview of the boost regulator controller.
The boost regulator uses an external switching MOSFET, current sense resistor, inductor, rectier, and input and output capacitors for
this purpose (Figure 8.1). Because the MOSFET and current sense resistor are external, the boost regulator operates over a wide range
of input and output voltage, and LED current congurations. It includes a 2.5V reference voltage, xed slope compensation and external
voltage regulator compensation to optimize the control loop for each conguration. Because the boost regulator components are external,
it is useful for a number of topologies such as SEPIC, yback, and single-switch forward converters. The boost regulator includes soft-
start, adjustable cycle-by-cycle current limiting, and output over-voltage fault detection.
Figure 8.1. Power section of MSL3086/MSL3088
8.4 Error Amplier
The internal error amplier compares the external divided output voltage at FB to the internal 2.5V reference voltage to set the regulated
output voltage. The error amplier output voltage at COMP is externally accessible and is used in conjunction with an external RC
network to compensate the voltage regulator. FB also drives the integrated boost over-voltage comparator that detects if the output
voltage exceeds the regulation voltage, to generate a fault condition. The error amplier internally controls the current mode PWM
regulator.
MSL3086/MSL3087/MSL3088 Datasheet
Page 13 of 26
© Atmel Inc., 2011. All rights reserved.
CS
GATE
FB
COMP
MSL3086
V
LED
V
PWR
L1
R
CS
C
COMP1
R
COMP
R
BOTTOM
C
OUT
C
COMP2
C
IN
GND
STRn
LED
CURRENT
SINK
EFFICIENCY
OPTIMIZER
SOFT
START
SENSE
LIMIT
CURRENT
GATE DRIVE
+
REF
SLOPE
COMP
0.4V
R
ESR
R
TOP
D1
Q1
DROPOUT DETECT
COMPENSATION:
C
COMP2
= POLE
C
COMP1
, R
COMP
= ZERO
A = 11
0.1 V
PVIN
5V
1.1V
1.5V
Figure 3. Power section of MSL3086/MSL3088
ERROR AMPLIFIER
The internal error amplifier compares the external divided output voltage at FB to the internal 2.5V reference voltage to set
the regulated output voltage. The error amplifier output voltage at COMP is externally accessible and is used in
conjunction with an external RC network to compensate the voltage regulator. FB also drives the integrated boost over-
voltage comparator that detects if the output voltage exceeds the regulation voltage, to generate a fault condition. The
error amplifier internally controls the current mode PWM regulator.
GATE DRIVER
The gate driver drives the gate of the external boost regulator switching MOSFET. The drain of the switching MOSFET in
turn drives the boost inductor and rectifier to boost the input voltage to the regulated output voltage. The gate driver
sources and sinks up to 1A allowing fast switching speed and allows the use of MOSFETs with high gate capacitance.
The gate driver power is separated from the internal circuitry power to reduce internal noise and to allow separate gate
driver bypassing for optimal performance.
SOFT-START
The boost regulator includes a built in soft-start to prevent excessive input current overshoot at turn-on. The soft-start
ramps the output regulation voltage from 0V at turn-on to the as-configured regulation output voltage over 1.6ms. Note
that the boost regulator only controls output voltages greater than the input voltage; when the soft-start sets the regulation
voltage below the input voltage, the actual output voltage remains at approximately the input voltage.