LT3599FE Просмотр технического описания (PDF) - Linear Technology

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LT3599FE

4-String 120mA LED Driver with ±1.5% Current Matching

Linear Technology 

LT3599

Operation

The LT3599 uses a constant-frequency, peak current

mode control scheme to provide excellent line and load

regulation. Operation can be best understood by referring

to the Block Diagram in Figure 1.

To turn on the LT3599, the VIN pin must exceed 3.1V and

the SHDN/UVLO pin must exceed 1.4V. The SHDN/UVLO

pin threshold allows programming of an undervoltage

lockout (UVLO) threshold for the system input supply us-

ing a simple resistor divider. A 4µA current flows into the

SHDN/UVLO pin before the part turns on and is removed

after the part turns on. This current hysteresis allows the

programming of hysteresis for the UVLO threshold. See

“Shutdown Pin and Programming Undervoltage Lockout”

in the Applications Information section. For part switching,

the PWM pin must exceed 1V (typical). For micropower

shutdown, the SHDN/UVLO pin at 0V reduces VIN supply

current to approximately ~0µA.

LT3599 has a built-in boost converter which converts the

input voltage to a higher output voltage for driving 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 LED4 voltages are monitored for

output voltage regulation. During normal operation, when

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

LED4) is used to regulate the output voltage to ensure all

LED strings have enough voltage to run the programmed

current.

If the user prefers only three strings, then LED string 4 can

be disabled through the DISABLE4 pin and by connecting

LED4 to any other LED pin. If the user prefers only two

strings, then two pins are connected in parallel (i.e., LED1,2

and LED3,4 can be connected together in operation).

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) is proportional to the sum of the switch

current and oscillator ramp. When this signal exceeds the

VC voltage, 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 capacitor. 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 either PWM dim-

ming or analog dimming. PWM dimming is achieved by

pulsing the LED current using the PWM pin. For constant

color LED dimming, the LT3599 provides up to a 3000:1

wide PWM dimming range by allowing the duty cycle of the

PWM pin to be reduced from 100% to as low as 0.033%.

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.

In applications where the user can sacrifice OPENLED,

SHORTLED fault flag diagnostics, the dimming ratios can

be as high as 3000:1. Analog dimming of LED currents is

accomplished by varying the level of CTRL pin voltage.

This method, however, changes LED color since dimming

is achieved by changing LED current. For CTRL pin voltage

less than 1V, LED current is defined as:

ILED

=

VCTRL

•

⎛

⎜

⎝

1330

RISET

⎞

⎟

⎠

(Amps)

The LT3599 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 LED1-4 pin voltage is below 0.3V, the string is treated

as an open LED string. As a result, an OPENLED flag is

set. If a LED string is opened during regular operation,

the output voltage will regulate to the optimum voltage

for the remaining connected strings.

If a short occurs between VOUT and any LED pin during

operation, the LT3599 immediately turns off the LED pin

current of the shorted string and sets a SHORTLED flag.

Disabling the LED pin current protects the LT3599 from high

power thermal dissipation and ensures reliable operation.

3599ff

For more information www.linear.com/LT3599

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