LTC3524

11

3524f

OPERATION

output voltage is not set to a fixed voltage, but rather

controlled to produce the programmed current in the

LED strings. The output (VLED) is rated for a maximum

of 21V which will support two strings of up to five series

LED in most cases.

The boost output is used to power one or two white LED

strings with a common ground. If only one string is en-

abled (ELED1 or ELED2) the voltage on that string (LED1

or LED2) will be controlled to regulate the LED current

set at the PROG pin. The voltage on VLED will be slightly

greater due to the overhead needed for the internal sense

element and share circuitry. For example, a single string

application with four white LEDs programmed at 20mA

would require 14.4V on LED1 if the forward drop on each

LED is 3.6V. The voltage on VLED may need to be 15V to

support the drops on the internal share circuitry. For ap-

plications with five or fewer LED elements, a single-string

operation will provide better efficiency.

If both strings are enabled, the boost output (VLED)

will generate the voltage required to regulate current in

the higher voltage string. Voltage on the lower string is

controlled by the internal share circuit to provide the pro-

grammed current. The LTC3524 achieves current matching

between the strings while minimizing the voltage drop

between VLED and the higher voltage string (to maintain

high efficiency). For example, an application with four

LEDs on LED1 and five LEDs on LED2 is programmed for

forward drop, LED1 is 14.4V, LED2 is 18V, and VLED is

18.6V. The drop between VLED and LED1 is 4V at 20mA,

resulting in lower efficiency. For this reason, it is recom-

mended when possible to keep the number of LEDs in

each string matched.

Analog Dimming:

The LTC3524’s white LED driver allows both analog and

PWM dimming to be implemented. Analog dimming

provides a lower noise solution but a reduced dynamic

range. Analog dimming can be implemented by resis-

tively summing a current into the PROG pin. The LED

I

V

R

VV

R

LED

PROG

SUM

SUM

=+

−

⎛

⎝⎜

⎞

⎠⎟

1625

1 225

1 225

•

..

produce LED currents between 3mA and 20mA.

True Color PWM Dimming:

PWM dimming can be implemented by enabling and dis-

abling the LED strings with ELED1 and ELED2. A PWM

frequency between 100Hz and 500Hz is generally recom-

mended to get wide dimming range while operating at a

frequency faster than the eye can detect. For best results,

the LCD bias portion of the device should be enabled (to

keep the device out of shutdown) and ELED1 and ELED2

should be driven with a common low frequency PWM

signal. PWM dimming waveforms are shown in the Typical

Performance Characteristics section of this datasheet.

The achievable dimming range is dependant on the PWM

cycle (or light output) that the strings can be controlled

to is given by:

For example, if the settling time is 50μS and the PWM

frequency is 100Hz, the minimum duty cycle is 0.5%

which corresponds to a 200:1 dimming range.

Open LED:

The LTC3524 has internal over voltage protection in the

event that one of the white LED strings becomes open

circuited. If VLED reaches 24V (nominal) due to an open

circuit on either string, the boost converter will regulate

at 24V while current in the remaining string (if enabled)

is controlled to the programmed value.

3524 F03

PROG

0V – 3V

300k

150k