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LTM8005 Datenblatt(PDF) 14 Page - Linear Technology |
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LTM8005 Datenblatt(HTML) 14 Page - Linear Technology |
14 / 26 page LTM8005 14 8005f For more information www.linear.com/LTM8005 APPLICATIONS INFORMATION PWM Dimming Control for Brightness There are two methods to control the LED current for dimming using the LTM8005. One method uses the CTRL pins to adjust the current regulated in the LEDs. A sec- ond method uses the PWM pin to modulate the LED cur- rent between zero and full current to achieve a precisely programmed average current, without the possibility of color shift that occurs at low current in LEDs. To make PWM dimming more accurate, the switch demand cur- rent is stored on the VC node during the quiescent phase when PWM is low. This feature minimizes recovery time when the PWM signal goes high. To further improve the recovery time, a disconnect MOSFET switch has been implemented to open the LED current path to prevent the output capacitor from discharging during the PWM signal low phase. The minimum PWM on or off time depends on the choice of operating frequency set by the RT input. For best current accuracy, the minimum recommended PWM high time should be at least three switching cycles (3µs for fSW = 1MHz). A low duty cycle PWM signal can cause excessive start-up times if it is allowed to interrupt the soft-start sequence. Therefore, once start-up is initiated by a PWM signal, the LTM8005 will ignore a logical disable by the external PWM input signal. The device will continue to soft-start with switching and TG enabled until either the voltage at SS reaches about 1V or the output current reaches one-fourth of the full-scale current. At this point the device will begin following the dimming control as designated by PWM. If at any time an output overcurrent is detected, the internal MOSFETs will be disabled even as SS continues to charge. Programming the Switching Frequency The RT frequency adjust pin allows the user to program the switching frequency from 100kHz to 1MHz to opti- mize efficiency/performance or external component size. Higher frequency operation yields smaller component size but increases switching losses and gate driving cur- rent, and may not allow sufficiently high or low duty cycle operation. Lower frequency operation gives better perfor- mance at the cost of larger external component size. For an appropriate RT resistor value see Table 1. An external resistor from the RT pin to GND is required—do not leave this pin open. Table1. Typical Switching Frequency vs RT Value (1% Resistor) fOSC(kHz) RT(k) 1000 6.65 900 7.50 800 8.87 700 10.2 600 12.4 500 15.4 400 19.6 300 26.1 200 39.2 100 82.5 Spread Spectrum Frequency Modulation Switching regulators can be particularly troublesome for applications where electromagnetic interference (EMI) is a concern. To improve the EMI performance, the LTM8005 includes a spread spectrum frequency fea- ture. If there is a capacitor (CRAMP) at the RAMP pin, a triangle wave sweeping between about 1V and 2V is gen- erated. This signal is then fed into the internal oscillator to modulate the switching frequency between about 70% of the base frequency and the base frequency, which is set by the RT resistor. The modulation frequency is set by 12µA/(2 • CRAMP). The results of EMI measurements are sensitive to the RAMP frequency selected with the capacitor. 1kHz is a good starting point to optimize peak measurements, but some fine tuning of this selection may be necessary to get the best overall EMI results in a particular system. Consult factory applications for more detailed information about EMI reduction. The Typical Performance Characteristics section contains plots that show the LTM8005 conducted and radiated emissions with and without Spread Spectrum enabled. |
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