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LM3508

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SNVS494C – APRIL 2007 – REVISED MAY 2013

Start-Up

The LM3508 features a soft-start to prevent large inrush currents during start-up that can cause excessive

voltage ripple on VIN. During start-up the average input current is ramped up at a controlled rate. For the typical

application circuit, driving 4LED’s from a 3.6V lithium battery at 30mA, when EN is driven high the average input

current ramps from zero to 160mA in 470µs. See plot of Soft Start functionality in the TYPICAL PERFORMANCE

CHARACTERISTICS.

DIM Operation

DIM is the input to the gate of an internal switch that accepts a logic level PWM waveform and modulates the

filter is approximately 500Hz. DIM frequencies higher than 100kHz cause the LED current to drastically deviate

vs DIM duty cycle (see TYPICAL PERFORMANCE CHARACTERISTICS) show the typical variation of the

current source set point voltage.

Enable Input and Output Isolation

Driving EN high turns the device on while driving EN low places the LM3508 in shutdown. In shutdown the

supply current reduces to less than 1µA, the internal synchronous PFET turns off as well as the current source

(N2 in Figure 27). This completely isolates the output from the input and prevents leakage current from flowing

through the LED’s. In shutdown the leakage current into SW and IN is typically 400nA. EN has an internal 383k

Ω

pull-down to PGND.

Peak Current Limit/Maximum Output Current

The LM3508 boost converter provides a peak current limit. When the peak inductor current reaches the peak

current limit the duty cycle is terminated. This results in a limit on the maximum output power and thus the

(1)

PERFORMANCE CHARACTERISTICS.

Output Current Accuracy

The LM3508 provides highly accurate output current regulation of ±5% over the 3V to 5.5V input voltage range.

Voltage Head Room at ILED

If the LED current is increased to a point where the peak inductor current is reached, the boost converter's on-

time is terminated until the next switching cycle. If the LED current is further increased the 500mV regulated

the current sink FET (see N2 in Figure 27) will be fully on, appearing as a 5

Ω resistor between ILED and SET.

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Product Folder Links: LM3508