AP8803 datasheet(4/10 Pages) DIODES

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AP8803

Document number: DS31970 Rev. 5 - 2

4 of 10

www.diodes.com

August 2012

AP8803

Application Information

AP8803 Operation

The AP8803 is a hysteretic LED current switching regulator sometimes known as an equal ripple switching regulator. In normal operation, when

voltage is applied at +VIN (See Figure 1), the AP8803 internal switch is turned on. Current starts to flow through sense resistor R1, inductor L1,

and the LEDs. The current ramps up linearly, and the ramp rate is determined by the input voltage +VIN, and the inductor L1 (See Figure 2).

RSET

AP8803

CTRL

VIN

SET

GND

12V_DC

Figure 1 Typical configuration

Figure 2 Typical switching waveform

This rising current produces a voltage ramp across RSET. The internal circuit of the AP8803 senses the voltage across RSET and applies a

proportional voltage to the input of the internal comparator.

When this voltage reaches an internally set upper threshold, the internal switch is turned off. The inductor current continues to flow through

RSET, L1, the LEDs and the schottky diode D1, and back to the supply rail, but it decays, with the rate of decay determined by the forward

voltage drop of the LEDs and the schottky diode.

This decaying current produces a falling voltage at RSET, which is sensed by the AP8803. A voltage proportional to the sense voltage across

RSET is applied at the input of the internal comparator. When this voltage falls to the internally set lower threshold, the internal switch is turned

on again. This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor RSET, with a switching current

determined by the input voltage and LED chain voltage.

In normal operation the off time is relatively constant (determined mainly by the LED chain voltage) with only the on-time varying as the input

voltage changes. At duty cycles up to around 80% the ramp of the LED/switch current is very linear; however, as the duty cycle approaches

95% the LED current ramp starts to become more exponential. This has two effects:

The overall on time starts to increase lowering the overall switching frequency.

The average LED current starts to increase – which may impact accuracy.

LED Current Control

The LED current is controlled by the resistor RSET in Figure 1.

Connected between VIN and SET the nominal average output current in the LED(s) is defined as:

SET

LED

where VTH is nominally 100mV

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