LT3475IFE-PBF データシートの表示（PDF） - Linear Technology

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LT3475IFE-PBF

Dual Step-Down 1.5A LED Driver

Linear Technology 

LT3475/LT3475-1

OPERATION

The LT3475 is a dual constant frequency, current mode

regulator with internal power switches capable of gen-

erating constant 1.5A outputs. Operation can be best

understood by referring to the Block Diagram.

If the SHDN pin is tied to ground, the LT3475 is shut

down and draws minimal current from the input source

tied to VIN. If the SHDN pin exceeds 1V, the internal bias

circuits turn on, including the internal regulator, reference

and oscillator. The switching regulators will only begin to

operate when the SHDN pin exceeds 2.6V.

The switcher is a current mode regulator. Instead of directly

modulating the duty cycle of the power switch, the feedback

loop controls the peak current in the switch during each

cycle. Compared to voltage mode control, current mode

control improves loop dynamics and provides cycle-by-

cycle current limit.

A pulse from the oscillator sets the RS flip-flop and turns

on the internal NPN bipolar power switch. Current in the

switch and the external inductor begins to increase. When

this current exceeds a level determined by the voltage at

VC, current comparator C1 resets the flip-flop, turning

off the switch. The current in the inductor flows through

the external Schottky diode and begins to decrease. The

cycle begins again at the next pulse from the oscillator.

In this way, the voltage on the VC pin controls the current

through the inductor to the output. The internal error

amplifier regulates the output current by continually

adjusting the VC pin voltage. The threshold for switching

on the VC pin is 0.8V, and an active clamp of 1.8V limits

the output current.

The voltage on the VADJ pin sets the current through the

LED pin. The NPN, Q3, pulls a current proportional to the

voltage on the VADJ pin through the 100Ω resistor. The gm

amplifier servos the VC pin to set the current through the

0.067Ω resistor and the LED pin. When the voltage drop

across the 0.067Ω resistor is equal to the voltage drop

across the 100Ω resistor, the servo loop is balanced.

Tying the REF pin to the VADJ pin sets the LED pin current

to 1.5A. Tying a resistor divider to the REF pin allows the

programming of LED pin currents of less than 1.5A. LED

pin current can also be programmed by tying the VADJ pin

directly to a voltage source.

An LED can be dimmed with pulse width modulation

using the PWM pin and an external NFET. If the PWM

pin is unconnected or is pulled high, the part operates

nominally. If the PWM pin is pulled low, the VC pin is dis-

connected from the internal circuitry and draws minimal

current from the compensation capacitor. Circuitry draw-

ing current from the OUT pin is also disabled. This way,

the VC pin and the output capacitor store the state of

the LED pin current until the PWM is pulled high again.

This leads to a highly linear relationship between pulse

width and output light, allowing for a large and accurate

dimming range.

The RT pin allows programming of the switching frequency.

For applications requiring the smallest external components

possible, a fast switching frequency can be used. If low

dropout or very high input voltages are required, a slower

switching frequency can be programmed.

During startup VOUT will be at a low voltage. The NPN,

Q3, can only operate correctly with sufficient voltage

of ≈1.7V at VOUT, A comparator senses VOUT and forces

the VC pin high until VOUT rises above 2V, and Q3 is op-

erating correctly.

The switching regulator performs frequency foldback

during overload conditions. An amplifier senses when

VOUT is less than 2V and begins decreasing the oscillator

frequency down from full frequency to 15% of the nominal

frequency when VOUT = 0V. The OUT pin is less than 2V

during startup, short circuit, and overload conditions.

Frequency foldback helps limit switch current under these

conditions.

The switch driver operates either from VIN or from the

BOOST pin. An external capacitor and Schottky diode

are used to generate a voltage at the BOOST pin that

is higher than the input supply. This allows the driver

to saturate the internal bipolar NPN power switch for

efficient operation.

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