LM3405 datasheet(7/29 Pages) TI1

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VIN

-VD1

TON

Inductor

Current

D = TON/TSW

VSW

TOFF

TSW

Voltage

'iL

ILPK

LM3405

www.ti.com

SNVS429B – OCTOBER 2006 – REVISED MAY 2013

APPLICATION INFORMATION

THEORY OF OPERATION

The LM3405 is a PWM, current-mode control switching buck regulator designed to provide a simple, high

efficiency solution for driving LEDs with a preset switching frequency of 1.6MHz. This high frequency allows the

LM3405 to operate with small surface mount capacitors and inductors, resulting in LED drivers that need only a

minimum amount of board space. The LM3405 is internally compensated, simple to use, and requires few

external components.

The following description of operation of the LM3405 will refer to the Simplified Block Diagram (Figure 13) and to

the waveforms in Figure 14. The LM3405 supplies a regulated output current by switching the internal NMOS

power switch at constant frequency and variable duty cycle. A switching cycle begins at the falling edge of the

reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the

internal NMOS power switch. During this on-time, the SW pin voltage (VSW) swings up to approximately VIN, and

the inductor current (IL) increases with a linear slope. IL is measured by the current sense amplifier, which

generates an output proportional to the switch current. The sense signal is summed with the regulator’s

corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the

feedback voltage and VREF. When the PWM comparator output goes high, the internal power switch turns off until

the next switching cycle begins. During the switch off-time, inductor current discharges through the catch diode

D1, which forces the SW pin to swing below ground by the forward voltage (VD1) of the catch diode. The

regulator loop adjusts the duty cycle (D) to maintain a constant output current (IF) through the LED, by forcing FB

pin voltage to be equal to VREF (0.205V).

Figure 14. SW Pin Voltage and Inductor Current Waveforms of LM3405

BOOST FUNCTION

Capacitor C3 and diode D2 in Figure 13 are used to generate a voltage VBOOST. The voltage across C3, VBOOST -

VSW, is the gate drive voltage to the internal NMOS power switch. To properly drive the internal NMOS switch

during its on-time, VBOOST needs to be at least 2.5V greater than VSW. Large value of VBOOST - VSW is

recommended to achieve better efficiency by minimizing both the internal switch ON resistance (RDS(ON)), and the

switch rise and fall times. However, VBOOST - VSW should not exceed the maximum operating limit of 5.5V.

When the LM3405 starts up, internal circuitry from VIN supplies a 20mA current to the BOOST pin, flowing out of

the BOOST pin into C3. This current charges C3 to a voltage sufficient to turn the switch on. The BOOST pin will

continue to source current to C3 until the voltage at the feedback pin is greater than 123mV.

There are various methods to derive VBOOST:

1. From the input voltage (VIN)

2. From the output voltage (VOUT)

3. From a shunt or series zener diode

4. From an external distributed voltage rail (VEXT)

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