Diodes AP8803 - Manual (Page 4)

AP8803
Document number: DS31970 Rev. 5 - 2
4 of 10
www.diodes.com
August 2012
© Diodes Incorporated
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 +V IN
(See Figure 1), the AP8803 internal switch is turned on. Current starts to flow through sense resistor R 1 , inductor L1,
and the LEDs. The current ramps up linearly, and the ramp rate is determined by the input voltage +V
IN
, and the inductor L1 (See Figure 2).
L1
R
SET
C1
AP8803
CTRL
V
IN
SET
SW
GND
12V_DC
Figure 1 Typical configuration
Figure 2 Typical switching waveform
This rising current produces a voltage ramp across R
SET
. The internal circuit of the AP8803 senses the voltage across R
SET
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
R
SET
, 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 R
SET
, which is sensed by the AP8803. A voltage proportional to the sense voltage across
R
SET
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 R
SET,
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:
1. The overall on time starts to increase lowering the overall switching frequency.
2. The average LED current starts to increase – which may impact accuracy.
LED Current Control
The LED current is controlled by the resistor R
SET
in Figure 1.
Connected between V
IN
and SET the nominal average output current in the LED(s) is defined as:
SET
TH
LED
R
V
I
=
where V
TH
is nominally 100mV