LC5200 datasheet(7/15 Pages) ALLEGRO

7 / 15 page

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

through the ION path (shown in red in figure 3, panel A).

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

the red waveform in panel B, and when it reaches the VSENSE

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

during the on-time is deenergized by the current IOFF running

through the path in blue of panel A.

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

external capacitor and resistor at the Toff pin, the MOSFET turns

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

the load current and VSen, across the sensing resistor RS, increase.

Comp2 compares VSen and VRef and its output is inverted at VSen

> VRef (see point A in figure 5). This resets the latter RS latch and

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

several logic circuits. At the same time, Coff at the Toff pin is dis-

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

ing input (linked to the Toff pin) voltage becomes lower than 2 V,

Comp1 output is inverted and it sets the RS latch. This turns off

the MOS switch and initiates the charging process of Coff by Roff.

Coff voltage (Toff pin) increases by it and when its voltage reaches

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

VBB

OUT

SENSE

IOFF

GND

VSENSE

ION

Figure 3. Output current control circuit

Figure 4. Current Control Circuit

Figure 5. Current Control Circuit Timing Chart

(A)

(B)

toff

Ioff

Ion

VSENSE

VREF

Ion

ILED

Logic

OUT

Sen

GND

Reg

Toff

Ref

ROff

COff

VRef

Comp1

Comp2

Gate Driver

Blank Pulse

Toff

VSen

VRef

VToff

Comp1

-IN

Comp2

Comp1

OUT

Comp2

OUT

OFF

Valid

Invalid

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