Circuit Description

The LM3520 is designed for White LED & OLED backlighting

in mobile phone applications. It has a main display loop

which can drive up to 5 white LEDS in series and a sub

display loop which is designed to drive OLED up to 20V/50

mA. The main display loop employs a fixed frequency cur-

rent mode scheme to regulate the LED current. The sub

display loop employs a fixed frequency gated oscillator

scheme to regulate the output voltage. The device has two

independent control pins to enable the Main or Sub displays.

Note that both displays can not be ON at the same time.

PWM Operation

The LM3520 utilizes a synchronous Current Mode PWM

control scheme to regulate the feedback voltage over all load

and line conditions for the main display. The LM3520 is

internally compensated preventing the need for external

compensation components yielding a compact solution. The

operation can best be understood referring to the functional

block diagram. The LM3520 operates as follows: During the

first cycle, the oscillator sets the driver logic and turns on the

NMOS power device conducting current through the inductor

and reverse biases the external diode 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 de-

crease for smaller loads limiting the peak current in the

inductor. The sum of the EAMP voltage and voltage ramp is

compared with the sensed switch voltage. Once these volt-

ages are equal, the PWM COMP will then reset the logic

turning off the NMOS power device and forward biasing the

external diode to the white LED load and flows through the

diode to the white LED load and output capacitor. The induc-

tor current recharges the output capacitor and supplies the

current for the white LED branches. The oscillator then sets

the driver logic again repeating the process.

PFM Operation

The LM3520 utilizes a gated oscillator control scheme for the

sub-display. There is a hysteresis window to regulate the

output voltage. The oscillator frequency is the same as the

frequency in PWM control. The Duty cycle of the oscillator

signal is always set to maximum. During the first part of each

switching cycle, the internal NMOS switch is turned on until

the PFM current limit is reached. When the NMOS is off, the

voltage of the inductor reverses and forces current through

the diode to the output capacitor. This process continues

until the upper comparator hysteresis is reached at which

point the NMOS is disabled until the lower comparator

threshold is reached and the process repeats again.

Current Limit Protection

The LM3520 has current limiting protection to prevent exces-

sive stress on itself and external components during over-

load conditions. The internal current limit comparator will

disable the NMOS power device at a typical switch peak

current limit of 700 mA.

Output Over-Voltage Protection

The LM3520 contains dedicated circuitry for monitoring the

output voltage. In the event that the primary LED network is

disconnected the output will increase and be limited to 23.2V

(typ.). There is a ~1V hysteresis associated with this cir-

cuitry, which will turn the NMOS off when the output voltage

is at 24.2V(max.) until the output voltage reach 22.5V(typ.)

or lower. The 23.5V limit allows the use of 25V 1 µF ceramic

output capacitors creating an overall small solution for white

LED applications.

Under Voltage Protection

The LM3520 has an UVP comparator to turn the NMOS

power device off in case the input voltage or battery voltage

is too low preventing an on state of the power device con-

ducting large amounts of current.

Reliability and Thermal Shutdown

The LM3520 has an internal thermal shutdown function to

protect the die from excessive temperatures. The thermal

shutdown trip point is typically 160˚C, Normal operation re-

sumes when the temperature drops below 140˚C.

Startup

The LM3520 does not include a power on reset circuit and

relies on external signal to monitor enable signal. In the

event of under voltage condition, the device enable pin must

be brought low until the input voltage is above the minimum

guarantee voltage (2.7V).

Application Information

SETTING LED CURRENT

The White LED current is set using the following equation:

For main display:

(1)

PWM CONTROL

The LED current can be controlled using a PWM signal on

the enable pin with frequencies in the range of 100 Hz to

1 kHz. LM3520 LED current can be controlled with PWM

signal frequencies above 1 kHz but LED current is not

linearly porportional to the duty cycle. The maximum LED

current would be achieved using the equation above with

100% duty cycle, ie. The enable pin is always high.

SETTING SUB VOLTAGE

Sub-display voltage is be set by choosing R

as illustrated in Figure 4.V

(2)

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