LM3509SDX National Semiconductor Corporation, LM3509SDX Datasheet - Page 13

• Manufacturer Part Number: LM3509SDX
• Description: High Efficiency Boost For White Led S And/or Oled Displays With Dual Current Sinks And I2c Compatible Brightness Control
• Manufacturer: National Semiconductor Corporation

room at the input to the current sinks for proper current
regulation.
In the instance when there are unequal numbers of LEDs or
unequal currents from string to string, the string with the high-
est voltage will be the regulation point.
UNISON/NON-UNISON MODE
Within White LED mode there are two separate modes of op-
eration, Unison and Non-Unison. Non-Unison mode provides
for independent current regulation, while Unison mode gives
up independent regulation for more accurate matching be-
tween LED strings. When in Non-Unison mode the LED cur-
rents I
registers BMAIN and BSUB respectively (see Brightness
Registers (BMAIN and BSUB) section). When in Unison
mode BSUB is disabled and both I
trolled via BMAIN only.
START-UP
The LM3509 features an internal soft-start, preventing large
inrush currents during start-up that can cause excessive volt-
age ripple on the input. For the typical application circuits
when the device is brought out of shutdown the average input
current ramps from zero to 450mA in 1.2ms. See Start Up
Plots in the Typical Performance Characteristics.
OLED MODE
When the LM3509 is configured for a single White LED bias
+ OLED display bias (OLED mode), the non-inverting input of
the error amplifier is connected to the internal 1.21V reference
via MUX S2. MUX S1 switches SUB/FB to the inverting input
of the error amplifier while disconnecting the internal current
sink at SUB/FB. The voltage at MAIN is not regulated in OLED
mode so when the application requires white LED + OLED
panel biasing, ensure that at least 300mV of headroom is
maintained at MAIN to guarantee proper regulation of I
(see the Typical Performance Characteristics for a plot of
I
PEAK CURRENT LIMIT
The LM3509’s boost converter has a peak current limit for the
internal power switch of 770mA typical (650mA minimum).
When the peak switch current reaches the current limit the
duty cycle is terminated resulting in a limit on the maximum
output current and thus the maximum output power the
LM3509 can deliver. Calculate the maximum LED current as
a function of V
ƒ
be found in the efficiency and I
formance Characteristics.
OVER VOLTAGE PROTECTION
The LM3509's output voltage (V
by the Output Over-Voltage Protection Threshold (V
LED
SW
vs Current Source Headroom Voltage)
= 1.27MHz. Typical values for efficiency and I
MAIN
and I
IN
, V
SUB/FB
OUT
, L and I
are independently controlled via
PEAK
PEAK
OUT
) is limited on the high end
curves in the Typical Per-
MAIN
as:
and I
SUB/FB
PEAK
are con-
OVP
MAIN
can
) of
.
13
21.2V. In White LED mode during output open circuit condi-
tions the output voltage will rise to the over voltage protection
threshold (V
troller will stop switching causing V
output voltage drops below 19.7V (min) the device will resume
switching. If the device remains in an over voltage condition
the LM3509 will repeat the cycle causing the output to cycle
between the high and low OVP thresholds. See waveform for
OVP condition in the Typical Performance Characteristics.
OUTPUT CURRENT ACCURACY AND CURRENT
MATCHING
The LM3509 provides both precise current accuracy (% error
from ideal value) and accurate current matching between the
MAIN and SUB/FB current sinks. Two modes of operation af-
fect the current matching between I
mode (Non-Unison mode) is set by writing a 0 to bit 2 of the
General Purpose register (UNI bit). Non-Unison mode allows
for independent programming of I
BMAIN and BSUB respectively. In this mode typical matching
between current sinks is 1%.
Writing a 1 to UNI configures the device for Unison mode. In
Unison mode, BSUB is disabled and I
controlled via register BMAIN. In this mode typical matching
is 0.15%.
LIGHT LOAD OPERATION
The LM3509 boost converter operates in three modes; con-
tinuous conduction, discontinuous conduction, and skip mode
operation. Under heavy loads when the inductor current does
not reach zero before the end of the switching period the de-
vice switches at a constant frequency. As the output current
decreases and the inductor current reaches zero before the
end of the switching cycle, the device operates in discontin-
uous conduction. At very light loads the LM3509 will enter skip
mode operation causing the switching period to lengthen and
the device to only switch as required to maintain regulation at
the output.
ACTIVE LOW RESET/GENERAL PURPOSE I/O (RESET
\GPIO)
The RESET/GPIO serves as an active low reset input or as a
general-purpose logic input/output. Upon power-up of the de-
vice RESET/GPIO defaults to the active low reset mode. The
functionality of RESET/GPIO is set via the GPIO register and
is detailed in Table 6. When configured as an active low reset
input, (Bit 0 = 0), pulling RESET/GPIO low automatically pro-
grams all registers of the LM3509 with 0x00. Their state
cannot be changed until RESET/GPIO is pulled high. The
General Purpose I/O (GPIO) register is used to enable the
GPIO function of the RESET/GPIO pin. The GPIO register is
an 8-bit register with only the 3 LSB’s active. The 5 MSB’s are
not used. When configured as an output, RESET/GPIO is
open drain and requires an external pull-up resistor.
THERMAL SHUTDOWN
The LM3509 offers a thermal shutdown protection. When the
die temperature reaches +140°C the device will shutdown
and not turn on again until the die temperature falls below
+120°C.
OVP
= 21.2V min). When this happens the con-
MAIN
MAIN
OUT
MAIN
and I
and I
to droop. When the
and I
SUB/FB
SUB/FB
SUB/FB
via registers
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. The first
are both