Datasheet MAX1698, MAX1698A (Maxim) - 6

High-Efficiency Step-Up Current Regulator for LEDs
10µH VBATT = 3.3V MBR0540 10µF 1µF VCC = 3.3V VCC EXT 1µF MAX1698 MAX1698A LEDs SHDN CS OPTIONAL REF DZ ADJ FB GND PGND RFB 15Ω 15Ω 15Ω 15Ω
MAX1698/MAX1698A
Figure 2. Typical Operating Circuit
Detailed Description Soft-Start
The MAX1698/MAX1698A include a soft-start function The MAX1698/MAX1698As' high efficiency and small that eliminates input current surges at turn-on. They do size make them ideally suited to drive LEDs. They oper- this by extending the external FET driver (EXT) mini- ate as a boost DC-DC converter that controls output mum off-time during start-up. During the first 512 current rather than voltage. Losses are minimized by a switching cycles, the minimum off-time is 5µs. It is then low, 300mV current-sense threshold. In the standard allowed to drop to 2µs for the next 1500 switching configuration, a feedback resistor, RFB, sets the current cycles. After that time, the minimum off-time falls to the through the primary chain of LEDs. Additional chains of 1µs value used during normal operation. (See matching LEDs can be added with an equivalent resis- Shutdown/Soft-Start Timing in the Typical Operating tor. In matched LED arrays, the secondary chain cur- Characteristics section.) rents closely track the primary chain. An optional zener diode, D2, prevents overvoltage in the event that one of
Design Procedure
the LEDs in the primary chain becomes an open circuit. The LED brightness can be adjusted dynamically by a
Setting the Maximum LED Current
voltage input at ADJ. Resistor RFB sets the maximum current in the primary chain of LEDs:
Shutdown
In shutdown, the supply current is reduced below 1µA. mV 300 RFB = EXT goes low in shutdown, shutting off the external N- I LEDMAX channel FET. This leaves a current path between the input and the LEDs through the boost inductor and where ILEDMAX is the maximum LED current. catch diode. The minimum forward voltage of the LED
Adjusting LED Current
array must exceed the maximum VBATT to ensure that R the LEDs remain off in shutdown. Typical shutdown tim- FB sets the maximum LED current. This current can be reduced proportional to the voltage at the ADJ pin ing characteristics are shown in the Typical Operating (see Normalized Output Current vs. ADJ Voltage in the Characteristics. Typical Operating Characteristics section). Figure 3
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