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LT3598

6-String 30mA LED Driver with ±1.5% Current Matching

Linear Technology 

LT3598

OPERATION

The LT3598 uses a constant-frequency, peak current mode

control scheme to provide excellent line and load regulation.

Each string can drive up to 30mA with 1.5% matching ac-

curacy between strings. Operation can be best understood

by referring to the Block Diagram in Figure 1.

LT3598 has a built-in boost converter which converts the

input voltage to a higher output voltage to drive LEDs.

The LED strings are connected to current sources where

the current level is set with an external resistor on the

ISET pin. The LED1 to LED6 voltages are monitored for

output voltage regulation. During normal operation, when

all LEDs are used, the lowest LED pin voltage (LED1 to

LED6) is used to regulate the output voltage to ensure all

LED strings have enough voltage to run the programmed

current.

For any unused LED strings, tie their LED pins to VOUT.

An unused LED string is no longer in the regulation loop,

nor does it affect open LED detection. Never allow unused

LED strings to be left open.

The basic loop uses a pulse from an internal oscillator

to set the SR latch and turn on the internal power NPN

switch Q1. The signal at the noninverting input of the PWM

comparator (A2 slope) is proportional to the sum of the

switch current and oscillator ramp. When slope exceeds

VC (the output of the gm amplifier), the PWM comparator

resets the latch. The switch is then turned off, causing the

inductor current to lift the SW pin and turn on an external

Schottky diode connected to the output. Inductor current

flows via the Schottky diode charging the output capaci-

tor. The switch is turned on again at the next reset cycle

of the internal oscillator. During normal operation, the VC

voltage controls the peak switch current limit and, hence,

the inductor current available to the output LEDs.

Dimming of the LEDs is accomplished by pulsing the LED

current using the PWM pin. When the PWM pin is low,

switching is disabled and the error amplifier is turned off

so that it does not drive the VC pin. Also, all internal loads

on the VC pin are disabled so that the state of the VC pin

is maintained on the external compensation capacitor.

This feature reduces transient recovery time. When the

PWM input again transitions high, the peak switch current

returns to the correct value.

The LT3598 uses the FB pin to provide overvoltage protec-

tion when all LED strings are open. There is an internal

PMOS switch between VOUT and VO_SW that is controlled

by the PWM signal. During the PWM off-period, this

PMOS is turned off, allowing for higher dimming range

and lower current during shutdown. A resistor divider is

connected between the VO_SW pin and ground, which sets

the overvoltage protection voltage.

If the LED1-6 pin voltage is below 0.2V (for a certain delay

after 80% of the programmed output voltage is reached),

the string is treated as an open LED string. As a result,

OPENLED flag is set. If a LED string is open in the middle

of the operation, the regulation will continue.

OPENLED detection is disabled during the start-up phase

to avoid erratic flag generation. An LED string that is

disabled by connecting its LED pin to VOUT is not an open

LED condition. During normal operation, if an LED string

is open and has the lowest LED pin voltage, the output

voltage will regulate itself to find another LED string that has

the lowest LED pin voltage at about 0.8V. If the open LED

string has an LED voltage above 0.8V, the output voltage

will remain the same. When the LED string is open, it is

no longer in the regulation loop. The OPENLED detection

is active only when the PWM signal is enabled. To avoid

spurious OPENLED detection and high PWM dimming

ratio, more output capacitance is recommended to allow

less voltage drop on VOUT.

During start-up, 10μA of current charges the external

soft-start capacitor. The SS pin directly limits the rate of

voltage rise on the VC pin, which in turn, limits the peak

switch current. Soft-start also enables the switching

frequency foldback to provide a clean start-up for the

LT3598. Current limit protects the power switch and

external components.

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