ADD5205 Data Sheet
Rev. 0 | Page 10 of 16
THEORY OF OPERATION
CURRENT MODE, STEPUP SWITCHING
REGULATOR OPERATION
The ADD5205 uses a current mode PWM boost regulator to
provide the minimal voltage needed to enable the LED string at
the programmed LED current. The current mode regulation
system allows fast transient response while maintaining a stable
output voltage. The regulator response can be optimized for a
wide range of input voltages, output voltages, and load conditions.
The ADD5205 can provide a 28 V fixed overvoltage protection
voltage and drive up to 6 LEDs (3.4 V/20 mA type of LEDs) for
four channels from a supply of 3 V or up to 7 LEDs (3.4 V/20 mA
type of LEDs) for four channels from a supply of 5 V to 18 V.
CURRENT SOURCE
The ADD5205 contains four current sources to provide accurate
current sinking for each LED string. String-to-string tolerance
is kept within ±2% at 20 mA. Each LED string current is adjusted
up to 25 mA by an external resistor.
The ADD5205 contains an LED open fault protection circuit for
each channel. The ADD5205 recognizes that the current source
has an open load fault for the current source, and the current
source is disabled.
Programming the LED Current
As shown in Figure 2, the ADD5205 has an LED current set pin
(ISET). A resistor (R
SET
) from this pin to ground adjusts the
LED current up to 25 mA (see Figure 15). LED current level can
be set by
)(
2600
A
R
I
SET
LED
=
PWM DIMMING MODE
The ADD5205 supports PWM input. The internal resistor and
external capacitor change the PWM input duty to analog level,
and the low-pass filter output adjusts each current source sink
current level.
DC Current Dimming
In this mode, the maximum LED current is set by the value of R
SET
.
Once the maximum LED current is set, the LED current can be
changed through PWM input.
DUTY = 80%
PWMI
I
LED MAX
I
LED
0A
DUTY = 60% DUTY = 40% DUTY = 20%
0.8 × I
LED MAX
0.6 × I
LED MAX
0.4 × I
LED MAX
0.2 × I
LED MAX
09858-014
Figure 14. DC Current Dimming Timing
SAFETY FEATURES
The ADD5205 contains several safety features to provide stable
operation, such as soft start, open load protection (OLP), under-
voltage lockout (UVLO), and thermal protection.
Soft Start
The ADD5205 contains an internal soft start function to reduce
inrush current at startup. The soft start time is typically 1.5 ms.
OLP
The ADD5205 contains a headroom control circuit to minimize
power loss at each current source. Therefore, the minimum feed-
back voltage is achieved by regulating the output voltage of the
boost converter. If any LED string is open during normal operation,
the current source headroom voltage (V
HR
) is pulled to GND. In
this condition, OLP is activated.
UVLO
An undervoltage lockout circuit is included with built-in hysteresis.
The ADD5205 turns on when V
IN
rises above 2.5 V (typical) and
shuts down when V
IN
falls below 2 V (typical).
Thermal Protection
Thermal overload protection prevents excessive power dissipation
from overheating the ADD5205. When the junction temperature
(T
J
) exceeds 160°C, a thermal sensor immediately activates the
fault protection, which shuts down the device, allowing the IC to
cool. The device self starts when the T
J
of the die falls below 130°C.
