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ISL97632IRT26Z-TK Datasheet(PDF) 6 Page - Intersil Corporation |
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ISL97632IRT26Z-TK Datasheet(HTML) 6 Page - Intersil Corporation |
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6 / 9 page  6 FN9239.2 April 10, 2007 Detailed Description The ISL97632 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. There are three OVP models for driving 3, 4, and 6 LEDs and their OVP thresholds are set at 14V, 18V, and 26V respectively. The ISL97632 operates from an input voltage of 2.4V to 5.5V and ambient temperature from -40°C to +85°C. The switching frequency is around 1.45MHz and allows the driver circuit to employ small LC components. The forward current of the LED is set using the RSET resistor. In the steady state mode, the LED current is given by Equation 1: where S is the 5-bit Serial Interface Setting or Digital code from 0 to 31 programmed in the XSD single-wire interface. The default setting is 0 and the VFB is at minimum. Dimming Control The ISL97632 powers up to provide minimium current. By programming the digital code with the Intersil’s XSD single- wire interface as shown in Figure 1, the current can be changed linearly with the digital code from 0 to 31. Figure 6 shows LED current versus the programming codes. Overvoltage Protection The ISL97632 comes with overvoltage protection. The OVP trip points are at 14V, 18V, and 26V for ISL97632IRT14Z, ISL97632IRT18Z, and ISL97632IRT26Z respectively. The maximum numbers of LEDs and OVP threshold are shown in Table 1. When the device reaches the OVP, the LX stops switching, disabling the boost circuit until VOUT falls about 7% below the OVP threshold. At this point, LX will be allowed to switch again. The OVP event will not cause the device to shutdown There are three OVP options so that the 3 LEDs application should use the 14V OVP device and the 6 LEDs application should use the 26V OVP device. An output capacitor that is only rated for the required voltage range can therefore be used which will optimize the component costs in some cases. Shut-Down An active high EN pin is normally on but this pin can be used as a shutdown power saving function or zero brightness setting. When taken low the EN pin places the ISL97632 into power down mode down where the supply current is reduced to less than 1µA. The EN pin cannot be used as PWM input as the part resets to 0 whenever EN is low. To resume previous setting, the device needs to be reprogrammed. Output Disconnect The ISL97632 features a FBSW feedback disconnect switch that can be used in between the LED and Rset for an optional short-circuit protection. For example, the user may build an external short circuit detection to monitor the VOUT. If the VOUT goes low due to one or more LEDs which are shorted, the circuit can release the EN and FBSW switch to disconnect the LEDs. Components Selection The input capacitance is typically 0.22µF to 4.7µF. 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 the following equations (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 ISL97632 supports a wide range of inductance values (10µH~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, the number of LEDs and the LED current also affect whether the converter operates in continuous conduction or discontinuous conduction mode. Both operating modes are allowed and normal. The discontinuos 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 attributes degrade the phase margin but the ISL97632 has an internal compensation network that ensures the device operates reliabily under the specified conditions. The internal compensation and the highly integrated functions of the ISL97632 make it a design friendly device to be used in high volume high reliability applications. TABLE 1. PART NO. OVP MAX NO. OF LEDS MAX ILED ISL97632IRT14Z 14V 3 70mA ISL97632IRT18Z 18V 4 50mA ISL97632IRT26Z 26V 6 30mA I LED S () V FB S () R SET -------------------- 9.8mV 5.68mV S × + R SET -------------------------------------------------------- == (EQ. 1) I LAVG I LED VOUT ⋅ 0.8 V IN ⋅ --------------------------------- = (EQ. 2) I LPK I LAVG 1 2 --- I L Δ ⋅ + = (EQ. 3) I L Δ V IN V OUT V IN – () ⋅ LV OUT fOSC ⋅⋅ --------------------------------------------------- = (EQ. 4) ISL97632 |
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