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LP3942YQX Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LP3942YQX Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 20 page RGB Driver Electrical Characteristics Limits in standard typeface are for T J = +25˚C. Limits in boldface type apply over the full operating junction temperature range (−30˚C ≤ T J ≤ +105˚C). Unless otherwise noted, specifications apply to the LP3942 Typical Application Circuit (pg. 1) with: V VDD = 2.78V, VIN = 3.6V, CIN = 2.2 µF, C1 = 1.0 µF, C2 = 1.0 µF, COUT = 3.0 µF, RBIAS = 27k. (Note 9). Symbol Parameter Condition Min Typ Max Units I Leakage R1/2, G1/2, B1/2 Pin Leakage Current V RGB =5V 1 µA I MAX(RGB) Maximum Sink Current Constant Current Mode: Limited with external Rr, Rg, Rb resistors by user 3.20V < V IN < 5.0V 40 mA Constant Current Mode: 3.0 < V IN < 3.2V 30 mA Switch Mode: Limited with ballast resistors by user 50 mA Accuracy @ 20 mA V DD = 2.78V VRGB = 0.2V −10 ±310 % Current Mirror Ratio 1:100 RGB1 and RGB2 Mismatch 20 mA LED Current ±5% R RGB Switch mode 2 3.5 Ω f RGB RGB Switching Frequency Accuracy proportional to internal clock freq. 15 20 26 kHz RGB1 and RGB2 Output Description RGB1 and RGB2 outputs can be used in two modes: con- stant current and switch modes. There are few basic param- eters in the RGB1 and RGB2 outputs, which can cause lowered output current values and/or extra current mis- match, if not considered carefully in application. This chapter helps to do this analysis. CONSTANT CURRENT MODE Outputs can be understood as Ideal current sources with certain output resistance Rds. Additionally, outputs have a minimum voltage Vsat, which must be exceeded at the output. If voltage limit is not reached, the ideal current source’s current will be less than expected. Taking as an example the R1 output and defining the voltage between R1 pin and GND3 pin to Vr, and defining the ideal output current as Ir, one can estimate the true output current with following simple formula: Ireal A Ir + (Vr / Rds). This is true only, if Vr > Vsat. In the LP3942, the nominal value for Rds is 10 k Ω, and for Vsat is 200 mV. For example, if we have a LED that has 3.0V forward bias voltage at 10 mA and we have set the LED current to 10 mA. Then we can estimate the true LED current to be appr. 10 mA + ((4.5V-3.0V) / 10 k Ω) = 10.15 mA. And voltage on RGB output pin is 1.5V, which is high enough for the con- stant current generator. Note that this example does not consider the worst case tolerance of output current which results from tolerance of external bias resistor and current mismatch in the LP3942. SWITCH MODE In switch mode the function is more straightforward. De- signer needs to only consider the output resistance of the switch, which is typically 2 Ω. Using now the case from previous example and setting the LED current with an external ballast resistor of 150 Ω. Now, we get true output current of 1.5V/(150 + 2) A 9.9 mA. Note that this example does not consider the worst case tolerance of output current which results from tolerance of POUT voltage and LED forward voltage. RGB Functionality Both RGB outputs RGB1 and RGB2 have separate control for mode, brightness and color. The RGB LEDs are con- trolled through the SPI interface. The control register table is shown below. Addr. Reg. [7] [6] [5] [4] [3] [2] [1] [0] 00’h RGB1 Color[3] Color[2] Color[1] Color[0] Bright[2] Bright[1] Bright[0] Enable1 default 0 0 0 0 0 0 0 0 01’h RGB2 Color[3] Color[2] Color[1] Color[0] Bright[2] Bright[1] Bright[0] Enable2 default 0 0 0 0 0 0 0 0 02’h CTRL NSTBY CP_ON CC/SW2 CC/SW1 1.5x/2x reserved reserved reserved default 0 0 0 0 0 0 0 0 03’h OVL Overlap2 R2SW G2SW B2SW Overlap1 R1SW G1SW B1SW default 0 0 0 0 0 0 0 0 Enable1 and Enable2 bits are used to enable RGB1 and RGB2 outputs. OVL register can be used, if overlapping mode is needed, default mode is non-overlapping. If overlapping mode is www.national.com 9 |
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