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

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

Once the maximum LED current is set, the LED current can be

changed through PWM input.

DUTY = 80%

PWMI

0A

DUTY = 60%

DUTY = 40%

DUTY = 20%

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,

this condition, OLP is activated.

UVLO

An undervoltage lockout circuit is included with built-in hysteresis.

Thermal Protection

Thermal overload protection prevents excessive power dissipation

from overheating the ADD5205. When the junction temperature

fault protection, which shuts down the device, allowing the IC to