7

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

Current Control

PWM current control works as shown in figure 3.

• PWM On Period. During MOSFET on-time, the current runs

• MOSFET Turn Off. During on-time, the current increases as

threshold voltage, the MOSFET turns off.

• PWM Off Period. During MOSFET off-time, the back EMF

occurs on the coil and the energy which is charged on the coil

through the path in blue of panel A.

• MOSFET Turn On. After the fixed off-time, which is set by the

on again, and repeats the above operations.

Figure 4 shows the current control circuit and figure 5 shows the

timing diagram of that circuit. When the MOSFET turns on, both

it results in turning off the MOSFET after the signal goes through

charged by the internal MOS switch, and when the Comp1 invert-

3 V, Comp1 output becomes high and it turns on the MOSFET

(point B in figure 5). The Blank Pulse circuit creates periods that

mask surge or ringing noise, from turn off edge to just after the

turn on edge, for securing proper PWM operation.

Description of Operation

LED

Di

L

VBB

OUT

SENSE

GND

Figure 3. Output current control circuit

Figure 4. Current Control Circuit

Figure 5. Current Control Circuit Timing Chart

(A)

(B)

+

-

+

-

Logic

OUT

Sen

GND

Reg

Toff

Ref

6V

2V

3V

Comp1

Comp2

Gate Driver

R

Q

S

Blank Pulse

I

O

Toff

Comp1

-IN

Comp2

Comp1

OUT

Comp2

OUT

OUT

ON

OFF

ON

OFF

Valid

Valid

Invalid

AB