A8507 Datasheet

A8507
LED Backlight Driver for LCD Monitors and Televisions
Discontinued Product
This device is no longer in production. The device should not be
purchased for new design applications. Samples are no longer available.
Date of status change: December 3, 2013
Recommended Substitutions:
For existing customer transition, and for new customers or new applications, contact Allegro Sales.
NOTE: For detailed information on purchasing options, contact your
local Allegro field applications engineer or sales representative.
Allegro MicroSystems, LLC reserves the right to make, from time to time, revisions to the anticipated product life cycle plan
for a product to accommodate changes in production capabilities, alternative product availabilities, or market demand. The
information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its use; nor for any infringements of patents or other rights of third parties which may result from its use.
A8507
LED Backlight Driver for LCD Monitors and Televisions
Features and Benefits
Description
▪ Fixed frequency current mode control with integrated
gate driver
▪ Each individual current sink is capable of 80 mA
▪ Adjustable overvoltage protection (OVP)
▪ Active current sharing between LED strings for ±0.6%
accuracy and matching
▪ 250 kHz to 1 MHz adjustable switching frequency
▪ Open or shorted LED string protection
▪ Overtemperature, cycle-by-cycle current limit, and
undervoltage protection
▪ No audible MLCC noise during PWM dimming
▪ No pull-up resistors required for LED modules that use
ESD capacitors
▪ SOIC 24-pin package for easy single-side PCB
manufacturing or TSSOP 24-pin package with exposed
thermal pad for better thermal performance
The A8507 is a multi-output WLED/RGB driver for
backlighting LCD monitors and televisions. The A8507
integrates a boost controller to drive external MOSFET and
six internal current-sinks. The boost converter is constant
frequency current mode converter.
PWM dimming allows LED currents to be controlled in 500:1
ratio. The LED sinks are capable of sinking up to 80 mA
each, and can be paralleled together to achieve even higher
currents.
The A8507 provides protection against overvoltage, open or
shorted LED string, and overtemperature. A dual level cycleby-cycle current limit function provides soft start and protects
against overloads.
The device is provided in a 24-pin SOICW package (LB), with
internally fused pins for enhanced thermal dissipation, and a
24-pin TSSOP package (LP), with an exposed thermal pad
for enhanced thermal dissipation. Both packages are lead (Pb)
free, with 100% matte tin leadframe plating.
Packages:
24-pin TSSOP
with exposed thermal pad
(Package LP)
Not to scale
24-pin SOICW
with internally fused pins
(LB package)
Functional Block Diagram
VBAT
10 μH
4.7 μF
2.2 μF
P
RSC
P
ROVP
P
SENP
DRIVER
RFSET
OSC
FSET
OVP
Overvoltage
Protection
Ref
OCP SS
CCOMP
SENN
Current Mode
Boost
Controller
COMP
Control and
Feedback
LED1
6
RISET
LED2
ISET
Reference
Current
PWM
LED3
100 kΩ
EN
+5 V
10 kΩ
VIN
Control
Logic/
UVLO
LED Select
Logic
6
Open/Short
LED Detect
6
LED4
TSD
OVP
OCP
LED5
0.1 μF
LED6
FAULT
LB Only
LP Only
PGND
P
A8507-DS, Rev. 2
GND
GND
PAD
LGND
LGND
Figure 1. Functional block diagram
showing 6 parallel strings with 9 series
LEDs per channel
A8507
LED Backlight Driver for LCD Monitors and Televisions
Selection Guide
Part Number
Packing
Package
A8507ELBTR-T
1000 pieces per 13-in. reel
A8507ELPTR-T
4000 pieces per 13-in. reel
24-pin SOICW, with internally fused pins for enhanced
thermal dissipation
24-pin TSSOP, with exposed thermal pad for enhanced
thermal dissipation
Absolute Maximum Ratings
Characteristic
Rating
Unit
VLEDx
–0.3 to 40
V
OVP Pin Input Voltage
VOVP
–0.3 to 50
V
SENP and SENN Pin Input Voltage
VSENx
–0.3 to 1
V
LED Output Voltage
Symbol
–0.3 to 7
V
–40 to 85
ºC
TJ(max)
150
ºC
Tstg
–55 to 150
ºC
Remaining Pins Input Voltage
VIN
Operating Ambient Temperature
TA
Maximum Junction Temperature
Storage Temperature
Notes
Range E
Thermal Characteristics may require derating at maximum conditions, see application information
Characteristic
Package Thermal Resistance
Symbol
RθJA
Value
Unit
Package LB, on 2-layer PCB, 1-in.2 2-oz copper exposed area
Test Conditions*
51
ºC/W
Package LB, on 4-layer PCB, based on JEDEC standard
35
ºC/W
Package LP, 4-layer PCB, based on JEDEC standard
28
ºC/W
*Additional thermal information available on the Allegro website
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
A8507
LED Backlight Driver for LCD Monitors and Televisions
Pin-out Diagrams
PWM 1
PGND 2
24 EN
GND 1
24 EN
23 FAULT
PWM 2
23 FAULT
NC 3
22 LED6
NC 3
22 LED6
NC 4
21 LED5
PGND 4
21 LED5
SENN 5
20 LED4
NC 5
GND 6
19 LGND
SENN 6
GND 7
18 LGND
SENP 7
18 LED3
SENP 8
17 LED3
OVP 8
17 LED2
OVP 9
16 LED2
NC 9
16 LED1
DRIVER 10
15 LED1
DRIVER 10
20 LED4
PAD
19 LGND
15 COMP
VIN 11
14 COMP
VIN 11
14 FSET
ISET 12
13 FSET
ISET 12
13 GND
Package LB
Package LP
Terminal List Table
Number
Name
Function
LB
LP
1
2
PWM
PWM LED-current control; apply logic level PWM for dimming
2
4
PGND
Power ground for external FET gate driver; connect to common star ground and
RSC ground
3,4
3,5,9
NC
5
6
SENN
Connect ground side of current sense resistor RSC
6,7
1,13
GND
Connect to common star ground
8
7
SENP
Connect high side of current sense resistor RSC
9
8
OVP
10
10
DRIVER
No internal electrical connection to these pins
Connect this pin to output capacitor +ve node to enable overvoltage protection;
typical OVP level is 36 V, and this level can be increased by connecting through
an external resistor ROVP
Gate driver terminal to drive external MOSFET
11
11
VIN
12
12
ISET
Input supply for the IC; decouple with a 0.1 μF ceramic capacitor
Sets 100% Current through LED strings; connect RISET from ISET to GND
13
14
FSET
Sets switching frequency; connect RFSET from FSET to GND
14
15
COMP
Compensation pin; connect 1 μF capacitor to GND or common star ground
15,16,17
16,17,18
LEDx
LED sinks capable of 80 mA sink; connect unused LEDx pins to ground
18,19
19
LGND
Connect to common star ground
20,21,22
20,21,22
LEDx
LED sinks capable of 80 mA sink; connect unused LEDx pins to ground
23
23
¯Ā¯Ū¯L̄¯T̄
¯
F̄
During normal operation, this pin is high (high impedance); at a fault event, this
pin pulls low
24
24
EN
Device enable
–
PAD
PAD
Exposed pad for enhanced thermal dissipation, connect to common star ground
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
3
A8507
LED Backlight Driver for LCD Monitors and Televisions
ELECTRICAL CHARACTERISTICS Valid at VIN = 5 V; TA = 25°C, RFSET = 52 kΩ, RISET = 12.4 kΩ, except
specifications guaranteed over the full operating temperature range with TA = TJ , unless otherwise noted
Characteristics
Input Voltage Range
Min.
Typ.1
Max.
Unit
4.3
–
5.5
V
VIN Falling
–
–
4.0
V
–
0.1
–
V
Switching at no load
–
7
–
mA
Shutdown, EN = VIL, TA = 25°C
–
0.1
1
μA
Standby, EN = VIH, PWM = VIL, soft start
completed
–
1
2
mA
0.8
1
1.25
MHz
Symbol
Test Conditions
VIN
Undervoltage Lockout Threshold
VUVLO
Undervoltage Lockout Hysteresis
VUVLOHYS
Supply Current2
IVIN
indicates
Boost Controller
Switching Frequency
fSW
Minimum Switch Off-Time
toff(min)
Driver output
–
72
–
ns
Minimum Switch On-Time
ton(min)
Driver output
–
72
–
ns
Logic Input Levels (EN and PWM pins)
Input Voltage Level Low
VIL
–
–
0.4
V
Input Voltage Level High
VIH
1.5
–
–
V
Input Leakage Current2
IIN
EN = PWM = 5 V
–
100
–
μA
Driver Section
High Side Gate Drive On Resistance
RDS(on)H
Measured at VGATE = VIN / 2
–
9
–
Ω
Low Side Gate Drive On Resistance
RDS(on)L
Measured at VGATE = VIN / 2
–
10
–
Ω
Driver to GND Resistance During Shutdown
RSDOFF
–
125
–
kΩ
VSEN
VSENP – VSENN
80
95
110
mV
LEDx Pin Regulation Voltage
VLEDx
ILED = 80 mA
–
1
–
V
ISET to ILEDx Current Gain
AISET
ISET = 100 μA
–
640
–
A/A
ISET Pin Voltage
VISET
–
1.235
–
V
ISET
41
–
125
μA
Sense Overcurrent Threshold Voltage
LED Current Sinks
ISET Allowable Current
Range2
LEDx Accuracy3
ErrILEDX
LED1 through LED6 = 1 V, at 100% Current
–3
±0.6
3
%
LEDx Matching4
ΔILEDX
LED1 through LED6 = 1 V, ISET = 100 μA
–3
±0.6
3
%
VLEDx = 12 V, EN = 0
–
0.1
–
μA
LEDx Switch Leakage Current2
ISL
Soft Start
Soft Start Sense Threshold Voltage
VSENS
Sense voltage for boost switch current
sensing
–
28.5
–
mV
Soft Start LEDx Current Limit Relative to LED
100% Current
ILED(SS)
Current through enabled LEDx pins during
soft start
–
8
–
%
Continued on the next page…
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
4
A8507
LED Backlight Driver for LCD Monitors and Televisions
ELECTRICAL CHARACTERISTICS (continued) Valid at VIN = 5 V; TA = 25°C, RFSET = 52 kΩ, RISET = 12.4 kΩ, except
specifications guaranteed over the full operating temperature range with TA = TJ , unless otherwise noted
Characteristics
Symbol
Test Conditions
indicates
Min.
Typ.1
Max.
Unit
–
165
–
°C
Protection Features
Thermal Shutdown Threshold
TTSD
TJ rising
Short Circuit Detect Voltage
VSC
–
18.7
–
V
Output Overvoltage Threshold
VOVP
ROVP = 0
–
36
–
V
IOVPLK
VOVP = 22 V, EN = VIL
–
0.1
–
μA
–
240
–
μA
OVP Pin Leakage
Current2
Overvoltage Protection Sense Current2
IOVPH
¯Ā¯Ū¯L̄¯T̄
¯ Pin Output Leakage2
F̄
IFLT
V=5V
–
–
1
μA
¯Ā¯Ū¯L̄¯T̄
¯ Pin Output Voltage
F̄
VOL
I = 500 μA
–
–
0.4
V
1Typical
specifications are at TA = 25ºC.
input and output current specifications, negative current is defined as coming out of the node or pin (sourcing), positive current is defined as going
into the node or pin (sinking).
3LED accuracy is defined as (I
SET × 640 – ILED(av)) / (ISET × 640), ILED(av) measured as the average of ILED1 through ILED6.
4LED current matching is defined as (I
LEDx – ILED(av)) / ILED(av), with ILED(av) as defined in footnote 3.
2For
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
5
A8507
LED Backlight Driver for LCD Monitors and Televisions
Efficiency (%)
Characteristic Performance
99
98
97
96
95
94
93
92
91
90
89
88
87
86
85
6 parallel strings, 12 serial LEDs each, ILED = 55 mA
6 parallel strings, 9 serial LEDs each, ILED = 55 mA
6 parallel strings, 12 serial LEDs each, ILED = 80 mA
6 parallel strings, 9 serial LEDs each, ILED = 80 mA
10
12
14
16
18
20
VBAT (V)
22
24
26
28
Figure 2. Efficiency versus Battery Voltage at various LED configurations, RFSET = 105 kΩ
(500 kHz), RISET = 14.3 kΩ (55 mA) or 10 kΩ (80 mA), Q1 = FQB17N08L, L1 = 10 μH
100
1.007
Normalized Current Gain
Efficiency (%)
1.006
VBAT= 16 V
VBAT= 21 V
95
VBAT= 12 V
90
VBAT= 8 V
85
80
1.005
1.004
1.003
1.002
1.001
1.000
75
0
10
20
30
40
50
60
70
80
90 100
0.999
10
12
14
Figure 3. Efficiency versus PWM Duty Cycle at various VBAT levels,
5 parallel strings with 9 series LEDs each, ILED = 55 mA per channel
20
22
24
26
28
30
60
Average LED Current (mA)
LED Current (mA)
18
Figure 4. Normalized Current Gain versus RISET , normalized to 1 for
RISET = 12.4 kΩ
55.5
55.4
55.3
55.2
55.1
55.0
54.9
-40
16
RISET (kΩ)
PWM Duty Cycle (%)
-20
0
20
40
60
80
100
Temperature (°C)
Figure 5. LED Current versus Ambient Temperature, RISET = 14.3 kΩ
50
40
30
VBAT (V)
20
21
16
12
8
10
0
0
20
40
60
80
100
120
PWM Duty Cycle(%)
Figure 6. Average LED Current versus PWM Duty Cycle at various
VBAT levels, 6 parallel strings with 9 series LEDs each, ILED = 55 mA
per channel, fPWM = 200 Hz
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
A8507
LED Backlight Driver for LCD Monitors and Televisions
90
1.034
80
1.033
1.032
60
50
fSW (MHz)
LED Current (mA)
70
40
30
20
1.030
1.029
1.028
10
1.027
--35
0
10
1.031
15
20
25
30
-20
RISET (kΩ)
LED Current Matchiing Error (%)
LED Current Accuracy (%)
90
80
70
Uncompensated
Compensated (3 μs)
50
40
1
10
60
80
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
10
100
Figure 9. LED Current Accuracy versus PWM Duty Cycle,
LED Current Accuracy normalized to the 100% Current level,
VBAT = 12 V, 6 parallel strings with 9 series LEDs each, ILED = 55 mA
per channel fPWM = 200 Hz
20
30
40
50
60
70
80
90
100
PWM Duty Cycle (%)
PWM Duty Cycle (%)
Switching Frequency (kHz)
40
Figure 8. Switching Frequency versus Ambient Temperature
100
0.1
20
Ambient Temperature (°C)
Figure 7. LED Current versus RISET
60
0
Figure 10. LED Current Matching Error versus PWM Duty Cycle,
VBAT = 12 V, 6 parallel strings with 9 series LEDs each, ILED = 55 mA
per channel, fPWM = 200 Hz
1050
950
850
750
650
550
450
350
250
50 60 70 80 90 100 110 120 130 140 150 160 170 180
R F S E T (kΩ)
Figure 11. Switching Frequency versus RFSET
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
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7
A8507
LED Backlight Driver for LCD Monitors and Televisions
Typical Power Sequencing Waveforms
Turn-on sequence
VIN – VEN – VPWM
Turn-off sequence
VPWM – VEN – VIN
VIN
VIN
VEN
VEN
VEN
VPWM
VPWM
C2
C1
C4
VEN
C3
C3
VPWM
IOUT
VIN
C2
VIN
C1
IOUT
IOUT
C4
t
t
Turn-on sequence
VIN – VPWM – VEN
C3
Turn-off sequence
VEN – VPWM – VIN
VIN
VIN
VEN
VEN
VEN
C1
VPWM
IOUT
VIN
VPWM
C2
VPWM
IOUT
VIN
C1
IOUT
C4
VEN
C3
VPWM
C2
VPWM
IOUT
IOUT
C4
t
t
C1: VIN , 1 V / div.
C2: VPWM , 5 V / div.
C3: VEN , 5 V / div.
C4: IOUT , 200 mA / div.
Time: 2 ms / div.
Fig 12. Alternative Turn-On and Turn-Off sequences: (top) VEN –VPWM –VEN, (bottom) VVPWM –VEN –VPWM, with 6 parallel
strings with 9 series LEDs each, VBAT = 12 V, VPWM = 5 V (100% Current), VIN = 5 V, VEN = 5 V, ILED = 55 mA per channel
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
8
A8507
LED Backlight Driver for LCD Monitors and Televisions
Functional Description
Overview The A8507 is a multi-output WLED/RGB controller
for backlighting medium-size displays. It has a integrated gate
driver for driving an external N-channel boost MOSFET. The
boost controller is fixed frequency current mode converter. The
switching frequency can be set in the range from 250 kHz to
1 MHz, by an external resistor, RFSET , connected between FSET
and ground.
The external MOSFET switch is protected by pulse-by-pulse current limiting. The current limit is independent of duty cycle, and
is set using an external sense resistor, RSC.
The A8507 has six well-matched current sinks that provide regulated current through the LEDs for uniform display brightness.
The boost converter is controlled by monitoring all LEDx pins
simultaneously and continuously.
LED Current Setting The maximum LED current can be set,
at up to 80 mA/channel, through the ISET pin. Connect a resistor,
RISET , between this pin and ground to set the reference current
level, ISET . The value of ISET (mA) is determined by:
ISET = 1.235 / RISET (kΩ) .
(1)
widths less than 100 μs, increase the applied PWM pulse-width
by 3 μs to compensate for this delay.
Startup Sequence When EN is pulled high, the IC enters
soft start. The IC first tries to determine which LEDx pins are
being used, by raising the LEDx pin voltage with a small current.
After a duration of 512 switching cycles, the LEDx pin voltage
is checked. Any LEDx channel with a drain voltage smaller then
100 mV is removed from the control loop.
After the first PWM positive trigger, the boost current is limited
to 35% of normal value and all active LEDx pins sink 1/12 of
the set current until output voltage reaches sufficient regulation
level. When the device comes out of soft start, boost current and
the LEDx pin currents are set to normal operating level. Within
a few cycles, the output capacitor charges to the voltage required
to supply full LEDx current. After output voltage, VOUT , reaches
the required level, LEDx current toggles between 0% and 100%
with each PWM command signal.
In case of a heavy overload on VOUT at startup, the device will
stay in soft start mode indefinitely, as the output voltage cannot
rise to the LED regulation level.
The resulting current is multiplied internally with a gain of 640
and mirrored on all enabled LEDx pins. This sets the maximum
current through each LEDx, referred as the 100% Current. The
LEDx current can be reduced from the 100% Current value by
applying an external PWM signal on the PWM pin.
LED Short Detect Any LEDx pins that have a voltage exceeding the Short Circuit Detect Voltage, VSC , cause the device to
shut down and this condition is latched. This faults occurs when
multiple LEDs short. In case only a few LEDs short, the IC will
continue to work as long as power dissipation in the IC is limited.
Boost Switching Frequency Setting Connect an external
resistor between the FSET pin and GND, to set boost switching
frequency, fSW . The value of fSW (MHz) is determined by:
Overvoltage Protection The A8507 has an adjustable overvoltage protection feature to protect the external MOSFET
against output overvoltage. The overvoltage level can be set,
from 36 V to a higher voltage, with an external resistor, ROVP .
When the current though the OVP pin exceeds 240 μA, internal
OVP comparator goes high and the device shuts down. The OVP
fSW = 52 / RFSET ,
(2)
where fSW is in MHz and RFSET is in kΩ.
Enable The IC turns on when a high signal is applied on the EN
pin, and turns off when this pin is pulled low.
PWM Dimming The A8507 has a very wide range for PWM
signal input. It can accept a PWM signal from 100 Hz to 5 kHz.
When a PWM high signal is applied, the LEDx pins sink
100% Current. When the PWM signal is low, the LED sinks
turn off.
Referring to figure 13, there is a ramp-up delay between when
the PWM signal is applied and when the current reaches the
90% level. To improve current dimming linearity for PWM pulse
PWM
tD
ILED
Figure 13. Propagation delay from the PWM signal rising edge to ILEDx
reaching the 90% level
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
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9
A8507
LED Backlight Driver for LCD Monitors and Televisions
fault disables all LEDx strings that are below regulation, thus preventing them from controlling the boost output voltage.
Calculate the value for ROVP (Ω) as follows:
ROVP = (VOVP – 36) / 240 μA ,
(3)
where VOVP is the required OVP level in V.
Open LED Protection Unused LEDx pins should be connected
to GND. During normal operation, if any enabled LED string
opens, voltage on the corresponding LEDx pin goes to zero. The
boost loop operates in open loop till the OVP level is reached.
The A8507 identifies the open LED string when overvoltage is
detected. Open strings are then removed from the regulation loop.
Afterwards, the boost controller operates in normal manner, and
the output voltage is regulated to drive the remaining strings. If
the open LED string is reconnected, it will sink current up to the
programmed current level.
Note: Open strings are removed from boost regulation, but not
disabled. This keeps the string in operation if LEDs open for only
a short length of time, or reach OVP level on a transient event.
The disconnected string can be restored to normal mode by reenabling the IC. It can also be restored to normal operation if the
fault signal is removed from the corresponding LEDx pin, but an
OVP event occurs on any other LEDx pin.
Overcurrent Protection The IC provides pulse-by-pulse
current limiting for the boost MOSFET. The current limit level,
ISC (A), can be set by selecting the external resistor, RSC (Ω):
RSC = 0.095 / ISC .
Channel Selection The A8507 can be used to drive 1 to 6 LED
channels. During startup, the IC detects LED sink pins which are
shorted to ground, and disables the corresponding LED channel.
Therefore, any unused LED pins must be connected to ground,
otherwise the IC will go into overvoltage protection fault during
startup. LED pins can be paralleled together for higher current. For example for a 3 parallel string configuration, connect
LED1-2, LED3-4, and LED5-6 together to deliver up to 160 mA
per LED.
Thermal Shutdown (TSD) The IC shuts down when junction
temperature exceeds 165°C. It will recover automatically when
the junction temperature falls below 125°C.
VIN Undervoltage Lockout (UVLO) The device is shut down
when input voltage, VIN , falls below VUVLO.
Fault Mode The IC functions in various fault states:
Fault State
AutoRestart
Overvoltage
Protection
Yes
Fault occurs when OVP pin exceeds the VOVP
threshold. Used to protect the output voltage
from damaging the part.
Yes
Fault occurs when the current through the
external MOSFET increases exceeds such that
the voltage across the SENP and SENN pins
exceeds 95 mV typical. The MOSFET switch is
turned off on a cycle-per-cycle basis.
Overcurrent
Protection
Yes
Fault occurs when the COMP pin exceeds the
overcurrent detect threshold. Multiple pulse-bypulse current limits will result in the COMP pin
voltage to rise. After a time period determined
by the COMP pin current and the output
capacitor, COUT , the COMP voltage will exceed
the overcurrent detect threshold, forcing a fault.
Overtemperature
Protection
Yes
Fault occurs when the die temperature exceeds
the over-temperature threshold, 165°C typical.
LED Short
Protection
No
Fault occurs when the LED pin voltage exceeds
VSC , 18.7 V typical.
VIN UVLO
No
Fault occurs when VIN drops below VUVLO,
4.0 V typical. This fault resets all latched faults.
Pulseby-Pulse
Current Limit
(4)
If the boost output voltage is unable to reach the regulation target
even when the switch is operating at maximum current limit, the
boost control loop will force the compensating capacitor, CCOMP ,
to rise in voltage until it reaches the overcurrent fault level
(3.4V approximately). The overcurrent fault forces the device
into soft start.
Description
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
10
A8507
LED Backlight Driver for LCD Monitors and Televisions
Application Information
PCB Layout Guidelines As with any switching power supply,
care should be taken in laying out the board. A switching power
supply has sources of high dv/dt and high di/dt which can cause
malfunction. All general norms should be followed for board
layout. Refer to figure 14 for a typical application schematic. The
A8507 evaluation board provides a useful model for designing
application circuit layouts.
The following guidelines should be observed:
• Place the supply bypass capacitor (C5) close to the VIN pin and
the ground plane.
• Route analog ground, digital signal ground, LED ground
(LGND pin), and power ground (PGND pin) separately. Connect all these grounds at the common ground plane under the
A8507, serving as a star ground.
• Place the resistors RFSET and RISET, and the compensation components (Rz and Cz) close to the FSET, ISET, and COMP pins,
respectively. Connect the other ends to the common star ground.
• A8507 has 50 kΩ internal pull-down resistors on the EN and
PWM pins to keep these pins low while driving through tri-state
state (for example, shutdown). Add external resistors R2 and
R3 between the EN and PWM pins and ground, for added noise
immunity. Connect these resistors close to the pins and return to
the common star ground.
• Sense voltage across RSC with smaller length traces. Place the
SENP and SENN traces as close to each other as possible to
minimize noise pickup. Connect the SENN trace to the negative
end of the resistor and do not connect it to power ground plane.
• Provide a substantial copper plane near MOSFET Q1 and the
IC, to provide good thermal conduction.
• Place the input capacitors (C1, C2), inductor (L1), boost diode
(D1), MOSFET (Q1), and output capacitors (C3, C4) so that
they form the smallest loop practical. Avoid long traces for
these paths.
• Place ROVP as close as possible to the OVP pin. A long trace
between ROVP and the OVP pin may pick up switching noises
and cause overvoltage protection to trip prematurely.
VOUT
L1
VBAT
8 to 21 V
C1
P
10
RSC
7
C3
6
P
DRIVER SENP SENN
VBIAS
4.5 to 5.5 V
ROVP
8
OVP
C4
P
11 VIN
R1
R2
Q1
C2
8 LEDs per string
D1
C5
RFSET
23 FAULT
EN
24
PWM
2
COMP
15
FSET
14
RISET
R3
12
Rz1
Cz1
3
5
9
A8507
(LP package)
LED1
LED2
LED3
LED4
ISET
LED5
NC
NC
PAD
NC
GND GND
1
13
LED6
16
17
18
20
21
22
LGND PGND
19
4
P
R2, R3 optional (A8507 has
internal pull-down resistors)
Figure 14. Typical application circuit
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
11
A8507
LED Backlight Driver for LCD Monitors and Televisions
VOUT
L1
C1
Q1
C2
P
10
RSC
7
C3
6
P
DRIVER SENP SENN
VBIAS
4.5 to 5.5 V
8 LEDs per string
D1
ROVP
8
OVP
C4
P
100
11 VIN
R1
R2
C5
RFSET
23 FAULT
EN
24
PWM
2
COMP
15
FSET
14
RISET
R3
12
Rz1
3
5
Cz1
9
A8507
(LP package)
LED1
16
LED2
17
LED3
GND GND
1
13
21
0
20
21
LED6
NC
12
85
20
LED5
PAD
VBAT (V)
90
80
LED4
NC
95
18
ISET
NC
Efficiency (%)
VBAT
8 to 21 V
40
60
80
100
PWM duty cycle (%)
22
RISET = 10.0 kΩ, RFSET = 51 kΩ,
ROVP = 0 Ω, Q1=FQB17N08L
LGND PGND
19
4
P
R2, R3 optional (A8507 has
internal pull-down resistors)
Figure 15. Typical Application with 6 parallel strings, 8 series LEDs each, 80 mA per channel
12 LEDs per string
VOUT
L1
C1
P
10
D1
RSC
7
C3
6
P
DRIVER SENP SENN
VBIAS
4.5 to 5.5 V
ROVP
8
OVP
C4
P
100
11 VIN
R1
R2
Q1
C2
C5
RFSET
23 FAULT
EN
24
PWM
2
COMP
15
FSET
14
RISET
R3
12
Rz1
Cz1
3
5
9
A8507
(LP package)
LED1
LED2
LED3
LED4
ISET
LED5
NC
NC
PAD
NC
GND GND
1
13
LED6
LGND PGND
19
4
16
17
Efficiency (%)
VBAT
8 to 21 V
95
VBAT (V)
90
12
21
85
18
80
20
21
22
0
20
40
60
80
100
PWM duty cycle (%)
RISET = 10.0 kΩ, RFSET = 51 kΩ,
ROVP = 36 kΩ, Q1=FQB17N08L
P
R2, R3 optional (A8507 has
internal pull-down resistors)
Figure 16. Typical Application with 2 parallel strings, 12 series LEDs each, 240 mA per channel
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
12
A8507
LED Backlight Driver for LCD Monitors and Televisions
VOUT
C1
Q1
C2
P
RSC
7
10
C3
6
P
DRIVER SENP SENN
VBIAS
4.5 to 5.5 V
12 LEDs per string
D1
ROVP
8
OVP
C4
P
100
11 VIN
R1
C5
23 FAULT
EN
A8507
(LP package)
24
2
R2
RFSET 15
14
RISET
R3
12
Rz1
3
5
Cz1
9
16
LED1
17
LED2
PWM
FSET
95
VBAT (V)
90
12
21
85
18
LED3
COMP
Efficiency (%)
L1
VBAT
8 to 21 V
80
20
LED4
0
20
ISET
21
LED5
NC
NC
PAD
GND GND
1
13
60
80
100
22
LED6
NC
40
PWM duty cycle (%)
RISET = 10.0 kΩ, RFSET = 51 kΩ,
ROVP = 36 kΩ, Q1=FQB17N08L
LGND PGND
19
4
P
R2, R3 optional (A8507 has
internal pull-down resistors)
Figure 17. Typical Application with 3 parallel strings, 12 series LEDs each, 160 mA per channel
12 LEDs per string
VOUT
L1
C1
P
10
D1
RSC
7
C3
6
P
DRIVER SENP SENN
VBIAS
4.5 to 5.5 V
ROVP
8
OVP
C4
P
100
11 VIN
R1
R2
Q1
C2
C5
RFSET
23 FAULT
EN
24
PWM
2
COMP
15
FSET
14
RISET
R3
12
Rz1
Cz1
3
5
9
A8507
(LP package)
LED1
LED2
LED3
LED4
16
17
18
20
ISET
LED5
NC
NC
PAD
NC
GND GND
1
13
LED6
LGND PGND
19
4
P
21
22
Efficiency (%)
VBAT
8 to 21 V
95
VBAT (V)
90
12
21
85
80
0
20
40
60
80
100
PWM duty cycle (%)
RISET = 10.0 kΩ, RFSET = 51 kΩ,
ROVP = 36 kΩ, Q1=FQB17N08L
R2, R3 optional (A8507 has
internal pull-down resistors)
Figure 18. Typical Application with 4 parallel strings, 12 series LEDs each, 80 mA per channel
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
13
A8507
LED Backlight Driver for LCD Monitors and Televisions
VOUT
C1
P
10
RSC
7
C3
6
P
DRIVER SENP SENN
VBIAS
4.5 to 5.5 V
D2
8
OVP
C4
100
P
11 VIN
R1
R2
Q1
C2
18 LEDs per string
D1
Efficiency (%)
L1
VBAT
8 to 21 V
C5
RFSET
23 FAULT
EN
24
PWM
2
COMP
15
FSET
14
RISET
R3
12
Rz1
Cz1
3
5
9
A8507
(LP package)
LED1
LED2
LED3
LED4
16
17
LED5
NC
PAD
NC
GND GND
1
13
LED6
LGND PGND
19
4
12
21
85
0
20
21
40
60
80
100
RISET = 10.0 kΩ, RFSET = 51 kΩ,
D2 = 30 V Zener, Q1=FQB17N08L
R5
22
20
PWM duty cycle (%)
R4
R6
R7
P
R2, R3 optional (A8507 has
internal pull-down resistors)
VBAT (V)
90
80
18
ISET
NC
95
For higher output voltage, the voltage on the LEDx pins during
PWM off-time may exceed the rated voltage. Connect a resistor
from the LEDx pin to GND. Recommended values are:
• C3 = 2.2 μF / 100 V ceramic
• C4 = 10 μF / 100 V electrolytic
• R4 through R7 = 25 kΩ, 0603
Figure 19. Typical Application with 4 parallel strings, 18 series LEDs each, 80 mA per channel
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
14
A8507
LED Backlight Driver for LCD Monitors and Televisions
Package LB 24-Pin SOICW with Internally Fused Pins
15.40 ±0.20
8°
0°
24
24
0.33
0.20
7.50 ±0.10
2.20
10.30 ±0.33
9.60
A
1.40 REF
1
2
1.27
0.40
Branded Face
24X
SEATING
PLANE
0.10 C
0.51
0.31
1.27 BSC
1
2
0.65
1.27
0.25 BSC
SEATING PLANE
GAUGE PLANE
C
B
PCB Layout Reference View
2.65 MAX
0.30
0.10
Pins 6, 7, 18, and 19 internally fused for enhanced thermal dissipation
For Reference Only; not for tooling use (reference MS-013AD)
Dimensions in millimeters
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
A Terminal #1 mark area
B
Reference pad layout (reference IPC SOIC127P1030X265-24M)
All pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary
to meet application process requirements and PCB layout tolerances
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
15
A8507
LED Backlight Driver for LCD Monitors and Televisions
Package LP 24-Pin TSSOP with Exposed Thermal Pad
7.80±0.10
24
0.65
0.45
8º
0º
0.20
0.09
B
3 NOM
4.40±0.10
3.00
6.40±0.20
6.10
0.60 ±0.15
A
1
2
1.00 REF
4.32 NOM
0.25 BSC
24X
SEATING
PLANE
0.10 C
0.30
0.19
0.65 BSC
SEATING PLANE
GAUGE PLANE
C
1.65
4.32
C
PCB Layout Reference View
For Reference Only; not for tooling use (reference MO-153 ADT)
Dimensions in millimeters
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
1.20 MAX
0.15
0.00
A Terminal #1 mark area
B
Exposed thermal pad (bottom surface); dimensions may vary with device
C Reference land pattern layout (reference IPC7351
TSOP65P640X120-25M); all pads a minimum of 0.20 mm from all
adjacent pads; adjust as necessary to meet application process
requirements and PCB layout tolerances; when mounting on a multilayer
PCB, thermal vias at the exposed thermal pad land can improve thermal
dissipation (reference EIA/JEDEC Standard JESD51-5)
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Allegro MicroSystems, LLC reserves the right to make, from time to time, such departures from the detail specifications as may be required to
permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that
the information being relied upon is current.
Allegro’s products are not to be used in any devices or systems, including but not limited to life support devices or systems, in which a failure of
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use; nor for any infringement of patents or other rights of third parties which may result from its use.
For the latest version of this document, visit our website:
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Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
16
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