LM3500 Datasheet, PDF (1/18 Pages) National Semiconductor (TI) – Synchronous Step-up DC/DC Converter for White LED

February 2005
LM3500
Synchronous Step-up DC/DC Converter for White LED
Applications
General Description
The LM3500 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 (500mV), power efficiency of the
LM3500 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 LM3500 oper-
ates with a fixed 1MHz switching frequency. When used with
ceramic input and output capacitors, the LM3500 provides a
small, low-noise, low-cost solution.
Two LM3500 options are available with different output volt-
age capabilities. The LM3500-21 has a maximum output
voltage of 21V and is typically suited for driving 4 or 5 white
LEDs in series. The LM3500-16 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 LM3500 uses internal protec-
tion circuitry on the output to prevent a destructive over-
voltage 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 using a pulse width modulated (PWM) signal
on the shutdown pin. In shutdown, the LM3500 completely
disconnects the input from output, creating total isolation 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 Internal output over-voltage protection (OVP) circuitry,
with no external zener diode required
LM3500-16: 15.5V OVP; LM3500-21: 20.5V OVP.
n True shutdown isolation
n Input undervoltage lockout
n Requires only small ceramic capacitors 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 Backlighting
n Handheld Devices
n Digital Cameras
n Portable Applications
Typical Application Circuit
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