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

VBATT = 3.3V

10µH

10µF

VCC = 3.3V

1µF

VCC

EXT

MAX1698

SHDN

CS

REF

ADJ

FB

GND PGND

MBR0540

OPTIONAL

DZ

1µF

LEDs

RFB

15Ω

15Ω

15Ω

15Ω

Figure 2. Typical Operating Circuit

Detailed Description

The MAX1698’s high efficiency and small size make it

ideally suited to drive LEDs. It operates as a boost DC-

DC converter that controls output current rather than

voltage. Losses are minimized by a low, 300mV cur-

rent-sense threshold. In the standard configuration, a

feedback resistor, RFB, sets the current through the pri-

mary chain of LEDs. Additional chains of matching

LEDs can be added with an equivalent resistor. In

matched LED arrays, the secondary chain currents

closely track the primary chain. An optional zener

diode, D2, prevents overvoltage in the event that one of

the LEDs in the primary chain becomes an open circuit.

The LED brightness can be adjusted dynamically by a

voltage input at ADJ.

Shutdown

In shutdown, the MAX1698’s supply current is reduced

below 1µA. EXT goes low in shutdown, shutting off the

external N-channel FET. This leaves a current path

between the input and the LEDs through the boost

inductor and catch diode. The minimum forward volt-

age of the LED array must exceed the maximum VBATT

to ensure that the LEDs remain off in shutdown. Typical

shutdown timing characteristics are shown in the

Typical Operating Characteristics.

Soft-Start

The MAX1698 includes a soft-start function that elimi-

nates input current surges at turn-on. It does this by

extending the external FET driver (EXT) minimum off-

time during start-up. During the first 512 switching

cycles, the minimum off-time is 5µs. It is then allowed to

drop to 2µs for the next 1500 switching cycles. After

that time, the minimum off-time falls to the 1µs value

used during normal operation. (See Shutdown/Soft-

Start Timing in the Typical Operating Characteristics

section.)

Design Procedure

Setting the Maximum LED Current

Resistor RFB sets the maximum current in the primary

chain of LEDs:

RFB =

300mV

I LEDMAX

where ILEDMAX is the maximum LED current.

Adjusting LED Current

RFB sets the maximum LED current. This current can

be reduced proportional to the voltage at the ADJ pin

(see Normalized Output Current vs. ADJ Voltage in the

Typical Operating Characteristics section). Figure 3

6 _______________________________________________________________________________________

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