SP6699 datasheet(8/12 Pages) EXAR

8 / 12 page

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Rev. 2.1.0

THEORY OF OPERATION

DETAILED DESCRIPTION

The SP6699 is a boost DC-DC converter which

uses a constant frequency, current mode

control scheme to provide excellent line and

load

regulation.

Operation

can

best

understood by referring to the Figure 1. At the

start of each oscillator cycle, the SR latch is

set and the switch Q1 turns on. The switch

current will increase linearly. The voltage on

sense resistor is proportional to the switch

current. The output of the current sense

amplifier is added to a stabilizing ramp and

the result is fed into the non-inversion input of

the PWM comparator A2. When this voltage

exceeds the output voltage level of the error

amplifier A1, the SR latch is reset and the

switch is turned off. It is clear that the voltage

level at non-inversion input of A2 sets the

peak current level to keep the output in

regulation. This voltage level is the output

signal of error amplifier A1, and is the

amplified signal of the voltage difference

between feedback voltage and reference

voltage of 200mV. So, a constant output

current can be provided by this operation

mode.

Fig. 19: R1 Selection

Typical Application Circuit

LED CURRENT CONTROL

Refer to Figure 19, the LED current is

controlled by the feedback resistor R1. LEDs'

current

accuracy

determined

the

regulator's feedback threshold accuracy and is

independent of the LED's forward voltage

variation.

the

precise

resistors

are

preferred. The resistance of R1 is in inverse

proportion to the LED current since the

feedback reference is fixed at 200mV. The

relation for R1 and LED current can be

expressed as below:

R1=200mV/ILED

OVER VOLTAGE PROTECTION

The SP6699 has an internal open-circuit

protection

circuit.

When

the

LEDs

are

disconnected from the circuit or fail open, the

output voltage is clamped at 27V. The SP6699

will switch at a low frequency, and minimize

input current.

SOFT START

The SP6699 has an internal soft start circuit to

limit the inrush current during startup. The

startup delay is controlled by internal soft start

capacitor. Please refer to Figure 20.

Fig. 20: Soft Start Waveform

VIN=3.6V, 4 LEDs, ILED=20mA

DIMMING CONTROL

Two typical types of dimming control circuit

are present. First, controlling the CTRL Pin

voltage to change operation state is a good

choice. Second, changing the feedback voltage

to get appropriate duty and luminous intensity

is also useful.

Adding a control signal to CTRL pin

There are three methods to control CTRL pin

signal.

First, adding a PWM Signal to the CTRL pin

directly. The SP6699 is turned on or off by the

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