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ADD5203 データシート(PDF) 11 Page - Analog Devices

部品番号 ADD5203
部品情報  8-String,White LED Driver with SMBus and PWM Input for LCD Backlight Applications
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ホームページ  http://www.analog.com
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ADD5203 データシート(HTML) 11 Page - Analog Devices

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ADD5203
Rev. 0 | Page 11 of 24
THEORY OF OPERATION
CURRENT-MODE, STEP-UP SWITCHING REGULATOR
OPERATION
The ADD5203 uses a current-mode PWM boost regulator to
provide the minimal voltage needed to enable the LED string at
the programmed LED current. The current-mode regulation
system allows fast transient response while maintaining a stable
output voltage. By selecting the proper resistor-capacitor network
from COMP to GND, the regulator response can be optimized for
a wide range of input voltages, output voltages, and load conditions.
The ADD5203 can provide a 45 V maximum output voltage and
drive up to 13 LEDs (3.4 V/30 mA type of LEDs) for each channel.
INTERNAL 3.3 V REGULATOR
The ADD5203 contains a 3.3 V linear regulator. The regulator is
used for biasing internal circuitry. The internal regulator requires
a 1 μF bypass capacitor. Place this bypass capacitor between
VDDIO (Pin 4) and GND, as close as possible to Pin VDDIO.
BOOST CONVERTER SWITCHING FREQUENCY
The ADD5203 boost converter switching frequency is user
adjustable, between 350 kHz to 1 MHz, by using an external
resistor, RF. A frequency of 350 kHz is recommended to optimize
the regulator for high efficiency, and a frequency of 1 MHz is
recommended for small external components.
See Figure 14 for considerations when selecting a switching
frequency and an adjustment resistor (RF).
1100
1000
900
800
700
600
500
400
300
120
170
220
270
320
370
420
470
RF (kΩ)
Figure 14. Switching Frequency vs. RF
DIMMING FREQUENCY (fPWM)
The ADD5203 contains an internal oscillator to generate the
PWM dimming signal for LED brightness control. The LED
dimming frequency (fPWM) is adjustable, in the fPWM range of 200 Hz
to 10 kHz, by using an external resistor (RFPWM) and capacitor
(CFPWM). The RFPWM should be in the range of 13 kΩ to 110 kΩ,
and the CFPWM should be in the range of 20 pF to 390 pF.
Table 8. RFPWM and CFPWM Recommendation
Dimming Freq (fPWM)
RFPWM (kΩ)
CFPWM (pF)
200 Hz
110
390
500 Hz
75
200
1 kHz
50
150
5 kHz
18
47
10 kHz
13
20
CURRENT SOURCE
The ADD5203 contains eight current sources to provide accurate
current sinking for each LED string. String-to-string tolerance is
kept within ±1.5% at 20 mA. Each LED string current is adjusted
up to 30 mA by an external resistor.
The ADD5203 contains an LED open and short fault protection
circuit for each channel. If the headroom voltage of the current
source remains below 200 mV while the boost converter output
reaches the OVP level, the ADD5203 recognizes that the current
source has an open load fault for the current source, and the current
source is disabled. If the headroom voltage of the current source
goes above 7.2 V, the current source is disabled for short protection.
If an application requires four LED strings, each LED string
should be connected using FB1 to FB4. Tie the unused FB pins
(FB5 to FB8) to GND.
The ADD5203 contains hysteresis to prevent the LED current
change that is caused by a ±0.195% jitter of the PWM input.
Programming the LED Current
As shown in Figure 2, the ADD5203 has an LED current set pin
(ISET). A resistor (RSET) from this pin to ground adjusts the LED
current up to 30 mA. LED current level can be set with the
following equation:
)
(
2831
A
R
I
SET
LED
=
DIMMING CONTROL INTERFACE
The ADD5203 dimming control interface method is selectable
between the SMBus serial input and/or the external PWM input.
The LED dimming modes supported by ADD5203 can be
controlled externally through these dimming control interfaces.
The SEL1 and SEL2 pins should be set based on the application
conditions (see Table 9).
Table 9. Brightness Control Mode Selection
SEL1
SEL2
Dimming Mode
Interface
High
High
Fixed delay PWM
SMBus
Low
No delay PWM
SMBus
Open
High
Fixed delay PWM
PWM
Low
No delay PWM
PWM
Low
High
DC current
SMBus
Open
DC current
PWM
Low
Direct PWM
PWM


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