Diodes AP3033 Manuel d'utilisateur télécharger pdf (Page 9)

Preliminary Datasheet

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

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

=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

CTRL

2V/div

500mA/div

Time 80µs/div

5V/div

ISET

PWM

LED

DmV

200

LED

ISET

200

200mV

Ctrl

1 2 3 4 5 6 7 8 9 10 11 12 13

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Diodes AP3033 Manuel d'utilisateur Page 9

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