LM3508TLEV National Semiconductor, LM3508TLEV Datasheet - Page 10

• Manufacturer Part Number: LM3508TLEV
• Description: BOARD EVALUATION FOR LM3508TL
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3508TLEV

• Current - Output / Channel: 20mA
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 17V
• Features: Dimmable
• Voltage - Input: 2.7 ~ 5.5V
• Utilized Ic / Part: LM3508
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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DIM Operation
DIM is the input to the gate of an internal switch that accepts
a logic level PWM waveform and modulates the internal
200mV reference through an internal RC filter. This forces the
current source regulation point (V
(D) of the DIM waveform making I
The cutoff frequency for the filter is approximately 500Hz. DIM
frequencies higher than 100kHz cause the LED current to
drastically deviate from their nominal set points. The graphs
of SET voltage vs DIM frequency, SET voltage vs V
voltage vs DIM duty cycle (see Typical Performance Charac-
teristics) show the typical variation of the current source set
point voltage.
Enable Input and Output Isolation
Driving EN high turns the device on while driving EN low
places the LM3508 in shutdown. In shutdown the supply cur-
rent reduces to less than 1µA, the internal synchronous PFET
turns off as well as the current source (N2 in figure 1). This
completely isolates the output from the input and prevents
leakage current from flowing through the LED’s. In shutdown
the leakage current into SW and IN is typically 400nA. EN has
an internal 383kΩ pull-down to PGND.
Peak Current Limit/Maximum
Output Current
The LM3508 boost converter provides a peak current limit.
When the peak inductor current reaches the peak current limit
the duty cycle is terminated. This results in a limit on the max-
imum output power and thus the maximum output current the
LM3508 can deliver. Calculate the maximum LED current as
a function of V
and f
efficiency and I
acteristics.
Output Current Accuracy
The LM3508 provides highly accurate output current regula-
tion of ±5% over the 3V to 5.5V input voltage range. Accuracy
depends on various key factors. Among these are; the toler-
ance of R
internal to the LM3508 controller and current sink. For best
accuracy, use a 1% resistor for R
1kHz and 100kHz. Refer to the Typical Performance Char-
acteristics for V
cycle.
Voltage Head Room at ILED
If the LED current is increased to a point where the peak in-
ductor current is reached, the boost converter's on-time is
terminated until the next switching cycle. If the LED current is
further increased the 500mV regulated voltage at ILED begins
to drop. When V
voltage (V
SW
= 850kHz. Efficiency and I
SET
HR
= 400mV typ.) the current sink FET (see N2 in
, the frequency at DIM (ƒ
IN
PEAK
SET
, V
ILED
OUT
vs V
curves in the Typical Performance Char-
drops below the current sink headroom
, L and I
IN
, V
SET
PEAK
vs ƒ
SET
SET
LED
DIM
PEAK
as:
) to vary by the duty cycle
and keep ƒ
, and V
= D × 200mV / R
DIM
can be found in the
), and the errors
SET
DIM
vs DIM duty
IN
and SET
between
SET
.
10
figure 1) will be fully on, appearing as a 5Ω resistor between
ILED and SET.
Output Short Circuit Protection
The LM3508 provides a short circuit protection that limits the
output current if OUT is shorted to PGND. During a short at
OUT when V
The PMOS will turn into a current source and limit the output
current to 35mA. The LM3508 can survive with a continuous
short at the output. The threshold for OUT recovering from a
short circuit condition is typically V
Output Over-Voltage Protection
When the load at the output of the LM3508 goes high
impedance the boost converter will raise V
maintain the programmed LED current. To prevent over-volt-
age conditions that can damage output capacitors and/or the
device, the LM3508 will clamp the output at a maximum of
21.8V. This allows for the use of 25V output capacitors avail-
able in a tiny 1.6mm × 0.8mm case size.
During output open circuit conditions when the output voltage
rises to the over voltage protection threshold (V
typical) the OVP circuitry will shut off both the NMOS and
PMOS switches. When the output voltage drops below 18.6V
(typically) the converter will begin switching again. If the de-
vice remains in an over voltage condition the cycle will be
repeated resulting in a pulsed condition at the output. See
waveform for OVP condition in the Typical Performance Char-
acteristics.
Light Load Operation
During light load conditions when the inductor current reaches
zero before the end of the switching period, the PFET will turn
off, disconnecting OUT from SW and forcing the converter
into discontinuous conduction. At the beginning of the next
switching cycle, switching will resume. (see plot of discontin-
uous conduction mode in the Typical Performance Charac-
teristics graphs).
Boost converters that operate in the discontinuous conduc-
tion mode with fixed input to output conversion ratios (V
V
switch on-times as the load decreases. As the load is de-
creased the duty cycle will fall until the converter hits its
minimum duty cycle (typically 15%). To prevent further de-
creases in the load current altering the V
LM3508 will enter a pulsed skip mode. In pulse skip mode the
device will only switch as necessary to keep the LED current
in regulation.
Thermal Shutdown
The LM3508 provides a thermal shutdown feature. When the
die temperature exceeds +150°C the part will shutdown, turn-
ing off both the NMOS and PMOS FET’s. The part will start-
up again with a soft-start sequence when the die temperature
falls below +115°C.
Applications Information
Brightness Adjustment
A logic high at DIM forces SET to regulate to 200mV. Adjust
the maximum LED current by picking R
SET to GND) such that:
IN
) have load dependent duty cycles, resulting in shorter
OUT
falls to below V
IN
× 0.93, switching will stop.
IN
× 0.95.
SET
OUT
(the resistor from
OUT
/V
OVP
IN
to try and
ratio, the
= 19.8V
OUT
/