A8504 Datasheet

A8504
WLED/RGB Backlight Driver for Medium Size LCDs
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, 2012
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.
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Features and Benefits
Description
▪ Active current sharing among 8 LED strings for ±1.3%
typical current matching
▪ ±1.2% typical current accuracy
▪ Drive up to 11 series × 8 parallel = 88 LEDs
(Vf = 3.5 V, If = 40 mA)
▪ External PWM control for LED dimming
▪ An external resistor for LED current setting
▪ Boost converter with integrated 50 V, 2 A DMOS
▪ LED sinks rated for 45 mA
▪ 200 kHz to 2 MHz switching frequency
▪ Open LED disconnect
▪ Boost current limit, thermal shutdown, and soft start
▪ No audible ceramic capacitor noise during PWM dimming
▪ Adjustable overvoltage protection (OVP)
▪ No pull-up resistors required for LED modules that use
ESD capacitors
The A8504 is a multi-output WLED driver for medium display
backlighting. The A8504 integrates a boost converter and eight
current-sinks to provide a WLED/RGB backlight driver. The
boost converter can provide output voltage up to 47 V. The
flexible channel selection control and high voltage capability
allow a wide range of LED backlight applications. The boost
converter is a constant frequency current-mode converter. Each
LED channel can sink 45 mA, and channels can be paralleled
for higher currents. LED current can be controlled with external
PWM duty cycle.
The A8504 is available in a 26 pin, 4 mm × 4 mm QFN/MLP
package that is only 0.75 mm nominal in height. Applications
include:
▪ Thin notebook displays
▪ LCD TV
▪ RGB backlight
▪ GPS systems
▪ Portable DVD players
Package: 26 pin QFN/MLP (suffix EC)
Approximate Scale 1:1
Typical Application
PWM pin dimming:
digital PWM input
VIN
5 V ±10%
CIN
0.1 μF/ 10 V
VBAT
5 to 24 V
L1
10 μH
CBAT
2.2 μF/ 50 V
VIN
VPWM
SKIP
ROVP
SW SW
PWM
IOUT
VOUT
D1
COUT
OVP
GND
AGND
A8504
PGND
COMP
CC
RFSET
2.2 μF
50 V
FSET
SEL3
ISET
SEL2
VIN
SEL1
RISET
LED1
LED3 LED5 LED7
LED2
LGND
LED8 LED6 LED4
Figure 1. LCD monitor backlight, driving 11 green, blue, or white LEDs with Vf= 3.5 V, or 18 red LEDs with Vf = 2.2 V,
per LED string. Overvoltage protection set to 45 V nominal (40.5 V minimum). See also: Recommended Components
table, page 16.
8504-DS, Rev. 3
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Selection Guide
Part Number
Package
Packing*
A8504EECTR-T
4 mm × 4 mm QFN/MLP
*Contact Allegro for additional packing options
1500 pieces / 7-in. reel
Device package is lead (Pb) free, with 100% matte tin leadframe plating.
Absolute Maximum Ratings
Characteristic
Symbol
Rating
Units
SW and OVP Pins
–0.3 to 50
V
LED1 through LED8 Pins
–0.3 to 23
V
–0.3 to 6
V
VIN Pin
Notes
VIN
–0.3 to VIN+ 0.3
V
–40 to 85
ºC
TJ(max)
150
ºC
Tstg
–55 to 150
ºC
Remaining Pins
Operating Ambient Temperature
TA
Maximum Junction Temperature
Storage Temperature
Range E
Package Thermal Characteristics*
Characteristic
Symbol
Note
Rating
Units
RθJA
Measured on 3 in. × 3 in., 2-layer PCB
48.5
°C/W
Package Thermal Resistance
*Additional information is available on the Allegro website
Allegro MicroSystems, LLC
115 Northeast Cutoff
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1.508.853.5000; www.allegromicro.com
2
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Functional Block Diagram
VIN
5 V ±10%
VBAT
5 to 25 V
CIN
VIN
D1
L1
CBAT
SW SW
COUT
COMP
CC
VOUT
Reference
and
Soft Start
+
+
–
–
R Q
S
ROVP
+
∑
–
PGND
OSC
SKIP
FSET
100 kΩ
RFSET
OVP
Feedback
Loop
AGND
Current Sinks
GND
LED1
LED2
On/Off
SEL1
PWM Generator
SEL2
IOUT_SET
LED3
LED4
LED5
SEL3
LED6
PWM
LED7
100 kΩ
LED8
ISET
RISET
PGND
LGND
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3
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
ELECTRICAL CHARACTERISTICS, valid at TA = –40°C to 85°C, typical values at TA = 25°C, VIN = 5 V, unless otherwise noted
Characteristics
Input Voltage Range
Undervoltage Lockout Threshold
UVLO Hysteresis Window
Supply Current
Symbol
Test Conditions
VIN
VUVLO
VIN falling
VUVLOhys
ISUP
Min.
Typ.
Max.
Units
4.2
–
5.5
V
–
–
4
V
–
0.2
–
V
Switching at no load, TA = 25°C
–
5
–
mA
Shutdown PWM = VIL
–
0.1
1
μA
–
60
–
dB
Error Amplifier
Error Amplifier Open Loop Gain
AVEA
Error Amplifier Unity Gain Bandwidth
UGBEA
–
3
–
MHz
Error Amplifier Transconductance
GmEA
∆ICOMP = ±10 μA
–
850
–
μA/V
IEAsink
VLED1-8 = 1 V
–
280
–
μA
IEAsource
VLED1-8 = 0 V
–
–280
–
μA
1.8
2
2.2
MHz
Error Amplifier Output Sink Current
Error Amplifier Output Source Current
Boost Controller
RFSET = 13 kΩ, SKIP = VIL
Switching Frequency
Minimum Switch Off-Time
fSW
RFSET = 26.1 kΩ, SKIP = VIL
–
1
–
MHz
RFSET = 32.4 kΩ, SKIP = VIH
–
200
–
kHz
–
70
–
ns
tOFFmin
Logic Input Levels (PWM, SELx, and SKIP pins unless otherwise specified)
Input Voltage Level Low
VIL
–
–
0.4
V
Input Voltage Level High
VIH
1.5
–
–
V
Input Leakage Current (PWM, and SKIP
pins)
IIleak
–
–
100
μA
ISELleak
–
–
1
μA
Output Overvoltage Rising Limit
VOVP
28
–
34
V
OVP Sense Current
IOVPH
–
49
–
μA
OVP Release Current
IOVPL
–
44
–
μA
OVP Leakage Current
IOVPleak
VVOP = 21 V
–
0.1
–
μA
Switch On Resistance
Rds(on)
ISW = 1.5 A
–
225
–
mΩ
Switch Leakage Current
ISWleak
VSW = 5 V, TA = 25°C
–
–
1
μA
VSW = 21 V
–
1
–
μA
Switch Current Limit
ISWlim
1.6
2
–
A
Input Leakage Current (SELx pins)
VI(pin) = 5 V, TA = 25°C
Over Voltage Protection (OVP)
Boost Switch
Continued on the next page…
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4
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
ELECTRICAL CHARACTERISTICS (continued), valid at TA = –40°C to 85°C, typical values at TA = 25°C, VIN = 5 V, unless
otherwise noted
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Units
–
800
–
mV
LED Current Sinks
LEDx Regulation Voltage
VLEDx
ISET to ILEDx Current Gain
AISET
–
460
–
–
Voltage on ISET Pin
VISET
–
1.23
–
V
ISET Allowable Current Range
ISET
40
–
100
μA
ISET = 83 μA
ErrLEDx
RISET = 14.7 kΩ; 100% current ratio,
measured as average of LED1 to LED8;
LED1 to LED8 = 0.8 V
–
±1.2
–
%
∆LEDx1
LED1 to LED6; ISET = 83 μA , 100% current ratio;
LED1 to LED6 = 0.8 V; SEL1=SEL3=VIH;
SEL2=VIL
–
±1.3
–
%
∆LEDx2
LED1 to LED8; ISET = 83 μA , 100% current ratio;
LED1 to LED8 = 0.8 V; SEL1=SEL2=SEL3=VIH
–
–1.7
to 2.5
–
%
LEDx Switch Leakage Current
ILSleak5
VLEDx= 5 V, PWM = 0, TA = 25°C
–
–
1
μA
LEDx Switch Leakage Current
ILSleak21
VLEDx= 21 V, PWM =0
–
1
–
μA
ISWSS
Initial soft start current for boost switch
-
1.2
-
A
Soft Start LEDx Current Limit
ILEDSS
Current through enabled LEDx pins during soft
start, RISET= 14.7 kΩ
-
3
-
mA
Thermal Shutdown Threshold
TSHDN
40°C hysteresis
–
165
–
°C
LEDx Accuracy
LEDx Matching
Soft Start
Soft Start Boost Current Limit
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5
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Performance Characteristics
Efficiency with PWM dimming: PLED/PBAT, VIN = 5 V
100
95
95
Efficiency (%)
Efficiency (%)
VIN = 5 V, 6 ch. with 7 LEDs per ch., 40 mA per ch., fSW = 1 MHz
100
90
85
VBAT (V)
9
15
21
80
75
90
85
VBAT (V)
9
15
21
80
75
70
70
0
20
40
60
80
100
0
2
4
6
8
10
Expanded View, Lower Range: Duty Cycle (%)
Duty Cycle (%)
95
95
90
90
85
Efficiency (%)
Efficiency (%)
VIN = 5 V, 6 ch. with 7 LEDs per ch., 40 mA per ch., fSW = 2 MHz
VBAT (V)
9
15
21
80
75
85
VBAT (V)
9
15
21
80
75
70
70
0
20
40
60
80
100
0
2
4
6
8
10
Expanded View, Lower Range: Duty Cycle (%)
Duty Cycle (%)
95
95
90
90
85
Efficiency (%)
Efficiency (%)
VIN = 5 V, 8 ch. with 8 LEDs per ch., 40 mA per ch., fSW = 1 MHz
VBAT (V)
9
15
21
80
75
85
VBAT (V)
9
15
21
80
75
70
70
0
20
40
60
80
0
100
2
4
6
8
10
Expanded View, Lower Range: Duty Cycle (%)
Duty Cycle (%)
95
90
90
85
Efficiency (%)
Efficiency (%)
VIN = 5 V, 8 ch. with 8 LEDs per ch., 40 mA per ch., fSW = 2 MHz
95
VBAT (V)
9
15
21
80
75
85
VBAT (V)
9
15
21
80
75
0
20
40
60
Duty Cycle (%)
80
100
0
2
4
6
8
10
Expanded View, Lower Range: Duty Cycle (%)
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A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Performance Characteristics
Turn-On with PWM Signal
VIN = 5 V, VBAT = 15 V; fPWM= 100 Hz; fSW= 1 MHz
8S8P configuration, 40 mA per channel
50% Duty Cycle
C1
C2
1% Duty Cycle
PWM
C1
IIN
C2
VOUT
PWM
IIN
VOUT
C3
C3
C4
C4
IOUT
IOUT
t
Symbol
C1
C2
C3
C4
t
Parameter
VPWM
IIN
VOUT
IOUT
time
t
Units/Division
5V
500 mA
10 V
200 mA
5 ms
Symbol
C1
C2
C3
C4
t
Parameter
VPWM
IIN
VOUT
IOUT
time
Units/Division
5V
500 mA
10 V
200 mA
20 ms
Soft Start Operation
VIN = 5 V, VBAT = 15 V; fSW= 1 MHz
8S8P configuration, 40 mA per channel
PWM
C1
IIN
C2
VOUT
C3
C4
IOUT
t
Symbol
C1
C2
C3
C4
t
Parameter
VPWM
IIN
VOUT
IOUT
time
Units/Division
5V
500 mA
10 V
200 mA
200 μs
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A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Functional Description
The A8504 is a multioutput WLED driver for medium display
backlighting. The A8504 works with 4.2 to 5.5 V input supply,
and it has an integrated boost converter to boost battery voltage
up to 47 V, 40 mA per LED string. An inductor can be connected
to a separate power supply, VBAT , from 5 to 25 V, with the A8504
IC powered from a 5 V source. The LED sinks can sink up to a
45 mA current.
The boost converter is a constant frequency current-mode converter. The integrated boost DMOS switch is rated for 50 V at
2 A. This switch has pulse-by-pulse current limiting, with the current limit independent of duty cycle. The switch also has output
overvoltage protection (OVP), with the OVP level adjustable,
typically from 30 to 47 V, as described in the Device Internal
Protection section.
The A8504 has individual open LED detection. If any LED
opens, the corresponding LED pin is removed from regulation
logic. This allows the remaining LED strings to function normally, without excessive power dissipation.
The switching frequency, fSW, can be set from 600 kHz to 2 MHz
by a single resistor, RFSET, connected across the FSET and
AGND pins, and with the SKIP pin set to logic low (see figure 2).
The switching frequency is set as:
FSW = 26.03 / RFSET ,
where FSW is in MHz and RFSET is in kΩ When the SKIP pin is
connected to logic low, switching frequency is as set by RFSET.
2.5
The IC offers a wide-bandwidth transconductance amplifier with
external COMP pin. External compensation offers optimum performance for the desired application.
The A8504 has eight well-matched current sinks to provide regulated current through LEDs for uniform display brightness. The
quantity of LEDx pins used is determined by the SELx pins. Refer
to table 1 for further description.
The boost converter is controlled such that the minimum voltage
on any LEDx pin is 800 mV. In a typical application, the LEDx
pin connected to the LED string with the maximum voltage drop
controls the boost loop, so the remaining pins will also have the
higher voltage drop. All LED sinks are rated for 21 V, to allow
PWM dimming control.
LED Current Setting
The maximum LED current can be set at up to 45 mA per channel, by using the ISET pin. To set the reference current, ISET ,
connect a resistor, RISET, between this pin and ground, valued
according to the following formula:
ISET = 1.23 / RISET ,
where ISET is in mA and RISET is in kΩ.
Table 1. LEDx Channel Enable Table
2.0
fSW (MHz)
When the SKIP pin is connected to logic high, the switching
frequency is divided by 4. The SKIP pin can be used to reduce
switching frequency in order to reduce switching losses and
improve efficiency at light loads.
1.5
1.0
0.5
0
10
20
30
40
SEL1
SEL2
SEL3
0
0
0
Only LED1 on
LEDx Outputs
1
0
0
LED1 through LED2 on
0
1
0
LED1 through LED3 on
1
1
0
LED1 through LED4 on
0
0
1
LED1 through LED5 on
1
0
1
LED1 through LED6 on
0
1
1
LED1 through LED7 on
1
1
1
LED1 through LED8 on
50
RFSET (kΩ)
Figure 2. Switching frequency setting by value of RFSET.
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A8504
WLED/RGB Backlight Driver for Medium Size LCDs
This current is multiplied internally with a gain of 480, and then
mirrored on all enabled LEDx pins. This sets the maximum current through the LEDs, referred to as “100% current.” The LED
current can be reduced from 100% by on/off control (PWM) with
an external PWM signal on the PWM pin
On/off Control (PWM) with an External PWM Signal on the
PWM Pin. When the PWM pin is pulled high, the A8504 turns on
and all enabled LEDx pins sink 100% current. When the PWM
pin is pulled low, the IC shuts down with the LEDx pins disabled.
External PWM applied to the PWM pin should be in the range of
100 to 400 Hz for optimal accuracy.
At startup, the output capacitor is discharged and the IC enters
soft start. The boost current is limited to 1 A, and all active LEDx
pins sink 1/16 of the set 100% current until all of the enabled
LEDx pins reach 0.8 V. After the IC comes out of soft start, the
boost current and the LEDx pin currents are set to 100% current.
The output capacitor charges to the voltage level required to supply full LEDx current within a few cycles. The startup sequence
is shown in the Soft Start chart in the Performance Characteristics
section. The IC is shut down immediately when PWM goes low.
Device Internal Protection
Overcurrent Protection (OCP). The A8504 has a pulse-by-pulse
current limit of 2 A on the boost switch. This current limit is
independent of duty cycle.
Thermal Shutdown Protection (TSD). The IC shuts down when
junction temperature exceeds 165°C and restarts when the junction temperature falls by 40°C.
Overvoltage Protection (OVP). The A8504 has overvoltage
VPWM
C1
protection to protect the IC against output overvoltage. The
overvoltage level can be set, from 30 to 45 V typical, with an
external resistor, ROVP, as shown in figure 5. When the current
though the OVP pin exceeds 49 μA, the OVP comparator goes
high. When the OVP pin current falls below 44 μA, OVP is reset.
Calculate the value for ROVP as follows:
VOUT
C2
IOUT
C3
ROVP = (VOVP – 30) / 49 μA ,
t
Symbol
C1
C2
C3
t
Parameter
VPWM
VOUT(ac)
IOUT
time
Units/Division
2.00 V
500 mV
200 mA
5 ms
Figure 3. Output Voltage Ripple During PWM Dimming. VIN= 5 V,
VBAT= 15 V, fSW= 1 MHz, fPWM= 100 Hz, PWM duty cycle = 50%,
8S8P configuration, 40 mA per channel.
where VOVP is the desired typical OVP level in V, and ROVP is
in Ω. For tighter OVP limits, a low–leakage-current Zener diode,
DZ, can be used, instead of ROVP, to set OVP at up to 47 V. For
redundancy, DZ can be connected across ROVP to provide additional protection, if ROVP should open. Select a 17 V low-leakage Zener diode for DZ.
VPWM
C1
VOUT
Turn-on Delay
6 μs Typical
Shutdown
(0 μA)
VPWM
100% Current Level
Shutdown
(0 μA)
IOUT
C2
IOUT
t
Figure 4. Timing of turn-on delay and turn-off delay when using the
PWM pin. VIN= 5 V, VBAT= 15 V, fSW= 1 MHz, fPWM= 100 Hz, 8S8P
configuration, 40 mA per channel.
Symbol
C1
C2
t
Parameter
VPWM
IOUT
time
Units/Division
2.00 V
200 mA
5 μs
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9
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Open LED Protection. The A8504 has protection against open
LEDs. 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 A8504 identifies the open
LED string when overvoltage on the OVP pin is detected. This
string is then removed from the boost controlling loop. The boost
circuit is then controlled in the normal manner, and the output
voltage is regulated, to provide the output required 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.
VPWM
C1
Startup
C2
VPWM
Overvoltage condition
detected
ROVP
DZ
COUT
OVP
A
28.8 V
OVP
Disable
–
22 kΩ
+
1.23 V
4.4 kΩ
Figure 5. Overvoltage protection circuit. Three alternative
configurations at (A) are available, as follows:
External Component
OVP Rating
ROVP only
up to 45 V
DZ only
up to 47 V
both ROVP and DZ
redundancy
VPWM
C1
IIN
IIN
LED opens
VOUT
IOUT
VOUT
SW SW
C2
Normal operation
established
D1
VIN
Normal
operation
Overvoltage condition
detected
VOUT
VOUT
Normal operation restored
with open LED string
removed from control loop
IOUT
IOUT
C4
C3
C4
C3
t
Symbol
C1
C2
C3
C4
t
Parameter
VPWM
IIN
vout
Iout
time
Units/Division
5.00 V
500 mA
5.00 V
200 mA
200 μs
t
Symbol
C1
C2
C3
C4
t
Parameter
VPWM
IIN
vout
Iout
time
Units/Division
5.00 V
500 mA
5.00 V
100 mA
200 μs
Figure 4. Timing of overvoltage protection (OVP) function when open
LEDs are detected at startup (left ) and during normal operation (right).
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A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Application Information
Design Example
This section provides a method for selecting component values
when designing an application using the A8504.
Assumptions For the purposes of this example, the following are
given as the application requirements:
•
•
•
•
•
•
•
•
VBAT: 8 to 21 V
VIN: 5 V
Quantity of LED channels: 6
Quantity of LEDs per channel: 8
LED current per channel, ILED: 40 mA
Vf at 40 mA: 3 to 3.4 V
fSW: 2 MHz
TA(max): 65°C
With a 15% margin, to set the output OVP level, given an
IOVPH of 49 μA typical, and VOVP = 30 V:
(4)
6. Select inductor L1. This should assume a maximum duty
cycle, D(max), at VBAT(min) and 90% efficiency.
D = 1– (VBAT × η) / VOUT
(5)
D(max) = 1– (8 × 0.9) / 27.7 = 74% .
Procedure The procedure consists of selecting the appropriate
configuration and then the individual component values, in an
ordered sequence.
1. Identify the SELx pins to use. For 6 channels:
• connect pins SEL1 and SEL3 to VIN
• connect pin SEL2 to AGND
2. Connect LEDs to pins LED1 through LED6 (leave pins
LED7 and LED8 open).
3. Select resistor RISET (connected between pin ISET and
AGND). Given ILED = 40 mA and VISET = 1.23 V typical,
then:
(1)
Select a common value: 14.3 kΩ, 1%.
4. Select resistor RFSET (connected between pin FSET and
AGND). Given:
for a 2 MHz switching frequency, select:
RFSET = 26.03 / 2 = 13 kΩ .
(3)
Select a common value: 41.2 kΩ.
A8504 can work with wide range of PWM frequencies, taking
about 6 μs typical (10 μs maximum) to turn on. This delay may
have a noticeable effect at high PWM frequencies combined with
low duty cycles. For example, at 100 Hz and 10% duty cycle, the
PWM on-period is 1 ms. In that period, a delay of 6 μs causes
only a 0.6% error. If the PWM frequency is 1 kHz, this error
is 6%. However, error due to turn-on delay can be nullified by
increasing the applied PWM duty cycle.
RFSET = 26.03 /fSW ,
VOUT(max) = 3.4 × 8 + 0.5 = 27.7 V .
ROVP = (32 – 30) / 49 = 40.8 kΩ .
Dimming Use a 100 Hz PWM signal on the PWM pin. The
RISET = 1.23 / (40/460) = 14.2 kΩ .
5. Select resistor ROVP (connect to the OVP pin to set the OVP
level, VOUT(max)). Given Vf (max) = 3.4 V, then:
(2)
Then calculate maximum switch on-time:
ton(max) = D(max) / fSW
(6)
= 0.74 / 2 MHz = 370 ns .
Maximum input current can be calculated as:
IBAT = (VOUT × IOUT) / (VBAT(min) × η)
(7)
IBAT(max) = [27.7 (40 × 6)] / (8 × 0.9) = 923 mA .
Set inductor ripple at 60% of IBAT(max):
∆IL = 0.6 × 923 = 554 mA
Given, during switch on-time:
VBAT = L × ∆IL × fSW / D
(8)
8 = L × 0.554 × 2 / 0.74, and
L = 5.3 μH .
Select a common value, 6.8 μH.
It is recommended to select an inductor that can handle a DC
current level that is greater than 923 mA, at the peak current
level (saturation) of 923 mA + 554 / 2 = 1200 mA. This is
to ensure that the inductor does not saturate at any steady
state or transient condition, within specified temperature and
tolerance ranges. Inductor saturation level decreases with
increasing temperature. It is advisable to use a inductor with
a saturation level of 1.6 A, because the switch current limit
is 1.8 A typical. The inductor should have a minimum DC
resistance and core loss for better efficiency.
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11
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
The rms current through capacitor should be selected such
7. Select output capacitor COUT (connect between the A8504
and the LEDs), given:
COUT = IOUT × ton / ∆VOUT ,
that internally-generated temperature rise is limited to 10°C.
(9)
The rms current through CBAT is given by:
where IOUT is the total output current, and ∆VOUT is the
output voltage ripple, 0.5% of VOUT (0.05 × 27.7 = 0.14 V).
Then:
IBATrms = (IOUT × r) / [(1 – D) × 3.46 ],
= [(40 mA × 6 )× 0.6] /
[(1 – 0.74) × 3.46] = 160 mA .
COUT = (40 mA × 6) × 370 ns / 0.14 = 0.63 μF .
Select a ceramic capacitor with a 35 or 50 V rating, X5R or
X7R grade. Usually capacitance at 35 V drops significantly
compared to the 0 V specification. Typically, this requires the
selection of a 2.2 μF capacitor to compensate for DC voltage
bias derating.
9. Select the boost diode D1 (connect between the SW pins and
the output). D1 should be a Schottky diode with low forward
drop and junction capacitance.
The diode reverse voltage rating should be greater than VOUT.
A 50 V diode rating is recommended.
The rms current through capacitor should be selected such
that internally-generated temperature rise is limited to 10°C
to 20°C. The rms current through COUT is given by:
ICOUTrms = IOUT × [(D+r2 / 12) / (1 – D)] 1/2 ,
The diode DC current rating should be greater than IOUT
and the peak repetitive current rating should be greater than
(10)
IBAT + ∆IL / 2.
where r = ∆IL / IBAT = 0.554 mA /0.923 mA = 0.6.
10. Select the compensation capacitor CC (connect between
COUT should have an rms current rating greater than:
(40 mA × 6 )× {[0.74 + (0.36 /
12)]/(1–0.74)}1/2
(12)
the COMP pin and ground. Typically, use a 0.1 to 0.47 μF
= 0.48 A.
capacitor for stability.
8. Select input capacitor CBAT (connect to battery input),
given:
CBAT = ∆IL / (VBAT(min) × fSW × ∆VINripple ,
High Output Current Operation
LED strings can be paralleled for higher current. The A8504
(11)
can sink up to 40 mA through each sink. These outputs can be
connected together with various possibilities for higher current
where ∆VINripple is the input ripple voltage, which can be assumed to be 1% of VBAT. Then:
as shown in figure 7. As an example, for an application with up
CIN = 0.554 mA/(8 × 2 MHz × 0.08) = 0.4 μF .
to 50 mA using 3 parallel strings: LED1 connected with LED2,
Select a 1 μF or higher, 25 or 35 V, ceramic capacitor, X5R or
X7R grade.
ILED(max)
Quantity
of Strings
SEL1
SEL2
SEL3
25
25
25
25
25
25
25
25
50
50
50
50
100
100
200
8
7
6
5
4
3
2
1
4
3
2
1
2
1
1
H
L
H
L
H
L
H
L
H
H
H
H
H
H
H
H
H
L
L
H
H
L
L
H
L
H
L
H
H
H
H
H
H
H
L
L
L
L
H
H
L
L
H
L
H
LED1
LED3 with LED4, and LED5 with LED6; LED7 and LED8 open;
SEL1 and SEL3 set logic high, and SEL2 set low.
LED2
NC
LED3
NC
NC
LED4
NC
NC
NC
LED5
NC
NC
NC
NC
LED6
LED7
LED8
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
Connect
Connect
Connect
Connect
Connect
Connect
Connect
Connect
Connect
NC
NC
NC
NC
NC
NC
NC
NC
Connect
Connect
Connect
NC
NC
NC
NC
NC
NC
Connect
NC
NC
Connect
Figure 7. LED strings can be combined to allow various maximum current levels to be applied.
The “Connect” notes indicate LED strings connected together.
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12
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Typical Application Circuits
A typical application circuit for dimming an LCD monitor backlight with multiple LED strings is shown in figure 1. Figure 8
shows two dimming methods: digital PWM control (PWM signal
on the PWM pin) and analog PWM control, with the analog signal, VA , applied to the ISET pin through a resistor, RA.
ISET
RISETP
RISET
Q1
The current flowing through RA can be calculated as:
IA = VA/ RA .
Figure 9. Configuration for 1000:1 dimming.
This current changes the reference current, ISET, as follows:
100
ISET = VSET / RSET – (VA – VSET) / RA .
LED current can be changed by changing VA. ISET can be
changed in the range from 40 μA to 100 μA.
Application Circuit for 1000:1 Dimming Level
A wider dimming range can be achieved by changing the reference current, ISET, while using PWM dimming. For higher output,
current levels turn on Q1. RISET and RISETP set the 100% current
level. This current level can be set to 45 mA, and then it can be
dimmed by applying 100% to 0.25% duty cycle on the PWM pin.
The reference current can be reduced by turning off Q1. LED
current can be dimmed to 18 mA by reducing reference current
through ISET pin. This provides 1000:1 combined dimming level
range. Figure 10 shows the accuracy, ErrLEDX , results using this
circuit.
85
80
75
0.1
SW SW
COUT
OVP
PWM
GND
SKIP
AGND
COMP
FSET
100.0
Figure 10. Typical accuracy, normalized to the 100% current level,
versus dimming level, with FPWM = 100 Hz.
ROVP
VIN
1.0
10.0
Dimming Level (%)
VOUT
CIN
RFSET
90
VBAT
24 V
VIN
5 V ±10%
CC
Accuracy (%)
95
A8504
PGND
SEL3
VIN
SEL2
RA
VA
RISET
ISET
SEL1
LED2
LED1
LED3 LED5 LED7 LGND LED8 LED6 LED4
Figure 8. Typical application circuit for PWM dimming, using digital PWM (on the PWM pin).
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13
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
VBAT 24 V
VIN
5 V ±10%
CIN
CBAT
VIN
ROVP
SW SW
COUT
OVP
PWM
GND
SKIP
AGND
A8504
COMP
CC
VOUT
PGND
SEL3
FSET
VIN
SEL2
RFSET
SEL1
ISET
LED1
LED8
LED7 LED6 LED5 LGND LED4 LED3 LED2
RISET
Figure 11. Typical application circuit for PWM dimming, using digital PWM (on the PWM
pin). Showing configuration of 16 WLEDs at 160 mA, in two strings of 8 LEDs each.
VIN
5 V ±10%
VBAT
24 V
CIN
PWM
VIN
L1
10 μH
D1
CBAT
ROVP
SW
SW
VOUT
All ESD capacitors across LED arrays are 0.1 μF
COUT
2.2 μF
OVP
SKIP
PGND
COMP
CC
0.1 μF
A8504
FSET
RFSET
RISET
SEL3
AGND
GND
SEL2
ISET
LED1
VIN
SEL1
LED8 LED7 LED6 LED5 LGND LED4 LED3 LED2
Figure 12. Typical application circuit for LED modules with ESD capacitors.
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14
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
PCB Layout Guidelines
The A8504 evaluation board provides a useful model for designing application circuit layouts. The following guidelines should
be observed:
• Place the supply bypass capacitor, C8, 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 pad for the exposed thermal pad
under the A8504, serving as a star ground.
• Place the input capacitors, C2 and C7, the inductor, L1, the
boost diode D1, and the internal MOSFET and output capaci-
L1
tor, C4, so that they form the smallest loop practical. Avoid
long traces for these paths.
• Place the RISET, RFSET, and OVP resistors and the compensation capacitor, C5, close to the ISET, FSET, OVP, and COMP
pins, respectively.
• Provide a substantial solder pad under the exposed thermal
pad on the bottom side of the A8504, to provide good thermal
conduction. Connect the PCB solder pad to the PCB ground
plane with multiple thermal vias. For a thermal via specification, please refer to JEDEC guidelines.
• For best thermal performance, avoid thermal stresses.
VBAT
CBAT
VIN
COUT
CIN
VOUT
D1
22
OVP
23
SW
24
25
AGND
SW
SEL2
18
17
16
15
14
13
RPWM
LED4
LED2
LED6
LED1
LED8
SEL1
12
RISET
SEL3
A8504
GND
8
RFSET
ISET
LED3
7
FSET
LGND
6
COMP
LED7
5
21
20
PWM
GND 19
11
4
ROVP
PGND
SKIP
LED5
3
PAD
PGND
10
2
9
1
CC
VIN
26
GND
Figure 13. Schematic diagram of A8504 typical application circuit and composite view of typical PCB
layout. In the composite view, the red line superimposed represents the current loop during switch
on-time (return through the A8504 device and the PCB ground plane). The green line represents the
current loop during off-time. Both of these loops should be designed to be as short as practicable.
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115 Northeast Cutoff
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15
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Figure 14. A8504 typical PCB layout silkscreen layer.
Figure 15. A8504 typical PCB layout top signal layer (left) and bottom ground plane layer (right)
Recommended Components Table (for application shown in figure 1)
Component
Capacitor
Capacitor
Capacitor
Diode
IC
Reference
Designator
CBAT
COUT
CIN, CC
D1
A8504
Inductor
L1
Resistor
Resistor
Resistor
RISET
RFSET
ROVP
Value
2.2 μF / 50 V
2.2 μF / 50 V
0.1 μF / 6.3 V
60 V / 1.5 A
–
10 μH
4.7 μH
4.7 μH
14.3 kΩ
24 kΩ
270 kΩ
Part Number
Vendor
TDK
TDK
IR 10MQ060NTRPBF
A8504
SLF6028T-100M1R3-PF
VLS4012T-4R7M1R1
NR4012T4R7M
International Rectifier
Allegro MicroSystems
TDK
TDK
Taiyo Yuden
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16
A8504
WLED/RGB Backlight Driver for Medium Size LCDs
22 OVP
23 SW
24 SW
25 VIN
26 AGND
Pin-out Diagram
PGND
1
21 PGND
SKIP
2
20 PWM
COMP
3
FSET
4
ISET
5
17 SEL2
GND
6
16 SEL1
LED1
7
15 LED2
19 GND
LED4 14
LED6 13
18 SEL3
LED8 12
LGND 11
LED7 10
9
LED5
LED3
8
EP
(Top View)
Terminal List Table
Number
Name
1
PGND
2
SKIP
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
COMP
FSET
ISET
GND
LED1
LED3
LED5
LED7
LGND
LED8
LED6
LED4
LED2
SEL1
SEL2
SEL3
GND
20
PWM
21
PGND
22
OVP
23
24
25
26
–
SW
SW
VIN
AGND
EP
Description
Power ground pin.
Reduces boost switching frequency in case of light load to improve frequency. Normally, this pin should be low; when
high, fSW is divided by 4.
Compensation pin; connect external compensation network for boost converter.
Sets boost switching frequency. Connect RFSET from FSET to GND to set frequency. Range for RFSET is 13 to 40 kΩ.
Sets 100% current through LED string. Connect RISET from ISET to GND. Range for RISET is 8.45 to 30 kΩ.
Connect to AGND.
LEDx capable of 45 mA.
Power ground pin for LED current sink.
LEDx capable of 45 mA.
SEL1, SEL2, and SEL3 decide active LED strings.
Connect to AGND.
On/off and on/off LED current control with external PWM. Apply logic level PWM for PWM controlled dimming mode.
When unused, connect to AGND.
Power ground pin.
Connect to this pin to output capacitor +Ve node through a resistor to enable OVP (overvoltage protection). Default OVP
level with 0 Ω resistor is 30 V, and it can be programmed up to 47 V.
DMOS drain node.
Input supply for the IC. Decouple with a 0.1 μF ceramic capacitor.
Circuit ground pin.
Exposed pad. Electrically connected to PGND and LGND; connect to PCB copper plane for heat transfer.
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A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Package EC, 4 × 4 mm 26-Pin QFN/MLP
0.20
4.00
1
2
0.40
26
26
0.95
A
1
2
C
1.10
4.00
4.00
1.23
Top View
2.45
4.00
27X
D
SEATING
PLANE
0.08 C
0.20
C
PCB Layout Reference View
0.75
0.40
0.40
B
1.23
1.10
2
1
26
2.45
Bottom View
All dimensions nomianl, not for tooling use
(reference JEDEC MO-220WGGE)
Dimensions in millimeters
Exact case and lead configuration at supplier discretion within limits shown
A Terminal #1 mark area
B Exposed thermal pad (reference only, terminal #1
identifier appearance at supplier discretion)
C Reference land pattern layout (reference IPC7351
QFN40P400X400X80-29M)
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)
D Coplanarity includes exposed thermal pad and terminals
Copyright ©2007-2013, Allegro MicroSystems, LLC
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
Allegro’s product can reasonably be expected to cause bodily harm.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its
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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