LT3599EFE-TRPBF(RevD) Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

Fabricante

LT3599EFE-TRPBF
(Rev.:RevD)

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

Linear Technology 

LT3599

Applications Information

a soft-start latch is set causing the SS pin to be discharged

(see the Soft-Start Pin Timing Diagram in Figure 11). All

faults are detected internally and do not require external

components. When all faults no longer exist and the SS

pin has been discharged to at least 0.25V, the soft-start

latch is reset and an internal 11µA supply charges the SS

pin. During start-up or recovery from a fault, the SS pin

ramp up controls the ramp up of switch current limit.

Soft-start ramp rate is given by:

( ) ∆VSS

∆T

=

ISS

CSS

ISS = 11µA typ

A 10nF capacitor from the SS pin to ground will therefore

provide a 1V/ms ramp rate on the SS pin.

In addition, during soft-start, switching frequency is re-

duced to protect the inductor from high currents.

SW

SS

FAULTS TRIGGERING

SOFT-START LATCH

WITH SW TURNED OFF

IMMEDIATELY:

VIN < 2.7V

OR

SHDN < 1.4V

SOFT-START

LATCH SET

0.3V (ACTIVE THRESHOLD)

0.25V (RESET THRESHOLD)

0.15V

SOFT-START

LATCH RESET:

SS < 0.25V

AND

VIN > 2.7V

AND

SHDN > 1.4V

AND

PWM > 1V (FOR >200ns)

3599 F12

Figure 11. Soft-Start Pin Timing Diagram

A useful feature of the LT3599 is that it waits for the first

PWM pin active high (minimum 200ns pulse width) before

it allows the soft-start of VC pin to begin. This feature

ensures that during start-up of the LT3599 the soft-start

ramp has not timed out before PWM is asserted high.

Without this ‘wait for PWM high’ feature, systems which

apply PWM after VIN and SHDN are valid, can potentially

turn on without soft-start and experience high inductor

currents during wake up of the converter’s output voltage.

It is important to note that when PWM subsequently goes

low, the soft-start ramp is not held at its present voltage

but continues to ramp upwards. If the soft-start ramp

voltage was held every time PWM goes low, this would

cause very slow start-up of LED displays for applications

using very high PWM dimming ratios.

OPENLED FLAG

The OPENLED pin is an open-collector output and needs

an external resistor tied to a supply (see Figure 12). If any

LED string is open during normal operation, the OPENLED

pin will be pulled down.

The open LED detection is enabled only when the PWM

signal is enabled. There is a delay for OPENLED flag genera-

tion when the PWM signal is enabled to avoid generating

a spurious flag signal.

During start-up (see the Operation section), the open LED

detection is disabled.

SHORTLED FLAG

The SHORTLED pin is an open-collector output, and needs

an external resistor tied to a supply (see Figure 12). If

any LED pin is shorted to VOUT during normal operation,

the SHORTLED pin will be pulled down. In addition, the

shorted LED string (channel) is immediately disabled,

thereby protecting the LT3599.

The short LED detection is enabled only when the PWM

signal is enabled. There is a delay for SHORTLED flag gen-

eration when the PWM signal is enabled to avoid spurious

signal being generated.

During start-up, the SHORTLED flag is disabled (see the

Operation section).

LT3599

OPENLED

SHORTLED

R1 R2

3599 F13

Figure 12. OPENLED and SHORTLED Connection

3599fd

16

Share Link: