LT3599(RevD) Просмотр технического описания (PDF) - Linear Technology

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LT3599
(Rev.:RevD)

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

Linear Technology 

LT3599

Applications Information

LED Current Dimming

Two different types of dimming control can be used with

the LT3599. The LED brightness can be set either by analog

dimming (CTRL pin voltage adjustment between 0V and 1V)

or PWM dimming (PWM pin duty cycle adjustment).

For some applications, the preferred method of brightness

control is to use a variable DC input voltage. The CTRL

pin voltage can be adjusted to set the dimming of the LED

string (see Figures 4 and 5). As the voltage on the CTRL

pin increases from 0V to 1V, the LED current increases

from 0 to the programmed LED current level. Once the

CTRL pin voltage increases beyond 1V, it has no effect on

the LED current.

For True Color PWM dimming, the LT3599 provides

up to a 3000:1 PWM dimming range by allowing the duty

cycle of the PWM pin to be reduced from 100% to as

120

ISET = 13.3k

100

80

60

40

20

0

0 0.2 0.4 0.6 0.8 1.0 1.2

CTRL PIN VOLTAGE (V)

3599 F04

Figure 4. LED Current vs CTRL Voltage

low as 0.033% at a PWM frequency of 100Hz (Figure 6).

Dimming by PWM duty cycle, allows for constant LED

color to be maintained over the entire dimming range.

For LT3599 PWM dimming control during startup and

normal operation, observe the following guidelines:

(1) STARTUP

LT3599 VOUT start-up requires the SHDN/UVLO pin to be

asserted from off to on and the PWM on-time to be above

a minimum value. The lowest PWM on-time allowed for

fault detection is ≈20µs. The lowest PWM on-time allowed

for reaching VOUT regulation is typically 20µs but might be

greater depending on external circuit parameters. Once LED

current is in regulation, PWM on-time can be reduced as

low as 3µs depending on external component selection.

(2) VOUT Collapse

If during normal operation VOUT collapses due to a fault

or because PWM on-time is too low, a re-start is required

(see STARTUP in item (1)).

VREF

R2

LT3599

CTRL

R1

3599 F05

Figure 5. LED Current vs CTRL

PWM

TPWM

TONPWM

(= 1/fPWM)

12

INDUCTOR

CURRENT

LED

CURRENT

MAX ILED

3599 F06

Figure 6. LED Current Using PWM Dimming

3599fd

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