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LT3497

3497f

OPERATION

Main Control Loop

The LT3497 uses a constant frequency, current mode con-

trol 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 band gap reference are shared between the two

converters. The control circuitry, power switch, Schottky

diode etc., are identical for both the converters.

At power up, the capacitors at CAP1 and CAP2 pins are

charged up to VIN (input supply voltage) via their respective

inductor and the internal Schottky diode. If either CTRL1

and CTRL2 or both are pulled higher than 100mV, the

bandgap reference, the start-up bias and the oscillator

are turned on.

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 VCAP1 and VLED1 voltage and

the bandgap reference. In this manner the error ampliﬁer,

A1, sets the correct peak current level in inductor L1 to

keep the output in regulation. The CTRL1 pin is used to

adjust the LED current.

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). The LT3497 enters into shutdown

when both CTRL1 and CTRL2 pins are pulled lower than

50mV. The CTRL1 and CTRL2 pins perform independent

dimming and shutdown control for the two converters.

Minimum Output Current

The LT3497 can drive a 4-LED string at 2mA 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

average LED current remains regulated down to zero. The

photo in Figure 2 details circuit operation driving 4 white

LEDs at 2mA. Peak inductor current is less than 50mA and

the regulator operates in discontinuous mode, meaning

the inductor current reaches zero during the discharge

phase. After the inductor current reaches zero, the SW

pin exhibits ringing due to the LC tank circuit formed

by the inductor in combination with the switch and the

diode capacitance. This ringing is not harmful; far less

spectral energy is contained in the ringing than in the

switch transitions.

Figure 2. Switching Waveforms

50mA/DIV

VSW

10V/DIV

VIN = 4.2V

ILED = 2mA

4 LEDs

200ns/DIV

3497 F02

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