Datasheet MAX1698, MAX1698A (Maxim) - 7

High-Efficiency Step-Up Current Regulator for LEDs MAX1698/MAX1698A Transistor Selection
The MAX1698/MAX1698A drive an external N-channel MOSFET. Since the gate drive voltage is derived from REF VCC, best performance is achieved with low-threshold NFETs that specify on-resistance with gate-source volt- ADJ 500k ages (VGS) at the voltage supplied at VCC or less. For MAX1698 best results, minimize the FET’s RDS(ON). The external MAX1698A GND NFET’s maximum drain-to-source voltage (VDS(MAX)) must exceed the output voltage.
Catch Diode (D1) Selection
The MAX1698/MAX1698As' high-switching frequency Figure 3. Adjusting LED Current demands a high-speed rectifier. Schottky diodes are recommended for most applications, due to their fast shows the standard method of setting the ADJ voltage. recovery time and low forward-voltage drop. Ensure Use the following equation to determine I that the diode’s average and peak current ratings LED: exceed the average output current and peak inductor VADJ current, respectively. In addition, the diode’s reverse ILED = . 4 1 6 ⋅ R FB breakdown voltage must exceed VOUT. For output volt- ages exceeding 40V, high-speed silicon rectifiers may where VADJ is the voltage at ADJ. Note that ADJ volt- be required for their higher breakdown voltages. ages below 50mV turn the LEDs off.
Zener Diode Inductor Selection
For applications requiring open-circuit protection if one Choose an inductor with low DC resistance (in the of the LEDs in the primary chain opens, add a zener neighborhood of 100mΩ) to minimize losses. A typical diode as shown in Figure 2. The zener diode protects inductance value for L is 10µH; however, values from the MOSFET and output capacitor if the current feed- 3.3µH to 100µH can also be used. Higher inductor val- back signal is lost. The zener voltage should exceed ues reduce the MAX1698’s switching frequency. The the maximum forward voltage of the LED network by at typical operating frequency is given by: least 2V. 0 6 7 ⋅ V f BATT =
Applications Information
L
PCB Layout
The MAX1698/MAX1698A limit peak inductor current to Due to fast switching waveforms and high-current 1.5A, but also contains a control loop that reduces paths, careful PCB layout is required. Protoboards and inductor current as a function of output power. For a wire-wrap boards should not be used for evaluation. An given output power, the required inductor peak current EV kit (MAX1698EVKIT) is available to aid most rating is approximately set by: designs. IL(PEAK) = 1.0
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POUT When laying out a board, minimize trace lengths to CS, where POUT is the output power to all LED banks in the inductor, diode, input capacitor, and output capaci- watts and IL(PEAK) is in amperes. tor. Keep traces short, direct, and wide. Keep noisy traces, such as the inductor’s traces, away from FB.
Capacitor Selection
VCC’s bypass capacitor should be placed as close to The exact value of output capacitance is not critical. the IC as possible. Typical values for the output capacitor are 0.1µF to Refer to the MAX1698 EV kit for an example of proper 10µF. Larger values help reduce output ripple at the layout. expense of size and higher cost. The requirements of the input capacitor depend on the type of the input voltage source. However, in many
Chip Information
applications, the same capacitor type and value are used for both the input and output capacitors. TRANSISTOR COUNT: 2180
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