LT3486 datasheet(8/20 Pages) LINER

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LT3486

3486f

OPERATIO

Main Control Loop

The LT3486 uses a constant frequency, current mode

control scheme to provide excellent line and load regulation.

It incorporates two identical, but fully independent PWM

converters. Operation can be best understood by referring

to the block diagram in Figure 1. The oscillator, start-up

bias and the bandgap reference are shared between the two

converters. The control circuitry, power switch, dimming

control etc., are all identical for both converters.

At power-up, the output capacitors of both converters are

charged up to VIN(inputsupplyvoltage)viatheirrespective

inductor and the Schottky diode. If the SHDN pin is taken

above 1.6V, the bandgap reference, start-up bias and the

oscillator are turned on. Grounding the SHDN pin shuts

down the part.

The CTRL1 and CTRL2 pins perform independent dimming

and shutdown control for the two converters. Taking

the CTRL pins high, enables the respective converters.

Connecting these pins to ground, shuts down each

converter by pulling their respective VC pin low.

Working of the main control loop can be understood by

following the operation of converter 1. At the start of

each oscillator cycle, the power switch Q1 is turned on.

A voltage proportional to the switch current is added to

a stabilizing ramp and the resulting sum is fed into the

positive terminal of the PWM comparator A2. When this

voltage exceeds the level at the negative input of A2, the

PWM logic turns off the power switch. The level at the

negative input of A2 is set by the error ampliﬁer A1, and

is simply an ampliﬁed version of the difference between

the feedback voltage and the 200mV reference voltage. In

this manner, the error ampliﬁer A1 regulates the feedback

voltage to 200mV reference voltage. The output of the

error ampliﬁer A1 sets the correct peak current level in

inductor L1 to keep the output in regulation. The CTRL1

pin voltage is used to adjust the reference voltage.

The PWM1, 2 control pins are used to extend the dimming

range for the individual converter. The LED current in each

string can be controlled down to µA levels by feeding

a PWM signal to these pins. Refer to the Applications

Information section for more detail.

If only one of the converters is turned on, the other converter

will stay off and its output will remain charged up to VIN

(input supply voltage).

Minimum Output Current

The LT3486 can drive an 8-LED string at 4mA LED current

without pulse skipping. As current is further reduced, the

device may begin skipping pulses. This will result in some

low frequency ripple, although the LED current remains

regulated on an average basis down to zero. The photo

in Figure 2 shows circuit operation with 8 white LEDs

at 4mA current driven from 3.6V supply. Peak inductor

current is less than 200mA and the regulator operates in

discontinuous mode implying that the inductor current

reached zero during the discharge phase. After the inductor

current reaches zero, the switch pin exhibits ringing due to

the LC tank circuit formed by the inductor in combination

with switch and diode capacitance. This ringing is not

harmful; far less spectral energy is contained in the ringing

than in the switch transitions. The ringing can be damped

by application of a 300Ω resistor across the inductors,

although this will degrade efﬁciency.

Figure 2. Switching Waveforms

VOUT2

10mV/DIV

VSW2

20V/DIV

IL2

200mA/DIV

0.5

µs/DIV

3486 F02

VIN = 3.6V

ILED2 = 4mA (8 LEDs)

CIRCUIT OF FRONT PAGE APPLICATION

Open-Circuit Protection

The LT3486 has internal open-circuit protection for both

the converters. Connect the overvoltage protection pins

(OVP1, OVP2) to the output of the respective converter.

When the LEDs are disconnected from the circuit or fail

open, the on-chip voltage detectors monitor the voltages at

the OVP1 and OVP2 pins and limits these voltages to 36V

(typ) by turning off the respective switcher. The converter

will then switch at a very low frequency to minimize the

input current. Output voltage and input current during

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