LT3596 查看數據表（PDF） - Linear Technology

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LT3596

60V Step-Down LED Driver

Linear Technology 

LT3596

Operation

The LT3596 uses a constant-frequency, peak current mode

control scheme to provide excellent line and load regula-

tion. Operation is best understood by referring to the Block

Diagram (Figure 1). The bias, start-up, reference, oscillator,

TSET amplifier and the buck regulator are shared by the

three LED current sources. The conversion and control,

PWM dimming logic, LED fault detection, and LED drive

circuitry are identical for all three current sources.

Enable and undervoltage lockout (UVLO) are both con-

trolled by a single pin. If the EN/UVLO pin falls below

1.51V (typical), an accurate comparator turns off the LED

drivers and the buck regulator. If the pin continues to fall

to less than 0.4V, the part enters a low quiescent current

shutdown mode.

The LT3596 contains three constant-current sink LED

drivers. These drivers sink up to 100mA with 1.5% match-

ing accuracy between LED strings. The LED strings are

powered from the buck converter.

The buck converter contains an adaptive loop that adjusts

the output voltage based on LED string voltage to ensure

maximum efficiency. External compensation and soft-start

components are not required, minimizing component

count and simplifying board layout. An external resistor

programs the buck’s switching frequency between 200kHz

and 1MHz. The frequency can also be synchronized to an

external clock using the SYNC pin.

Step-Down Adaptive Control

Adaptive control of the output voltage maximizes system

efficiency. This control scheme regulates the output voltage

to the minimum that ensures all three LED strings turn

on. This accounts for the variation in the forward voltage

of the LEDs, and minimizes the power dissipation across

the internal current sources.

Activation of the adaptive loop is set by the status of the

PWMn pins. If any channel’s PWM pin is low, then the buck

regulator output ascends to an externally programmed out-

put voltage. This voltage is always set above the maximum

voltage drop of the LEDs. This guarantees that the buck

output voltage is high enough to immediately supply the

LED current once the strings are reactivated. As soon as

all of the PWM pins transition high, the output voltage of

the buck drops until the adaptive loop regulates the output

with about 1V across the LED current sinks. This scheme

optimizes the efficiency for the system since the output

voltage regulates to the minimum voltage required for all

three LED strings.

LED Current

Each LED string current is individually programmed to

a maximum of 100mA with 1.5% matching accuracy

between the strings. An external resistor on the ISETn

pin programs the maximum current for each string. The

CTRLn pin is used for analog dimming. Digital dimming

is programmed using the PWMn pin. A dimming ratio of

10,000:1 is achievable at a frequency of 100Hz.

Fault Protection and Reporting

The LT3596 features diagnostic circuitry that protects

the system in the event that a LEDn pin is shorted to an

undesirable voltage. The LT3596 detects when the LEDn

voltage exceeds 12V or is within 1.2V of VOUT when the LED

string is sinking current. If either faulted condition occurs,

the channel is disabled until the fault is removed. The fault

is reported on FAULT until the fault has cleared.

The LT3596 also offers open-LED detection and report-

ing. If a LED string is opened and no current flows in the

string, then a fault is reported on FAULT. A fault is also

reported if the internal die temperature reaches the TSET

programmed derating limit. LED faults are only reported

if the respective PWM signal is high.

3596fa

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