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Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

48102.002

LED Drivers

LC5200 Series

LED Current Setting and Dimming

Output current level can be set using two alternative methods:

• Internal PWM Control. The LC5200 series provides fixed off-

time PWM current control operation, allowing implementation

of an LED constant-current control circuit with only a small

quantity of external components.

The output current is calculated by the formula below:

Based on this formula, there are two methods of LED current

control available:

▫ Analog control, varying the REF pin voltage as shown in

figure 6, panel A)

▫ PWM integrated control, inputting external PWM signal

through a low pass filter (LPF) and connecting the output to the

REF pin (figure 6, panel B)

In either method, the TOFF pin voltage is used to turn the

MOSFET on or off, therefore, the circuit in figure 6, panel C

also works to adjust the output current by the external signal. In

this application, when the external small signal MOSFET turns

on, LC5200 stops an output pulse.

• External PWM Control. In this method of control, the LC5200

allows direct on/off control of the MOSFET, for synchroniz-

ing PWM operation among LED arrays or for other reasons.

With this method, a pull-up shunt is connected from the REF

pin to the regulator output as shown figure 7. The capacitor and

resistor are removed from the TOFF pin, and instead the PWM

signal is input to the TOFF pin. Note that for this method, the

internal peak current control is disabled; therefore, it requires

an external current control circuit for constant current opera-

tion. However, overcurrent protection is still in active to protect

the LC5200 and LEDs from excessive current. The TOFF pin

threshold has hysteresis characteristics: from < 2 V to MOSFET

off, and from > 3 V to MOSFET on. Therefore, use 5 V CMOS

compatible input for the control.

LC5200

SENSE

GND

REG

TOFF

REF

LC5200

SENSE

GND

REG

TOFF

REF

LC5200

SENSE

GND

REG

TOFF

REF

PWM

LPF

PWM

MOSFET

100 Hz

Figure 6. Implementations of internal PWM control

Figure 7. Implementation of external PWM control

(A) Analog control

(B) Integrated PWM control

(C) External signal on TOFF pin

Internal PWM Truth Table

PWM

OUT

Low

Low (ON)

High

High (OFF)

External PWM Truth Table

TOFF

OUT

Low < 2 V

High (OFF)

High > 3 V

Low (ON)

PWM

LC5200

SENSE

GND

REG

TOFF

REF