LT3587 데이터 시트보기 (PDF) - Linear Technology

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LT3587

High Voltage Monolithic Inverter and Dual Boost

Linear Technology 

APPLICATIONS INFORMATION

VVIN

2.5V TO 5V

10μH

VIN

SW3

CAP3

LT3587

VOUT3

1μF

LED DRIVER

20mA

IFB3

EN/SS3

RIFB3

8.06k

PWM

FREQ

2.5V

0V

MN1

Si1304BDL

3587 F13

Figure 13. Six White LEDs Driver With PWM Dimming

IVOUT3

13mA/DIV 0mA

IL4

200mA/DIV

0mA

ENSS3 0V

5V/DIV

VVIN = 3.6V

6 LEDs

2ms/DIV

3587 F14

Figure 14. PWM Dimming Waveforms

LT3587

Dimming the LEDs via a PWM signal essentially involves

turning the LEDs on and off at the PWM frequency. The

typical human eye has a sensitivity limit of ~60Hz. By

increasing the PWM frequency to ~80Hz or higher, the eye

will interpret that the pulsed light source is continuously

on. Additionally, by modulating the duty cycle (amount of

“on-time”), intensity of the LEDs is controlled. The color

of the LEDs remains unchanged in this scheme since the

LED current is either zero or a constant value.

Figure 13 shows a partial application showing an LED

driver for six white LEDs. If the voltage at the CAP3 pin is

higher than 10V when the LEDs are on, direct PWM dim-

ming method requires an external NMOS. This external

NMOS is tied between the cathode of the lowest LED in

the string and ground as shown in Figure 13.

A Si1304 logic-level MOSFET can be used since its source

is connected to ground, and it is able to withstand the

open-circuit voltage at the VOUT3 pin across its drain and

source. The PWM signal must be applied to the EN/SS3

pin of the LT3587 and the gate of the NMOS. The PWM

signal should traverse between 0V to 2.5V, to ensure

proper turn on and off of the Boost3 regulation loop and

the NMOS transistor MN1. When the PWM signal goes

high, the LEDs are connected to ground and a current of

IVOUT3 = 160V/RIFB3 flows through the LEDs. When the

PWM signal goes low, the LEDs are disconnected and

turned off.

The output disconnect feature and the external NMOS

ensure that the LEDs quickly turn off without discharging

the output capacitor. This allows the LEDs to turn on faster.

Figure 14 shows the PWM dimming waveforms for the

circuit in Figure 13.

3587fc

17

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