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LM27964SQ-A Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM27964SQ-A Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 13 page Circuit Description (Continued) Note: DxA3-DxA0: Register Sets Current Level Supplied to DxA LED drivers DxB3-DxB0: Register Sets Current Level Supplied to DxB LED drivers Full-Scale Current set externally by the following equation: IDxx = 200 x 1.25V / RSETx Brightness Level Segments = 1/16th of Fullscale Note: DKEY1-DKEY0: Sets Brightness for DKEY pin (KEYPAD Driver). 11=Fullscale Bit7 to Bit 2: Not Used Full-Scale Current set externally by the following equation: IDKEY = 800 x 1.25V / RSETx Brightness Level are= 100% (Fullscale), 70%, 40%, 20% Application Information SETTING LED CURRENT The current through the LEDs connected to DxA, DxB and DKEY can be set to a desired level simply by connecting an appropriately sized resistor (R SETx) between the ISETx pin of the LM27964 and GND. The DxA and DxB LED currents are proportional to the current that flows out of the I SETA and I SETB pins and are a factor of 200 times greater than the I SETA/B currents. The DKEY current is proportional to the current that flows out of the I SETK pin and is a factor of 800 times greater than the I SETK current. The feedback loops of the internal amplifiers set the voltage of the I SETx pins to 1.25V (typ.). Separate R SETx resistor should be used on each I SETx pin. The statements above are simplified in the equations below: I DxA/B =200x(VISET /RSETA/B) R SETA/B = 200 x (1.25V / IDxA/B) I DKEY =800x(VISET /RSETK) R SETK = 800 x (1.25V / IDKEY) Once the desired R SETx values have been chosen, the LM27964 has the ability to internally dim the LEDs by Pulse Width Modulating (PWM) the current. The PWM duty cycle is set through the I 2C compatible interface. LEDs connected to BankA and BankB current sinks (DxA and DxB) can be dimmed to 16 different levels/duty-cycles (1/16th of full-scale to full-scale). The internal PWM frequency for BankA and BankB is a fixed 10kHz (LM27964SQ-I) or 23kHz (LM27964SQ-A) depending on the option. The DKEY current sink uses an analog current scaling method to control LED brightness. The brightness levels are 100% (Fullscale), 70%, 40%, and 20%. When connecting multiple LEDs in parallel to the DKEY current sink, it is recommended that ballast resistors be placed in series with the LEDs. The ballast resistors help reduce the affect of LED forward voltage mismatch, and help equalize the diode cur- rents. Ballast resistor values must be carefully chosen to ensure that the current source headroom voltage is sufficient to supply the desired current. Please refer to the I 2C Compatible Interface section of this datasheet for detailed instructions on how to adjust the brightness control registers. MAXIMUM OUTPUT CURRENT, MAXIMUM LED VOLTAGE, MINIMUM INPUT VOLTAGE The LM27964 can drive 4 LEDs at 30mA each (BankA) and 12 keypad LEDs at 5mA each (60mA total at DKEY) from an input voltage as low as 3.2V, so long as the LEDs have a forward voltage of 3.6V or less (room temperature). The statement above is a simple example of the LED drive capabilities of the LM27964. The statement contains the key application parameters that are required to validate an LED- drive design using the LM27964: LED current (I LEDx), num- ber of active LEDs (N x), LED forward voltage (VLED), and minimum input voltage (V IN-MIN). The equation below can be used to estimate the maximum output current capability of the LM27964: I LED_MAX = [(1.5 x VIN)-VLED -(IADDITIONAL xROUT)] / [(N x xROUT)+kHRx] (eq. 1) I LED_MAX = [(1.5 x VIN )-VLED -(IADDITIONAL x 2.75 Ω)] / [(N x x 2.75 Ω)+k HRx] I ADDITIONAL is the additional current that could be delivered to the other LED banks. R OUT – Output resistance. This parameter models the inter- nal losses of the charge pump that result in voltage droop at the pump output P OUT. Since the magnitude of the voltage droop is proportional to the total output current of the charge pump, the loss parameter is modeled as a resistance. The output resistance of the LM27964 is typically 2.75 Ω (V IN = 3.6V, T A = 25˚C). In equation form: 20138105 FIGURE 7. Brightness Control Register Description Internal Hex Address: A0h 20138104 FIGURE 8. Brightness Control Register Example 20138110 FIGURE 9. Internal Hex Address: B0h www.national.com 10 |
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