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MAX1698AEUB

High-Efficiency Step-Up Current Regulator for LEDs

Maxim Integrated 

High-Efficiency Step-Up

Current Regulator for LEDs

REF

ADJ

500k

MAX1698

MAX1698A

GND

Figure 3. Adjusting LED Current

shows the standard method of setting the ADJ voltage.

Use the following equation to determine ILED:

ILED =

VADJ

4.16 ⋅ RFB

where VADJ is the voltage at ADJ. Note that ADJ volt-

ages below 50mV turn the LEDs off.

Inductor Selection

Choose an inductor with low DC resistance (in the

neighborhood of 100mΩ) to minimize losses. A typical

inductance value for L is 10µH; however, values from

3.3µH to 100µH can also be used. Higher inductor val-

ues reduce the MAX1698’s switching frequency. The

typical operating frequency is given by:

f = 0.67 ⋅ VBATT

L

The MAX1698/MAX1698A limit peak inductor current to

1.5A, but also contains a control loop that reduces

inductor current as a function of output power. For a

given output power, the required inductor peak current

rating is approximately set by:

IL(PEAK) = 1.0 · POUT

where POUT is the output power to all LED banks in

watts and IL(PEAK) is in amperes.

Capacitor Selection

The exact value of output capacitance is not critical.

Typical values for the output capacitor are 0.1µF to

10µF. Larger values help reduce output ripple at the

expense of size and higher cost.

The requirements of the input capacitor depend on the

type of the input voltage source. However, in many

applications, the same capacitor type and value are

used for both the input and output capacitors.

Transistor Selection

The MAX1698/MAX1698A drive an external N-channel

MOSFET. Since the gate drive voltage is derived from

VCC, best performance is achieved with low-threshold

NFETs that specify on-resistance with gate-source volt-

ages (VGS) at the voltage supplied at VCC or less. For

best results, minimize the FET’s RDS(ON). The external

NFET’s maximum drain-to-source voltage (VDS(MAX))

must exceed the output voltage.

Catch Diode (D1) Selection

The MAX1698/MAX1698As' high-switching frequency

demands a high-speed rectifier. Schottky diodes are

recommended for most applications, due to their fast

recovery time and low forward-voltage drop. Ensure

that the diode’s average and peak current ratings

exceed the average output current and peak inductor

current, respectively. In addition, the diode’s reverse

breakdown voltage must exceed VOUT. For output volt-

ages exceeding 40V, high-speed silicon rectifiers may

be required for their higher breakdown voltages.

Zener Diode

For applications requiring open-circuit protection if one

of the LEDs in the primary chain opens, add a zener

diode as shown in Figure 2. The zener diode protects

the MOSFET and output capacitor if the current feed-

back signal is lost. The zener voltage should exceed

the maximum forward voltage of the LED network by at

least 2V.

Applications Information

PCB Layout

Due to fast switching waveforms and high-current

paths, careful PCB layout is required. Protoboards and

wire-wrap boards should not be used for evaluation. An

EV kit (MAX1698EVKIT) is available to aid most

designs.

When laying out a board, minimize trace lengths to CS,

the inductor, diode, input capacitor, and output capaci-

tor. Keep traces short, direct, and wide. Keep noisy

traces, such as the inductor’s traces, away from FB.

VCC’s bypass capacitor should be placed as close to

the IC as possible.

Refer to the MAX1698 EV kit for an example of proper

layout.

Chip Information

TRANSISTOR COUNT: 2180

_______________________________________________________________________________________ 7

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