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| 型號： | TPS650732TRSLRQ1 |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 電源管理 |
| 英文描述： | POWER SUPPLY SUPPORT CKT, PQCC48 |
| 封裝： | 6 X 6 MM, 0.4 MM PITCH, GREEN, PLASTIC, QFN-48 |
| 文件頁數(shù)： | 62/90頁 |
| 文件大小： | 1375K |
| 代理商： | TPS650732TRSLRQ1 |
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SLVSAP7 – JANUARY 2011
ISET1 to GND. Dimming is done with an internal PWM modulator by changing the duty cycle in the current sinks
from 1% to 100%. In order to set a LED current of less than 1% of the current defined at ISET1, a second current
range is set with a resistor at pin ISET2 to GND. By changing between the two current ranges and varying the
duty cycle, it is possible to achieve a dimming ratio of > 1:100. The main functions of the converter like enable /
disable of the converter, PWM duty cycle and dimming frequency are programmed in registers WLED_CTRL1
and WLED_CTRL2 – see the register description for details.
If only one string of white LEDs are used, ISINK1 and ISINK2 need to be connected in parallel.
Setup
In applications not requiring the wLED boost converter, the pins should be tied to a GND as stated below:
Pins L4, FB_wLED, ISINK1 and ISINK2 should be directly connected to GND. Each ISET1 and ISET2 should be
connected to GND with a 100k resistor. Optionally ISET1 and ISET2 can be used as analog inputs to the ADC.
In this case, these pins can be tied to a voltage source in the range from 0V to 2.25V.
Setting the LED Current
There are two resistors which set the default current for the current sinks at ISINK1 and ISINK2.
The resistor connected to ISET1 is used if Bit CURRENT LEVEL is set 1 in register 19h.
The resistor connected to ISET2 is used if Bit CURRENT LEVEL is set 0 in register 19h (default).
This allows switching between two different maximum values for the LED current with one Bit to extend the
resolution for dimming.
Dimming is done by an internal PWM signal that turns on and off the current sinks ISINK1 and ISINK2 at 200Hz
(default). The duty cycle range is 1% to 100% with a 1% resolution and a default duty cycle of 30%. In order to
get the full scale LED current, the PWM dimming needs to be set to 100% in register 19h. This is done by writing
63h to register 19h.
KISET is defined to be 1000 in the electrical spec, the reference voltage at ISET1 and ISET2 is 1.24V.
The current for each string is set by the resistor to:
ISINK1=ISINK2= KISET × 1.24V/RISETx
(8)
RISET1, RISET2 = KISET × 1.24V/10mA=124 k
(9)
A resistor value of 124 k
sets the current on each string to 10mA.
For one string of wLEDs, both strings need to be connected in parallel, so the current in the wLEDs is twice the
current programmed by the resistor at ISET1 or ISET2.
Connecting both strings in parallel is required because the wLED converter generates its output voltage
dependant on the current in ISINK1 and ISINK2. If the current falls below the target, the output voltage is
increased. If one string is open, the wLED driver will boost the output voltage to its maximum because it
assumes the voltage is not high enough to drive current into this string (there could be different numbers of
wLEDs in the two strings).
Inductor Design
The inductor in a boost converter serves as an energy storage element. The energy stored equals L × I2.
Therefore, the inductor must not be saturated as the inductance will drop and the energy stored will be reduced
causing bad efficiency. The converter operates with typically 15mH to 22mH inductors. A design example for an
application powering 6LEDs in one string given below:
Vin = 2.8 V — minimum input voltage for the boost converter
Vo = 6 × 3.2 V = 19.2 V — assuming a forward voltage of 3.2V per LED
Vf = 0.5 V — forward voltage of the Schottky diode
Io = 25 mA maximum LED current
Fsw = 1.125 MHz — switching frequency — T=890ns
Rds(on) = 0.6R — drain-source resistance of the internal NMOS switch
Vsw — voltage drop at the internal NMOS switch
IAVG — average current in NMOS when turned on
Copyright 2011, Texas Instruments Incorporated
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