LM3502SQX-44/NOPB National Semiconductor, LM3502SQX-44/NOPB Datasheet - Page 7

• Manufacturer Part Number: LM3502SQX-44/NOPB
• Description: IC LED DRIVR WHITE BCKLGT 16-LLP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Backlight, White LEDr

Specifications of LM3502SQX-44/NOPB

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 2
• Internal Driver: Yes
• Type - Primary: Backlight
• Type - Secondary: White LED
• Frequency: 800kHz ~ 1.2MHz
• Voltage - Supply: 2.5 V ~ 5.5 V
• Voltage - Output: 41 V ~ 42 V
• Mounting Type: Surface Mount
• Package / Case: 16-LLP
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 750mA
• Internal Switch(s): Yes
• Efficiency: 80%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM3502SQX-44

Detailed Description of Operation
The LM3502 utilizes an asynchronous current mode pulse-
width-modulation (PWM) control scheme to regulate the
feedback voltage over specified load conditions. The DC/DC
converter behaves as a controlled current source for white
LED applications. The operation can best be understood by
referring to the block diagram in Figure 2 for the following
operational explanation. At the start of each cycle, the oscil-
lator sets the driver logic and turns on the internal NMOS
power device, N1, conducting current through the inductor
and reverse biasing the external diode. The white LED cur-
rent is supplied by the output capacitor when the internal
NMOS power device, N1, is turned on. The sum of the error
amplifier’s output voltage and an internal voltage ramp are
compared with the sensed power NMOS, N1, switch voltage.
Once these voltages are equal, the PWM comparator will
then reset the driver logic, thus turning off the internal NMOS
power device, N1, and forward biasing the external diode.
The inductor current then flows through the diode to the
white LED load and output capacitor. The inductor current
recharges the output capacitor and supplies the current for
the white LED load. The oscillator then resets the driver logic
again repeating the process. The output voltage of the error
amplifier controls the current through the inductor. This volt-
age will increase for larger loads and decrease for smaller
loads limiting the peak current in the inductor and minimizing
EMI radiation. The duty limit comparator is always opera-
tional, it prevents the internal NMOS power switch, N1, from
being on for more than one oscillator cycle and conducting
large amounts of current. The light load comparator allows
the LM3502 to properly regulate light/small white LED load
currents, where regulation becomes difficult for the
LM3502’s primary control loop. Under light load conditions,
the LM3502 will enter into a pulse skipping pulse-frequency-
mode (PFM) of operation where the switching frequency will
vary with the load.
The LM3502 has 2 control pins, En1 and En2, used for
selecting which segment of a single white LED string net-
work is active for dual display applications. En1 controls the
main display (MAIN) segment of the single string white LED
network between pins V
sub display (SUB) segment of the single string white LED
OUT1
and V
OUT2
. En2 controls the
7
network between the V
the LM3502 control pin operational characteristics, see Fig-
ure 3.
When the Cntrl pin is ≥ 1.4V, the LM3502 will enter in low I
state if both En1 and En2 ≤ 0.3V. At this time, both the P1
and N2 FETs will turn off. The output voltage will be a diode
drop below the supply voltage and the soft-start will be reset
limiting the peak inductor current at the next start-up.
The LM3502 is designed to control the LED current with a
PWM signal without the use of an external RC filter. Utilizing
special circuitry, the LM3502 can operate over a large range
of PWM frequencies without restarting the soft-start allowing
for fast recovery at high PWM frequencies. Figure 4
reprsents a PWM signal driving the Cntrl pin where t
defined as the low time of the signal. The following is true:
The LM3502 has dedicated protection circuitry active during
normal operation to protect the integrated circuit (IC) and
external components. The thermal shutdown circuitry turns
off the internal NMOS power device, N1, when the internal
semiconductor junction temperature reaches excessive lev-
els. The LM3502 has a under-voltage protection (UVP) com-
parator that disables the internal NMOS power device when
battery voltages are too low, thus preventing an on state
where the internal NMOS power device conducts large
amounts of current. The over-voltage protection (OVP) com-
parator prevents the output voltage from increasing beyond
the protection limit when the white LED string network is
removed or if there is a white LED failure. OVP allows for the
use of low profile ceramic capacitors at the output. The
current though the internal NMOS power device, N1, is
monitored to prevent peak inductor currents from damaging
the IC. If during a cycle (cycle=1/switching frequency) the
peak inductor current exceeds the current limit for the
LM3502, the internal NMOS power device will be turned off
for the remaining duration of that cycle.
• If t
• If t
during this time and the soft-start will not be reset allow-
ing LED current PWM control.
soft-start will reset to prevent high peak currents at the
next startup. Both the N2 and P1 switches will turn off.
L
L
>
<
12ms (typical): The device will shutdown and the
12ms (typical): The device will stop switching
OUT2
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