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ISL97634 Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL97634 Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 10 page 7 FN6264.3 March 7, 2008 only rated for the required voltage range can therefore be used, which will optimize the component costs in some cases. Shutdown When PWM/EN is taken low the ISL97634 enters into the power-down mode where the supply current is reduced to less than 1µA. The device resumes normal when the PWM/EN goes high. Components Selection The input capacitance is typically 0.22µF. The output capacitor should be in the range of 0.22µF to 1µF. X5R or X7R type of ceramic capacitors of the appropriate voltage rating are recommended. When choosing an inductor, make sure the average and peak current ratings are adequate by using Equations 3, 4 and 5 (80% efficiency assumed): Where: • ΔIL is the peak-to-peak inductor current ripple in Amps • L is the inductance in H •fOSC is the switching frequency, typically 1.45MHz The ISL97634 supports a wide range of inductance values (10µH to ~82µH). For lower inductor values or lighter loads, the boost inductor current may become discontinuous. For high boost inductor values, the boost inductor current will be in continuous mode. In addition to the inductor value and switching frequency, the input voltage, number of LEDs and the LED current also affects whether the converter operates in continuous conduction or discontinuous conduction mode. Both operating modes are allowed and normal. The discontinuous conduction mode yields lower efficiency due to higher peak current. Compensation The product of the output capacitor and the load create a pole while the inductor creates a right half plane zero. Both of these attributes degrade the phase margin but the ISL97634 has internal compensation network that ensures the device operates reliably under the specified conditions. The internal compensation and the highly integrated functions of the ISL97634 make it a design friendly device to be used in high volume, high reliability applications. Applications Analog Dimming Analog dimming is usually not recommended because of the brightness non-linearity at low levels dimming. However, some systems are EMI or noise sensitive that analog dimming may be more suitable than PWM dimming under those situations. The ISL97632 is part of the same family as the ISL97634 and has been designed with a serial interface to give access to 32 separate dimming levels. Alternatively analog dimming can be achieved by applying a variable DC voltage (VDim) at FB pin (see Figure 14) to adjust the LED current. As the DC dimming signal voltage increases above VFB, the voltages drop on R1 and R2 increase and the voltage drop on RSET decreases. Thus, the LED current decreases as shown in Equation 6: If VDIM is taken below FB, the inverse will happen and the brightness will increase. The DC dimming signal voltage can be a variable DC voltage from a POT, a DCP (Digitally Controlled Potentiometer), or a DC voltage generated by filtering a high frequency PWM control signal. As brightness is directly proportional to LED currents, VDim may be calculated for any desired “relative brightness” (F) using Equation 7: Where F = ILED (dimmed)/ILED (undimmed). TABLE 1. PART NO. OVP MAX NO. OF LEDS MAX ILED ISL97634IRT14Z 14V 3 70mA ISL97634IRT18Z 18V 4 50mA ISL97634IRT26Z 26V 7 30mA I LAVG I LED VOUT ⋅ 0.8 V IN ⋅ --------------------------------- = (EQ. 3) I LPK I LAVG 1 2 --- I L Δ ⋅ + = (EQ. 4) I L Δ V IN V OUT V IN – () ⋅ LV OUT fOSC ⋅⋅ --------------------------------------------------- = (EQ. 5) I LED V FB R 1 R 2 + () V Dim R1 ⋅ – ⋅ R 2 R ⋅ SET -------------------------------------------------------------------------- = (EQ. 6) FIGURE 14. ANALOG DIMMING CONTROL APPLICATION CIRCUIT ISL97634 C1 VIN LX PWM FB GND L1 22µH C2 0.22µF RSET 4.75 Ω LEDs 1µF LX VIN 3.3V R1 R2 VDim VOUT 3.3k V Dim R 2 R 1 ------- V FB 1 R 1 R 2 ------- F – + ⎝⎠ ⎜⎟ ⎛⎞ ⋅⋅ = (EQ. 7) ISL97634 |
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