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ADM8845ACP-REEL Datasheet(PDF) 11 Page - Analog Devices |
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ADM8845ACP-REEL Datasheet(HTML) 11 Page - Analog Devices |
11 / 20 page ADM8845 Rev. 0 | Page 11 of 20 OUTPUT CURRENT CAPABILITY The ADM8845 can drive up to 30 mA of current to each of the six LEDs given an input voltage of 2.6 V to 5.5 V. The LED currents have a maximum current matching of 1% between any two LED currents. An external resistor, RSET, sets the output current, approximated by the following equation: RSET = 120 × (1.18 V/ILED) To regulate the LED currents properly, sufficient headroom voltage (compliance) must be present. The compliance refers to the minimum amount of voltage that must be present across the internal current sinks to ensure that the desired current and matching performance is realizable. To ensure that the desired current is obtained, use the following equation to find the minimum input voltage required: VOUT – VF ≥ Compliance where VF is the LED forward voltage. For 20 mA/LED, the compliance is 0.20 V typ and 0.30 V max (see Table 5). Table 5. ILED, RSET, and Compliance Table ILED RSET Typ. Compliance 15 mA 9.44 k Ω 0.17 V 20 mA 7.08 k Ω 0.20 V 30 mA 4.72 k Ω 0.34 V When the ADM8845 charge pump is loaded with 180 mA (six LEDs at 30 mA/LED), the ambient operating temperature is reduced (see Figure 21). AUTOMATIC GAIN CONTROL The automatic gain control block controls the operation of the charge pump by selecting the appropriate gain for the charge pump. This maintains sufficient drive for the LED anodes at the highest power efficiency over a 2.6 V to 5.5 V input supply range. The charge pump switching thresholds are described in Table 6. Table 6. Charge Pump Switching Thresholds Gain Threshold 1.5× to 2× 3.33 V 2× to 1.5× 3.36 V 1× to 1.5× 4.77 V 1.5× to 1× 4.81 V CURRENT MATCHING The 1% maximum current matching performance is defined by the following equations: IAVG = (IMAX + IMIN)/2 Max Matching Error = [(IMAX – IAVG)/IAVG] × 100 or Min Matching Error = [(IMIN – IAVG)/IAVG] × 100 where IMAX is the largest ILED current, and IMIN is the smallest ILED current. BRIGHTNESS CONTROL WITH A DIGITAL PWM SIGNAL PWM brightness control provides the widest brightness control method by pulsing the white LEDs on and off using the digital input control pins, CTRL1 and/or CTRL2. PWM brightness control also removes any chromaticity shifts associated with changing the white LED current, because the LEDs operate either at zero current or full current (set by RSET). The digital PWM signal applied with a frequency of 100 Hz to 200 kHz turns the current control sinks on and off using CTRL1 and/or CTRL2. The average current through the LEDs changes with the PWM signal duty cycle. If the PWM frequency is much less than 100 Hz, flicker could be seen in the LEDs. For the ADM8845, zero duty cycle turns off the LEDs, and a 50% duty cycle results in an average LED current ILED being half the pro- grammed LED current. For example, if RSET is set to program 20 mA/LED, a 50% duty cycle results in an average ILED of 10 mA/LED, ILED being half the programmed LED current. FB1 FB2 FB3 FB4 FB5 FB6 VOUT C1 1 µF CTRL1 CTRL2 C2 1 µF RSET ISET C3 2.2 µF ADM8845 PWM INPUT OR HIGH/LOW PWM INPUT OR HIGH/LOW Figure 24. Digital PWM Brightness Control Application Diagram By applying a digital PWM signal to the digital input control pins, CTRL1 and/or CTRL2 can adjust the brightness of the sub and/or main displays. The ADM8845’s six white LEDs are organized into two groups, main display (FB1 to FB4) and sub display (FB4 to FB6); refer to the Theory of Operation section. |
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