MAX1698(2000) View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX1698
(Rev.:2000)

High-Efficiency Step-Up Current Regulator for LEDs

Maxim Integrated 

High-Efficiency Step-Up

Current Regulator for LEDs

REF

ADJ

500k

MAX1698

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 turns 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 will reduce the MAX1698’s switching frequency.

The typical operating frequency is given by:

f = 0.67 ⋅ VBATT

L

The MAX1698 limits peak inductor current to 1.5A, but

also contains a control loop that reduces inductor cur-

rent 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 drives 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 voltages (VGS)

at the voltage supplied at VCC or less. For best results,

minimize the FET’s RDS(ON). The external NFET’s maxi-

mum drain-to-source voltage (VDS(MAX)) must exceed

the output voltage.

Catch Diode (D1) Selection

The MAX1698’s high switching frequency demands a

high-speed rectifier. Schottky diodes are recommend-

ed 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 voltages 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

PC Board Layout

Due to fast switching waveforms and high-current

paths, careful PC board layout is required. Protoboards

and wire-wrap boards should not be used for evalua-

tion. 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

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