LM3501 Datasheet, PDF (1/18 Pages) National Semiconductor (TI) – Synchronous Step-up DC/DC Converter for White LED Applications

May 2005
LM3501
Synchronous Step-up DC/DC Converter for White LED
Applications
General Description
The LM3501 is a fixed-frequency step-up DC/DC converter
that is ideal for driving white LEDs for display backlighting
and other lighting functions. With fully intergrated synchro-
nous switching (no external schottky diode required) and a
low feedback voltage (515 mV), power efficiency of the
LM3501 circuit has been optimized for lighting applications
in wireless phones and other portable products (single cell
Li-Ion or 3-cell NiMH battery supplies). The LM3501 oper-
ates with a fixed 1 MHz switching frequency. When used with
ceramic input and output capacitors, the LM3501 provides a
small, low-noise, low-cost solution.
Two LM3501 options are available with different output volt-
age capabilities. The LM3501-21 has a maximum output
voltage of 21V and is typically suited for driving 4 or 5 white
LEDs in series. The LM3501-16 has a maximum output
voltage of 16V and is typically suited for driving 3 or 4 white
LEDs in series (maximum number of series LEDs dependent
on LED forward voltage). If the primary white LED network
should be disconnected, the LM3501 uses internal protec-
tion circuitry on the output to prevent a destructive overvolt-
age event.
A single external resistor is used to set the maximum LED
current in LED-drive applications. The LED current can eas-
ily be adjusted by varying the analog control voltage on the
control pin or by using a pulse width modulated (PWM)
signal on the shutdown pin. In shutdown, the LM3501 com-
pletely disconnects the input from output, creating total iso-
lation and preventing any leakage currents from trickling into
the LEDs.
Features
n Synchronous rectification, high efficiency and no
external schottky diode required
n Uses small surface mount components
n Can drive 2-5 white LEDs in series (may function with
more low V_F LEDs)
n 2.7V to 7V input range
n True shutdown isolation, no LED leakage current
n DC voltage LED current control
n Input undervoltage lockout
n Internal output over-voltage protection (OVP) circuitry,
with no external zener diode required LM3501-16: 15.5V
OVP; LM3501-21: 20.5V OVP.
n Requires only a small 16V (LM3501-16) or 25V
(LM3501-21) ceramic capacitor at the input and output
n Thermal Shutdown
n 0.1µA shutdown current
n Small 8-bump thin micro SMD package
Applications
n LCD Bias Supplies
n White LED Back-Lighting
n Handheld Devices
n Digital Cameras
n Portable Applications
Typical Application Circuit
FIGURE 1. Typical 3 LED Application
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