Diodes AP3033 - Manual (Page 9)

Preliminary Datasheet
9
Jan. 2010 Rev. 1. 0 BCD Semiconductor Manufacturing Limited
WHITE LED STEP-UP CONVERTER AP3033
Application Information
Operation
The AP3033 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 Figure 3 and
Figure 17.
At the start of each oscillator cycle, 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 switch is turned off.
It is clear that the voltage level at 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.
LED Current Control
Refer to Figure 17, the LED current is controlled by the
feedback resistor R
ISET
. 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 preferred.
The resistance of R
ISET
is in inverse proportion to the
LED current since the feedback reference is fixed at
200mV. The relation for R
ISET
and LED current
(I
LED
)can be expressed as below:
Over Voltage Protection
The AP3033 has an internal open load protection
circuit. When the LEDs are disconnected from circuit
or fail open, the output voltage is clamped at about
17V. The AP3033 will switch at a low frequency, and
minimize current to avoid input voltage drop.
Soft Start
The AP3033 has an internal soft start circuit to limit
the inrush current during startup. If logic low time on
CTRL pin is more than about 0.45ms and then enable
the IC, the AP3033 will start smoothly to protect sys-
tem departments. The time of startup is controlled by
internal soft-start capacitor. Details please refer to
Figure 13.
Figure 13. Soft-start Waveform
V
IN
=5V, 3
×
8 LEDs, I
LED
=160mA
Dimming Control
For controlling LED brightness, the AP3033 provides
typically 200mV feedback voltage when the CTRL pin
is pulled constantly high. However, CTRL pin allows a
PWM signal to reduce this regulation voltage by
changing the PWM duty cycle to achieve LED
brightness dimming control. Detail circuit, as show in
Figure 14. The relationship between the duty cycle and
LED current can be expressed as below:
where D
PWM
is the duty cycle of PWM signal and
200mV is internal reference voltage.
Figure 14. Bolck Diagram of Programmable
FB Voltage Using PWM Signal
V
CTRL
2V/div
I
L
500mA/div
Time 80 µs/div
V
SW
5V/div
V
O
5V/div
ISET
PWM
LED R
D mV
I
×
=
200
LED
ISET I
mV
R
200
=
A
200mV
EA
FB
Ctrl