LTC3524_15 Datasheet, PDF (11/16 Pages) Linear Technology – Adjustable TFT Bias Supply with WLED Driver

LTC3524
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
20mA (R_PROG = 100k). In this instance, assuming a 3.6V
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
string currents with R_PROG, V_SUM, and R_SUM will be:
I_LED
=
1625
•
⎛
⎝⎜
1.225V
R_PROG
+
1.225V − V_SUM
R_SUM
⎞
⎠⎟
A 0V to 3V V_SUM with R_SUM = 300k and R_PROG = 150k will
produce LED currents between 3mA and 20mA.
V_SUM 0V – 3V
R_SUM
300k
PROG
R_PROG
150k
3524 F03
Figure 3. Analog Dimming Circuit Using V_SUM
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
dimming frequency (F_PWM) and the settling time of the
LED strings when enabled (T_SETTLE). The minimum duty
cycle (or light output) that the strings can be controlled
to is given by:
MinDuty = F_PWM • T_SETTLE
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.
3524f
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