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S P 6 6 9 9
2 7 V S t e p U p W h i t e L E D D r i v e r
THEORY OF OPERATION
D ETAILED 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 be 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
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/I LED
O VER V OLTAGE P ROTECTION
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.
S OFT S TART
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.
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. 20: Soft Start Waveform
V IN =3.6V, 4 LEDs, I LED =20mA
D IMMING C ONTROL
Two typical types of dimming control circuit
are present. First, controlling the CTRL Pin
Fig. 19: R1 Selection
Typical Application Circuit
LED C URRENT C ONTROL
Refer to Figure 19, the LED current is
controlled by the feedback resistor R1. LEDs'
current accuracy is determined by the
regulator's feedback threshold accuracy and is
independent of the LED's forward voltage
variation. So the precise resistors are
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
? 2011 Exar Corporation
8/12
Rev. 2.1.0
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