A6211 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver FEATURES AND BENEFITS

DESCRIPTION

• • • • • • • •

The A6211 is a single IC switching regulator that provides constant-current output to drive high-power LEDs. It integrates a high-side N-channel DMOS switch for DC-to-DC step- down (buck) conversion. A true average current is output using a cycle-by-cycle, controlled on-time method.

Supply voltage 6 to 48 V True average output current control 3 A maximum output over operating temperature range Cycle-by-cycle current limit Integrated MOSFET switch Dimming via direct logic input or power supply voltage Internal control loop compensation Undervoltage lockout (UVLO) and thermal shutdown protection • Low power shutdown (1 µA typical) • Robust protection against: □□ Adjacent pin-to-pin short □□ Pin-to-GND short □□ Component open/short faults

Package: 8-pin SOICN with exposed thermal pad (suffix LJ):

Output current is user-selectable by an external current sense resistor. Output voltage is automatically adjusted to drive various numbers of LEDs in a single string. This ensures the optimal system efficiency. LED dimming is accomplished by a direct logic input pulsewidth modulation (PWM) signal at the enable pin. The device is provided in a compact 8-pin narrow SOIC package (suffix LJ) with exposed pad for enhanced thermal dissipation. It is lead (Pb) free, with 100% matte-tin leadframe plating.

APPLICATIONS: • • • • •

General illumination Scanners and multifunction printers (light bars) Architectural lighting Industrial lighting Display case lighting / MR16

Not to scale

VIN (6 to 48 V) GND

C1

Enable/PWM Dimming (100 Hz to 2 kHz)

1

8 SW A6211 R1 2 7 TON BOOT 3 6 EN PAD GND 4 VCC 5 CS VIN

C4

LED+

D1 ...

EN

L1

C5 LED– RSENSE Typical Application Circuit

A6211-DS, Rev. 5

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver

A6211

SPECIFICATIONS Selection Guide Part Number

Operating Ambient Temperature, TA

Package

Packing

A6211GLJTR-T

–40°C to 105°C

8-pin SOICN with exposed thermal pad

3000 pieces per 13-in reel

Absolute Maximum Ratings Characteristic

Symbol

Supply Voltage Bootstrap Drive Voltage

Notes

Rating

Unit

VIN

–0.3 to 50

V

VBOOT

–0.3 to VIN + 8

V

–1.5 to VIN + 0.3

V

–0.3 to 7

V

Switching Voltage

VSW

Linear Regulator Terminal

VCC

Enable and TON Voltage

VEN , VTON

–0.3 to VIN + 0.3

V

VCS

–0.3 to 7

V

Current Sense Voltage

VCC to GND

Operating Ambient Temperature

TA

–40 to 105

°C

Maximum Junction Temperature

TJ(max)

150

°C

Tstg

–55 to 150

°C

Storage Temperature

G temperature range

Thermal Characteristics*: may require derating at maximum conditions; see application section for optimization Characteristic

Symbol

Package Thermal Resistance (Junction to Ambient)

RθJA

Package Thermal Resistance (Junction to Pad)

RθJP

Test Conditions*

Value

Unit

On 4-layer PCB based on JEDEC standard

35

°C/W

On 2-layer generic test PCB with 0.8 in.2 of copper area each side

62

°C/W

2

°C/W

*Additional thermal information available on the Allegro™ website.

8

VIN 1

7

TON 2

SW

BOOT

PAD EN 3

CS 4

6

5

Pinout Diagram

GND

VCC

Terminal List Table Number

Name

1

VIN

Supply voltage input terminals

Function

2

TON

Regulator on-time setting resistor terminal

3

EN

Logic input for Enable and PWM dimming

4

CS

Drive output current sense feedback

5

VCC

Internal linear regulator output

6

GND

Ground terminal

7

BOOT

DMOS gate driver bootstrap terminal

8

SW

Switched output terminals

–

PAD

Exposed pad for enhanced thermal dissipation; connect to GND

Allegro MicroSystems, LLC 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com

2

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver

A6211

CVCC

CBOOT

VCC

VIN

VIN

BOOT

L1

LED String

D1

SW

VREG 5.3 V VCC UVLO

Average On-Time Current Generator

TON

On-Time Timer

Off-Time Timer

Gate Drive UVLO Shutdown

RON

Level Shift EN

+ IC and Driver Control Logic

CCOMP

0.2 V

Current Limit Off-Time Timer

–

–

Buck Switch Current Sense

+

VIL = 0.4 V VIH = 1.8 V

+

+

VCC UVLO

–

–

Thermal Shutdown

ILIM

CS PAD

GND

RSENSE

Functional Block Diagram

Allegro MicroSystems, LLC 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com

3

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver

A6211

ELECTRICAL CHARACTERISTICS: Valid at VIN = 24 V, for TA = –40°C to 105°C, typical values at TA = 25°C, unless

otherwise noted

Characteristics

Symbol

Input Supply Voltage VIN Undervoltage Lockout Threshold VIN Undervoltage Lockout Hysteresis

VIN VUVLO IIN IINSD

Buck Switch Current Limit Threshold

ISWLIM

Buck Switch On-Resistance

R

 BOOT Undervoltage Lockout Threshold  BOOT Undervoltage Lockout Hysteresis

DS(on)

VBOOTUV

Typ.

Max.

Unit

6

–

48

V

VIN increasing

–

5.3

–

V

–

150

–

mV

VCS = 0.5 V, EN = high

–

5

–

mA

EN shorted to GND

–

1

10

µA

3.0

4.0

5.0

A

–

0.25

0.4

Ω

1.7

2.9

4.3

V

–

370

–

mV

–

110

150

ns

VBOOT = VIN + 4.3 V, TA = 25°C, ISW = 1 A VBOOT to VSW increasing

VBOTUVHYS VBOOT to VSW decreasing

Switching Minimum Off-Time

tOFFmin

Switching Minimum On-TIme

tONmin

Selected On-Time

Test Conditions

VUVLO_HYS VIN decreasing

VIN Pin Supply Current VIN Pin Shutdown Current

Min.

TA = 25°C

tON

VCS = 0 V VIN = 24 V, VOUT = 12 V, RON = 137 kΩ

–

110

150

ns

800

1000

1200

ns

187.5

200

210

mV

–

0.9

–

µA

5.1

5.4

5.7

V

5

20

–

mA

–

–

V

Regulation Comparator and Error Amplifier Load Current Sense Regulation Threshold 1

VCSREG

VCS decreasing, SW turns on

Load Current Sense Bias Current

ICSBIAS

VCS = 0.2 V, EN = low

Internal Linear Regulator VCC Regulated Output VCC Current Limit 2

VCC ICCLIM

0 mA < ICC < 5 mA, VIN > 6 V VIN = 24 V, VCC = 0 V

Enable Input Logic High Voltage Logic Low Voltage

VIH

VEN increasing

1.8

VIL

VEN decreasing

–

–

0.4

V

EN Pin Pull-down Resistance

RENPD

VEN = 5 V

–

100

–

kΩ

Maximum PWM Dimming Off-Time

tPWML

Measured while EN = low, during dimming control, and internal references are powered-on (exceeding tPWML results in shutdown)

10

17

–

ms

Thermal Shutdown Thermal Shutdown Threshold

TSD

–

165

–

°C

Thermal Shutdown Hysteresis

TSDHYS

–

25

–

°C

In test mode, a ramp signal is applied at CS pin to determine the CS pin regulation threshold voltage. In actual application, the average CS pin voltage is regulated at VCSREG regardless of ripple voltage. 2 The internal linear regulator is not designed to drive an external load 1

Allegro MicroSystems, LLC 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com

4

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver

A6211

CHARACTERISTIC PERFORMANCE

VIN

VIN

VOUT

VOUT

C1,C2

C3 C4

C1,C2

iLED

C3

VEN

C4

iLED VEN

t

t

Panel 1B. VIN = 24  V

Panel 1A. VIN = 19  V

VIN

VOUT C1,C2

C3 C4

iLED VEN t

Panel 1C. VIN = 30 V

Figure 1: Startup waveforms from off-state at various input voltages; note that the rise time of the LED current depends on input/output voltages, inductor value, and switching frequency • Operating conditions: LED voltage = 15 V, LED current = 1.3 A, R1 = 63.4 kΩ (frequency = 1 MHz in steady state), VIN = 19 V (panel 1A), 24 V (panel 1B) and 30 V (panel 1C) • Oscilloscope settings: CH1 (Red) = VIN (10 V/div), CH2 (Blue) = VOUT (10 V/div), CH3 (Green) = iLED (500 mA/div), CH4 (Yellow) = Enable (5 V/div), time scale = 50 µs/div

Allegro MicroSystems, LLC 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com

5

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver

A6211

VIN

VOUT C1,C2

C3 C4

iLED VEN t

Panel 2A: Duty cycle = 50% and time scale = 1 ms/div

VIN

VOUT C1,C2

C3 C4

iLED VEN t

Panel 2B: Duty cycle = 2% and time scale = 50 µs/div

Figure 2: PWM operation at various duty cycles; note that there is no startup delay during PWM dimming operation • Operating conditions: at 200 Hz, VIN = 24 V, VOUT = 15 V, R1 = 63.4 kΩ, duty cycle = 50% (panel 2A) and 2% (panel 2B) • CH1 (Red) = VIN (10 V/div), CH2 (Blue) = VOUT (10 V/div), CH3 (Green) = iLED (500 mA/div), CH4 (Yellow) = Enable (5 V/div), time scale = 1 ms/div (panel 2A) and 50 µs/div (panel 2B)

Allegro MicroSystems, LLC 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com

6

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver

A6211

95

95 VIN = 24 V, VOUT = 15 V VIN = 12 V, VOUT = 5.5 V

85

VIN = 12 V, VOUT = 3.5 V

80

fSW = 500 kHz

90

Efficiency, η (%)

Efficiency, η (%)

90

75

fSW = 1 MHz

85 fSW = 2 MHz 80

75

70

70 0

0.5

1.0

1.5

2.0

2.5

3.0

0

0.5

LED Current, iLED (A)

1.0

1.5

2.0

2.5

3.0

LED Current, iLED (A)

Figure 3: Efficiency versus LED Current at various LED voltages Operating conditions: fSW = 1 MHz

Figure 4: Efficiency versus LED Current at various switching frequencies. Operating conditions: VIN = 12 V, VOUT = 5.5 V

LED Current (A)

1

0.1

iLED = 3 A

iLED = 2 A iLED = 1.4 A

0.01

0.001 0.1

1

10

100

Duty Cycle (%) Figure 5: Average LED Current versus PWM dimming percentage Operating conditions: VIN = 12 V, VOUT = 3.5 V, fSW = 1 MHz, fPWM = 200 Hz, L = 10 µH

Allegro MicroSystems, LLC 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com

7

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver

A6211

FUCNTIONAL DESCRIPTION The A6211 is a buck regulator designed for driving a high-current LED string. It utilizes average current mode control to maintain constant LED current and consistent brightness. The LED current level is easily programmable by selection of an external sense resistor, with a value determined as follows: iLED = VCSREG / RSENSE

fSW = 1 / [ k × (RON + RINT )] where k = 0.0139, with fSW in MHz, tON in µs, and RON and RINT (internal resistance, 5 kΩ) in kΩ (see Figure 6).

Enable and Dimming

where VCSREG = 0.2 V typical.

Switching Frequency The A6211 operates in fixed on-time mode during switching. The on-time (and hence switching frequency) is programmed using an external resistor connected between the VIN and TON pins, as given by the following equation: 2.2 2.0 1.8 1.6

fsw (MHz)

tON = k × (RON + RINT ) × ( VOUT / VIN )

1.4 1.2

The IC is activated when a logic high signal is applied to the EN (enable) pin. The buck converter ramps up the LED current to a target level set by RSENSE. When the EN pin is forced from high to low, the buck converter is turned off, but the IC remains in standby mode for up to 10 ms. If EN goes high again within this period, the LED current is turned on immediately. Active dimming of the LED is achieved by sending a PWM (pulse-width modulation) signal to the EN pin. The resulting LED brightness is proportional to the duty cycle (tON / Period ) of the PWM signal. A practical range for PWM dimming frequency is between 100 Hz (Period = 10 ms) and 2 kHz. At a 200 Hz PWM frequency, the dimming duty cycle can be varied from 100% down to 1% or lower. If EN is low for more than 17 ms, the IC enters shutdown mode to reduce power consumption. The next high signal on EN will initialize a full startup sequence, which includes a startup delay of approximately 130 µs. This startup delay is not present during PWM operation.

1.0 0.8 0.6 0.4 0.2 0 0

20

40

60

80

100 120 140 160 180 200 220 240 260

RTON (kΩ)

Figure 6: Switching Frequency versus RTON Resistance • During SW on-time: iRIPPLE = [(VIN – VOUT) / L] × tON = [(VIN – VOUT) / L] × T × D where D = tON / T. • During SW off-time: iRIPPLE = [(VOUT – VD) / L] × tOFF = [(VOUT – VD) / L] × T × (1 – D) Therefore (simplified equation for Output Voltage): VOUT = VIN × D – VD × (1 – D) If VD