Diodes AP3032 - Manual (Page 9)

Data Sheet
WHITE LED STEP-UP CONVERTER AP3032
Dec. 2012 Rev. 1. 2 BCD Semiconductor Manufacturing Limited
9
Application Information
1. Operation
The AP3032 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 21.
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.
2. LED Current Control
Refer to Figure 21, 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:
LED
ISET
200
I
mV
R
=
3. Over Voltage Protection
The AP3032 has an internal open-load protection
circuit. When the LEDs are disconnected from circuit
or fail open, the output voltage is clamped at 27V.
The AP3032 will switch at a low frequency, and
minimize current to avoid input voltage drop.
4. Soft Start
The AP3032 has an internal soft start circuit to limit
th
the inrush current during startup. If logic low time on
CTRL pin is more than about 0.7ms and then enable
the IC, the AP3032 will start smoothly to protect
system departments. The time of startup is controlled
by internal soft start capacitor. Details please refer to
Figure 15.
Figure 15. Soft Start Waveform
5. Standby and Dimming
To avoid audio noise and achieve high frequency
dimming, AP3032 setup a standby function. If logic
low time on CTRL pin is less than about 0.7ms and
then enable the IC, AP3032 will hold on standby
mode and start directly to achieve high frequency
dimming. Details please refer to Figure 16.
Figure 16. Standby Waveform