Preliminary Datasheet EL7801 (Intersil) - 6

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EL7801 Theory of Operation General Description
The EL7801 is a flexible, highly integrated high-power LED VIN driver consisting of a PWM switching controller and integrated 36V NDMOS power FET. The device can drive up FB to 8 series high-power LED's at currents up to 1A. The Voltage LEVEL E control loop can be configured as either as a boost or buck S Feedback SHIFT N E 0.5 S regulator, providing an output voltage above or below the GND R input supply voltage, depending on the number of stacked + LED's. The controller operates from 2.7V to 16V and can be - powered by a single lithium ion battery, 5V or 12V regulated supplies or automotive electrical systems. LED Current is EL7801 VDC /2 sensed through a low value resistor in series with the LED. The resistor may be referenced to ground or the input rail,
FIGURE 3. FB REFERENCE AUTO SWITCH
allowing operation with supplies that span the output voltage, for example a lithium ion battery driving one LED.
Start Up
Load current can be adjusted using a thermistor to correct To maximize external PWM switching speed, the EL7801 for the reduction in optical efficiency of white LED's with doesn't include an internal soft-start circuit. When VDC increasing temperature. The thermistor is also used to exceeds the power on reset threshold, switching is delayed implement a thermal protection scheme to limit the for 1ms (TDELAY) allowing the output capacitor to charge maximum LED temperature to a preset customer level. through the inductor. If soft-start control is required, a suitable application circuit is shown in Figure 4.
Switching Regulator
The EL7801 employs a current mode PWM control scheme with a nominal switching frequency of 1MHz. This provides VBAT fast transient response and enables the use of low profile H u 1 VBAT FAULT L 10 inductors and compact multilayer ceramic capacitors. VOUT Settling time is optimized by the use of a simple control loop VIN without an error amplifier, relying instead on intrinsic gain SWD1 C1 20uF COUT SWD2 4.7nF within the direct summing path. Due to the lower loop gain, EL7801 offset must be accounted for when setting up initial LED bias R1 FB current. Refer to the applications section of the datasheet for E 100 SWS1 SWS2 S R2 N 2k E .5 0 further information. Figure 2 shows a block diagram of the S R system.
Application Configurations FIGURE 4. EXTERNAL SOFT START CIRCUIT Operating Modes
The EL7801 can operate as either a buck or boost regulator.
Light Level Control
Hardwire BUCK/BOOSTN to GND for boost mode or to VDC Two light control schemes are provided: for buck mode. In buck mode the power NDMOS drive circuit is "floated" (boot-strapped) allowing the NDMOS gate to be 1. An external PWM signal via the EN/PWM pin, providing driven above VIN to fully enhance the power NDMOS. An low frequency PWM dimming. internal Schottky diode between VDC (5V) and VHI reduces 2. Bias current level adjustment via the LEVEL input or fixed external component count. Use a ceramic capacitor of at internal bias. least 50nF between VHI and SWS1/2 to bootstrap VHI.
PWM Dimming LED Load Connection
LED color temperature varies with bias current. In EL7801 includes an auto-sensing FB level shift circuit that backlighting applications PWM dimming offers better control enables the LED load to be connected to either GND or VIN. of color temperature because current through the LED's is An internal sense circuit monitors the FB pin voltage. When kept constant. A 5V gate driver (ENL) synchronized to the level exceeds VDC/2, the feedback reference voltage is EN/PWM can be used to control an external FET and switched from GND to VIN. Refer to the application section disconnect the LED stack during the PWM off period. The of the datasheet for typical application schematics. switch prevents discharge of the output capacitor by the LED load, maintaining a constant bias independent of PWM duty cycle. Operation at 1kHz PWM rate is shown in Figure 5 and 6 FN7354.0 December 22, 2005