LM3500TL-21/NOPB National Semiconductor, LM3500TL-21/NOPB Datasheet - Page 12

• Manufacturer Part Number: LM3500TL-21/NOPB
• Description: IC LED DRVR WHT BCKLT 8-MICROSMD
• Manufacturer: National Semiconductor
• Type: Backlight, White LEDr

Specifications of LM3500TL-21/NOPB

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: Backlight
• Type - Secondary: White LED
• Frequency: 850kHz ~ 1.15MHz
• Voltage - Supply: 2.7 V ~ 7 V
• Voltage - Output: 19.5 V ~ 20.5 V
• Mounting Type: Surface Mount
• Package / Case: 8-MicroSMD
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 640mA
• Internal Switch(s): Yes
• Efficiency: 85%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM3500TL-21; LM3500TL-21TR

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Operation
The LM3500 utilizes a synchronous Current Mode PWM con-
trol scheme to regulate the feedback voltage over almost all
load conditions. The DC/DC controller acts as a controlled
current source ideal for white LED applications. The LM3500
is internally compensated preventing the use of any external
compensation components providing a compact overall solu-
tion. The operation can best be understood referring to the
block diagram in Figure 1. At the start of each cycle, the os-
cillator sets the driver logic and turns on the NMOS power
device conducting current through the inductor and turns off
the PMOS power device isolating the output from the V
The LED current is supplied by the output capacitor when the
NMOS power device is active. During this cycle, the output
voltage of the EAMP controls the current through the inductor.
This voltage will increase for larger loads and decrease for
smaller loads limiting the peak current in the inductor mini-
mizing EMI radiation. The EAMP voltage is compared with a
voltage ramp and the sensed switch voltage. Once this volt-
age reaches the EAMP output voltage, the PWM COMP will
then reset the logic turning off the NMOS power device and
turning on the PMOS power device. The inductor current then
flows through the PMOS power device to the white LED load
and output capacitor. The inductor current recharges the out-
FIGURE 1. LM3500 Block Diagram
SW
pin.
12
put capacitor and supplies the current for the white LED
branches. The oscillator then sets the driver logic again re-
peating the process. The Duty Limit Comp is always opera-
tional preventing the NMOS power switch from being on more
than one cycle and conducting large amounts of current.
The LM3500 has dedicated protection circuitry active during
normal operation to protect the IC and the external compo-
nents. The Thermal Shutdown circuitry turns off both the
NMOS and PMOS power devices when the die temperature
reaches excessive levels. The LM3500 has a UVP Comp that
disables both the NMOS and PMOS power devices when
battery voltages are too low preventing an on state of the
power devices which could conduct large amounts of current.
The OVP Comp prevents the output voltage from increasing
beyond 15.5V(LM3500-16) and 20.5V(LM3500-21) when the
primary white LED network is removed or if there is an LED
failure, allowing the use of small (16V for LM3500-16 and 25V
for LM3500-21) ceramic capacitors at the output. This com-
parator has hysteresis that will regulate the output voltage
between 15.5V and 14.6V typically for the LM3500-16, and
between 20.5V and 19.5V for the LM3500-21. The LM3500
features a shutdown mode that reduces the supply current to
0.1uA and isolates the input and output of the converter.
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