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PAM2701 Datasheet(PDF) 9 Page - Power Analog Micoelectronics |
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PAM2701 Datasheet(HTML) 9 Page - Power Analog Micoelectronics |
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9 / 14 page  9 Application Information Detailed Description As shown in the block diagram on page 2, the main components within the PAM2701 include a fractional charge pump, mode selection circuit, output selection logic, LED current setting detection circuit, and 4 current sense circuits. The fractional charge pump multiplies the input voltage a multiple of 1X and 1.5X times the input voltage. The charge pump switches at a fixed 1MHz when the mode is 1.5X. The charge pump does not switch during 1X mode, saving power and improving efficiency. The mode selection circuit automatically selects the mode as 1X or 1.5X based on circuit conditions such as LED voltage, input voltage and load current. 1X is the more efficient mode than 1.5X mode. Table 1 on page 2 shows the output selection logic control over the LED outputs for on and off functions with 8 different output states. The current set and detection circuit uses an external resistor and a 1.20V reference to program the LED current. 4 current regulating circuits sink matched currents from the LEDs. LEDs with matched forward voltage will produce the best possible matched currents. For best m atching performance it is recommended that the Vf between LEDs be under 250mV. The unused LED channels can be turned off by CTR0, CTR1 and CTR2, and connecting the respective LED pins to VOUT pin, in which case, the corresponding LED driver sink current is only about 20 A. There are 4 methods for setting and adjusting the LED current outlined here. The methods are: 1) R only 2) Analog Reference V 3) PWM Input at CTR0 4) PWM Input at EN The most basic means of setting the LED current is connecting a resistor from R to GND, as shown in the application circuit on Page 1. The resistor R establishes the reference current needed for a constant LED current. Values of R for a fixed LED current are given in Table 2, “Typical R Resistance vs. LED Current”. Table 2: R Resistance Selection example circuit is shown in Figure 1, employing a 14.7k resistor and an analog input DC voltage, V , which varies from 1.2V to 0V to control LED current from 1mA to 30mA. Table 3 shows the resulting output. If necessary, the analog V voltage can be sourced from a voltage higher than 1.20V, but the source must be divided down so that the V mode will not exceed 1.20V. For lower current and higher resolution , a larger resistor may be used instead. PWM applications are also possible with this circuit by application of RC filtering (Consult with PAM for detail application support). μ Methods for Setting LED Current SET SET SET SET SET SET SET SET SET SET Method 1: LED Current Setting with an External Resistor R Method 2: LED Current Setting with a DC Voltage to RSET Pin SET Method 2 is for setting the LED current to control brightness. An applications ILED(mA) RSET(kΩ) Standard Value (kΩ) Value % Difference 30 14.7 14.7 0..0% 20 21.8 22.0 0.9% 15 29.3 29.4 0.3% 10 44.1 44.2 0.2% 9 49.4 49.9 1.0% 8 55.7 56.0 0.5% 7 64.2 63.4 0.3% 6 75.0 75.0 0.0% 5 90.0 88.7 -0.3% 4 114.2 115.0 0.7% 3 156.5 158.0 0.9% 2 238.0 237.0 -0.4% 1 540.0 536.0 -0.7% , Power Analog Microelectronics Inc www.poweranalog.com 09/2008 Rev 1.3 PAM2701 4-Channel Regulated Charge Pump White LED Driver |
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