A8504EECTR-T Allegro Microsystems Inc, A8504EECTR-T Datasheet - Page 8

• Manufacturer Part Number: A8504EECTR-T
• Description: IC,Laser Diode/LED Driver,LLCC,28PIN,PLASTIC
• Manufacturer: Allegro Microsystems Inc
• Type: Backlight, White LED, RGBr

Specifications of A8504EECTR-T

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 8
• Internal Driver: Yes
• Type - Primary: Backlight
• Type - Secondary: RGB, White LED
• Frequency: 200kHz ~ 2MHz
• Voltage - Supply: 4.2 V ~ 5.5 V
• Voltage - Output: 47V
• Mounting Type: Surface Mount
• Package / Case: 26-WFQFN Exposed Pad
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 45mA
• Internal Switch(s): Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Efficiency: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 620-1215-2

Figure 2. Switching frequency setting by value of RFSET.
A8504
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, V
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 con-
verter. The integrated boost DMOS switch is rated for 50 V at
2 A. This switch has pulse-by-pulse current limiting, with the cur-
rent 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 nor-
mally, without excessive power dissipation.
The switching frequency, f
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:
where F
connected to logic low, switching frequency is as set by RFSET.
SW
is in MHz and R
2.5
2.0
1.5
1.0
0.5
0
10
F
SW
20
SW
= 26.03 / R
FSET
, can be set from 600 kHz to 2 MHz
BAT
R
FSET
30
is in kΩ When the SKIP pin is
, from 5 to 25 V, with the A8504
(kΩ)
WLED/RGB Backlight Driver for Medium Size LCDs
FSET
40
,
50
Functional Description
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.
The IC offers a wide-bandwidth transconductance amplifier with
external COMP pin. External compensation offers optimum per-
formance for the desired application.
The A8504 has eight well-matched current sinks to provide regu-
lated 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 chan-
nel, by using the ISET pin. To set the reference current, I
connect a resistor, RISET, between this pin and ground, valued
according to the following formula:
where I
Table 1. LEDx Channel Enable Table
SEL1
SET
0
1
0
1
0
1
0
1
is in mA and R
SEL2
0
0
1
1
0
0
1
1
I
SET
ISET
SEL3
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
= 1.23 / R
0
0
0
0
1
1
1
1
is in kΩ.
ISET
Only LED1 on
LED1 through LED2 on
LED1 through LED3 on
LED1 through LED4 on
LED1 through LED5 on
LED1 through LED6 on
LED1 through LED7 on
LED1 through LED8 on
LEDx Outputs
,
SET
,
8