A6268KLPTR-T [ALLEGRO]
The A6268 is a DC-to-DC converter controller, providing a programmable constant current output for driving high power LEDs in series.; 的A6268是DC到DC转换器的控制器，提供了一个可编程的恒定电流输出用于驱动高功率LED串联。
| 型号： | A6268KLPTR-T |
| 厂家： | 
ALLEGRO MICROSYSTEMS |
| 描述： | The A6268 is a DC-to-DC converter controller, providing a programmable constant current output for driving high power LEDs in series. |
| 文件： | 总19页 (文件大小：369K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
A6268
Automotive High Current LED Controller
Features and Benefits
▪ AEC Q100 Grade 0 Automotive Qualified
▪ Constant current LED drive
▪ 5 to 50 V supply
▪ Boost or buck-boost modes
▪ Drives up to 15 LEDs in series
▪ Programmable switching frequency 100 to 700 kHz
▪ Open LED overvoltage indication and protection
▪ Single and multiple LED short indication
▪ LED short to ground and supply protection
▪ PWM dimming control
Description
The A6268 is a DC-to-DC converter controller, providing a
programmable constant current output for driving high power
LEDs in series. Driving the LEDs in series ensures identical
currents and uniform brightness. For automotive applications,
optimumperformanceisachievedwhendrivingupto15LEDs
at currents up to 1 A.
TheA6268 provides a cost-effective solution using an external
logic-level MOSFET and minimum additional external
components. The maximum LED current is set with a single
external sense resistor and can be modified using a current
reference input. Direct PWM control is possible via the Enable
input, which also provides a shutdown mode.
▪ 10 μA shutdown current including LED leakage
Applications:
ThisDC-DCconvertercanbeconfiguredasaground-referenced
boost converter or as a supply-referenced boost converter
providingbuck-boostcapability.Thebuck-boosttopologyused
ensures that there is no leakage path through the LEDs when
in shutdown and no inrush current at power-up.
▪ Automotive high power LED lighting systems
▪ Fog lights, reversing lights, daytime running lights
▪ Headlights
Package: 16-pin TSSOP with exposed
thermal pad (suffix LP)
Integrated diagnostics and two fault outputs give indication of
VIN and VREG undervoltage, chip overtemperature, output
open circuit, LED short circuit and LED undercurrent, and can
be configured to provide short to supply and short to ground
protection for the LED connections, LED overcurrent and
shorted LED string protection. A unique feature is the ability
to detect one or more shorted LEDs.
Thedeviceisprovidedina16-pinTSSOPpackagewithexposed
thermal pad (suffix LP). It is lead (Pb) free, with 100% matte
tin leadframe plating.
Not to scale
Typical Application Diagrams
(50 V max)
12 V or 24 V
V
(50 V max)
12 V or 24 V
V
BAT
BAT
Power net
Power net
VIN
VIN
VREG
VREG
LN
LP
LN
LP
Buck Boost Mode
VBAT(min) Maximum
Boost Mode
VBAT(min)
Maximum
Quantity of
LEDs
FF1
FF2
FF1
FF2
(V)
Quantity of
LEDs
(V)
Fault
Flags
Fault
Flags
5
6
7
8
9
7
5
6
7
8
9
8
LF
LF
A6268
A6268
8
10
12
13
15
EN
EN
Enable
Enable
10
11
13
SG
SP
SG
SP
IREF
CKOUT
IREF
CKOUT
Vf of each LED = 3.5 V,
D(max) = 85%
Vf of each LED = 3.5 V,
D(max) = 85%
SN
LA
OSC
GND
OSC
GND
SN
LA
Buck-Boost Mode
(Supply-referenced boost)
Boost Mode
A6268-DS, Rev. 1
A6268
Automotive High Current LED Controller
Selection Guide
Part Number
Packing
Package
A6268KLPTR-T
4000 pieces per 13-in. reel
16-pin TSSOP with exposed thermal pad
Absolute Maximum Ratings With respect to GND at TA = 25°C, unless otherwise specified
Characteristic
Load Supply Voltage
Symbol
Notes
Rating
–0.3 to 50
–0.3 to 50
–0.3 to 6.5
–0.3 to 6.5
–0.3 to 6.5
–0.3 to 50
–6 to 6
Unit
V
VIN
Pins FF1, EN
Pins FF2, CKOUT
Pin OSC
V
V
V
Pin SG
V
Pins LA, LN
V
Pin LF
With respect to LA
With respect to LN
V
Pin LP
–6 to 6
V
Pin SP, SN
–0.3 to 5
–0.3 to 7
–0.3 to 7
150
V
Pin VREG
V
Pin IREF
V
Junction Temperature
Storage Temperature Range
Operating Temperature Range
TJ(max)
Tstg
°C
°C
°C
–55 to 150
–40 to 150
TA
Range K
Thermal Characteristics may require derating at maximum conditions, see application information
Characteristic
Symbol
Test Conditions*
Value Unit
On 4-layer PCB based on JEDEC standard
34
43
ºC/W
ºC/W
Package Thermal Resistance
(Junction to Ambient)
RθJA
On 2-layer PCB with 3.8 in.2 of copper area each side
Package Thermal Resistance
(Junction to Exposed Pad)
RθJP
2
ºC/W
*Additional thermal information available on the Allegro website
Allegro MicroSystems, Inc.
115 Northeast Cutoff
2
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
Pin-out Diagram
16
15
14
13
12
11
10
9
EN
FF1
LF
1
2
3
4
5
6
7
8
LA
FF2
LN
CKOUT
IREF
OSC
SN
LP
PAD
VIN
VREG
GND
SG
SP
Terminal List Table
Number
Name
EN
Function
1
2
Enable chip
Fault flag
Fault flag
FF1
FF2
CKOUT
IREF
OSC
SN
3
4
Oscillator output, with phase shift
Current reference
5
6
Oscillator input/frequency set
Switch current sense –ve input
Switch current sense +ve input
Switch gate drive
7
8
SP
9
SG
10
11
12
13
14
15
16
–
GND
VREG
VIN
Ground
Internal regulator capacitor
Main supply
LP
Load current sense +ve input
Load current sense –ve input
LED string voltage sense
Reference LED voltage sense
Exposed thermal pad
LN
LA
LF
PAD
Allegro MicroSystems, Inc.
115 Northeast Cutoff
3
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
Functional Block Diagrams
RSL
VBAT
VIN
V
LN
LP
LF
LA
Buck-Boost
VREG
LED Open and
Short Detect
REG
AL
AE
Shut-
AC
Fault
Detect
FF2
FF1
down
SG
R
Q
S
Control
Logic
SP
I
REF
EN
RSS
AS
SN
Temp
Monitor
Slope
Gen
Osc
GND
OSC
ROSC
IREF
CKOUT
VBAT
Boost
VIN
LF
LA
LN
LP
VREG
LED Open and
Short Detect
V
REG
AL
AE
Shut-
AC
Fault
Detect
down
FF2
FF1
SG
R
Q
S
SP
Control
Logic
I
REF
EN
RSS
AS
SN
RSL
Temp
Monitor
Slope
Gen
Osc
GND
IREF
OSC
ROSC
CKOUT
AL AE AC AS
See Functional Description section.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
4
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
ELECTRICAL CHARACTERISTICS1 Valid at TJ = –40°C to 150°C, VIN = 5 to 40 V; unless otherwise noted
Characteristics
Supply and Reference
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
VIN Functional Operating Range2
5
–
–
–
4
5
–
–
50
8
V
mA
μA
V
IINQ
IINS
SG open circuit
EN = GND
VIN Quiescent Current
–
10
5.15
–
I
REG = 0 to 2 mA, VIN ≥ 5.3 V
4.85
4.7
25
VREG Output Voltage
VREG
IREG = 2 mA, VIN = 5 V
V
VREG Current Limit
Gate Output Drive
Turn-On Time
VREGCL
–
mA
tr
tf
CLOAD = 1 nF, 20% to 80%
CLOAD = 1 nF, 80% to 20%
tON × fOSC
–
–
30
30
85
1.7
–
–
–
ns
ns
%
Ω
Turn-Off Time
Maximum Duty Cycle
D
80
–
–
TJ = 25°C, IGHx = –100 mA
TJ = 150°C, IGHx = –100 mA
TJ = 25°C, IGLx = 100 mA
TJ = 150°C, IGLx = 100 mA
–
Pull-Up On Resistance
RDS(on)UP
–
3.5
–
Ω
–
0.75
–
Ω
Pull-Down On Resistance
Output High Voltage
RDS(on)DN
–
1.5
Ω
VREG
0.1
–
VSGH
VSGL
ISG = –100 μA
ISG = 100 μA
–
–
VREG
0.1
V
V
Output Low Voltage
–
Logic Inputs and Outputs
Fault Output (Open Drain)
Fault Output FF1 Sink Current
Fault Output FF1 Leakage Current1
Fault Output FF2 Leakage Current1
Input Low Voltage
VOL
IOH(snk)
IOH1(lkg)
IOH2(lkg)
VIL
IOL = 1 mA, fault not asserted
0.4 V < VO < 50 V, fault not asserted
VO = 12 V, fault asserted
–
–
–
1.3
–
0.4
–
V
mA
μA
μA
V
–1
–5
–
1
VO = 5 V, fault asserted
–
5
–
0.8
–
Input High Voltage
VIH
2
–
V
Input Hysteresis
VIhys
120
–
180
8.4
200
–
–
mV
V
Enable Input Internal Clamp Voltage
Enable Input Current Limit Resistor
Boost Mode Select Voltage
Buck-Boost Mode Select Voltage
Disable Time
VENC
REN
–
Between EN and internal clamp
Defined by VLN
–
–
kΩ
V
VLNB
VLNBB
tDIS
–
0.8
–
Defined by VLN
3.5
–
–
V
fOSC = 350 kHz
94
–
ms
Continued on the next page…
Allegro MicroSystems, Inc.
115 Northeast Cutoff
5
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
ELECTRICAL CHARACTERISTICS1 (continued) Valid at TJ = –40°C to 150°C, VIN = 5 to 40 V; unless otherwise noted
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
Oscillator
ROSC = 43 kΩ
OSC = 62 kΩ
–
500
350
350
1.2
150
–
–
385
–
kHz
kHz
kHz
V
Oscillator Frequency
fOSC
R
315
ROSC = VREG
–
OSC Pin Voltage
VOSC
tDC
ROSC = 62 kΩ
1.15
1.25
–
CKOUT Output Delay
OSC input rise to CKOUT rise
–
ns
V
OSC Input Low Voltage
VOIL
–
0.8
–
OSC Input High Voltage
VOIH
VOihys
tOSWD
VCOH
VCOL
3.5
–
V
OSC Input Hysteresis
300
600
–
–
mV
μs
V
OSC Watchdog Period
Between successive rising edges
IOH = –1 mA
7
VREG – 1
–
–
CKOUT Output High Voltage
CKOUT Output Low Voltage
LED Current Sense
–
VREG
0.4
IOL = 1 mA
–
V
Input Bias Current LN (BB mode)3
Input Bias Current LP (BB mode)3
Input Bias Current LN (B mode)1,3
Input Bias Current LP (B mode)1,3
Differential Input Voltage (Active)
Input Common-Mode Range (BB mode)3
Input Common-Mode Range (B mode)3
Current Error
ILN
ILP
LP = LN = 12 V
LP = LN = 12 V
LP = LN = 0 V
–
–
130
125
–1.0
–12
100
–
–
μA
μA
μA
μA
mV
V
–
ILN
–
–
ILP
LP = LN = 0 V
–
–
VIDL
VCMLH
VCMLL
EISL
EN = High, VIDL = VLP – VLN
VLP = VLN
–
–
VIN
0
VIN + 1
VLP = VLN
–
1
5
V
[(10 × ILED × RSL) – 1] × 100
–5
–
%
Switch Current Sense
Input Bias Current
IBIASS
VIDS
IINS
SP = SN = 0 to 2 V
VIDS = VSP – VSN with D = 50%
VIDS = 120 mV
–30
110
–
–
–
200
–
μA
mV
μA
V
Maximum Differential Input Voltage4
Input Source Current
150
120
–
Input Common-Mode Range
Diagnostics and Protection
Fault Blank Timer5
VCMS
VSP = VSN
0
2
tFB
Start-up
–
3
–
–
ms
V
VIN Undervoltage Turn-Off
VIN Undervoltage Hysteresis
VREG Undervoltage Turn-Off
VREG Undervoltage Hysteresis
VINUV
Decreasing VIN
–
4.6
400
4.4
400
VINUVhys
VREGUV
VREGUVhys
200
2.9
170
–
mV
V
Decreasing VREG
3.65
300
mV
Continued on the next page…
Allegro MicroSystems, Inc.
115 Northeast Cutoff
6
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
ELECTRICAL CHARACTERISTICS1 (continued) Valid at TJ = –40°C to 150°C, VIN = 5 to 40 V; unless otherwise noted
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
Diagnostics and Protection (continued)
LED String Short Voltage
VSCL
VSCO
350
150
475
225
600
300
mV
mV
Non-Reference LED Short Offset
Voltage
Reference LED Short Offset Voltage
LED Open Voltage
VSCOR
VOCL
ILF
350
5
475
5.5
50
90
8
600
6
mV
V
LF Bias Current (BB mode)3
LA Bias Current (BB mode)3
LF Bias Current (B mode)3
LF = LA = VIN + 1.7 V
–
–
μA
μA
μA
μA
mV
mV
ILA
LF = LA = VIN + 1.7 V
LF = 1.7 V
–
–
ILF
–
–
LA Bias Current (B mode)3
ILA
LA = 1.7 V
–
24
1
–
LED Undercurrent Voltage Difference6
LED Overcurrent Voltage Difference7
VUCL
VOVCL
–
–
–
1
–
LED Sense Resistor Negative
Overcurrent Threshold8
VNOCL
VNOCL = VLP – VLN
–
–200
–
mV
Open Fault Time-Out
tOTO
TJF
fOSC = 350 kHz
–
–
–
94
170
15
–
–
–
ms
ºC
ºC
Overtemperature Warning Threshold
Overtemperature Hysteresis
Temperature increasing
Recovery = TJF – TJhys
TJhys
1For input and output current specifications, negative current is defined as coming out of (sourcing) the specified device pin.
2Function is correct but parameters are not guaranteed below the general limit (5 V).
3BB mode = buck-boost (supply-referenced) mode, B mode = boost (ground-referenced) mode.
4Parameters guaranteed by design.
5Fault Blank timer not enabled for open-LED condition.
6Undercurrent when VSENSEL < VIDL– VUCL, where VSENSEL is the voltage across the LED current sense resistor RSL
.
7Overcurrent when VSENSEL > VIDL+VOVCL, where VSENSEL is the voltage across the LED current sense resistor RSL
8Protection only provided in buck-boost mode with LED cathode (LP) connection shorted to ground.
.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
7
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
Functional Description
The A6268 is a DC-DC converter controller that is designed to
drive series-connected high power LEDs in automotive applica-
tions. It provides programmable constant current output at load
voltages and currents limited only by the external components.
For automotive applications optimum performance is achieved
when driving up to 15 LEDs at currents up to 1 A.
source of the PMOS FET used to isolate the load from the supply.
Table 1 defines when FF1 is active. If FF1 is pulled low when
an output short fault is indicated then the output disable will be
overridden.
FF2 Fault Flag output. Open drain output, when high impedance
indicates detection of a circuit fault. An external pull-up resistor
should be connected to a suitable logic supply. If VREG is not
used, then the logic supply should not be pulled 300 mV above
The A6268 can be configured as a standard boost converter or
as a supply referenced boost converter. In the supply referenced
configuration the load voltage is the difference between the boost VREG. Table 1 defines when FF2 is active. If FF2 is pulled low
voltage and the supply voltage. This difference can be greater
than, equal to, or less than the supply voltage, effectively provid-
ing a buck-boost capability. This configuration provides seam-
less, uninterrupted operation over the wide supply voltage range
possible in automotive applications and, because the output is ref-
erenced to the positive supply, there is no load current to ground.
This ensures that there is no leakage path through the LEDs when
in shutdown and no inrush current at power-up.
when an open LED fault is indicated then the output disable will
be overridden.
OSC Resistor to ground to set the internal oscillator or clock
input from external oscillator. When connected to VREG or GND
the oscillator runs at typically 350 kHz. Higher accuracy in the
frequency is possible by connecting a resistor from this pin to
ground or by driving this pin with an external precision oscillator.
CKOUT Logic output at the oscillator frequency with phase
shift. Used to drive succeeding controllers to interleave switching
instants.
The A6268 integrates all necessary control elements to pro-
vide a cost-effective solution using a single external logic-level
MOSFET and minimum additional external passive components.
IREF LED current reference modifier. A voltage input that can be
used to reduce the LED current sense voltage. When connected to
VREG, the current sense voltage, VIDL, and the value of the sense
resistor, RSL, define the maximum LED current.
The LED current is set by selecting an appropriate value for the
sense resistor value and using the EN input to provide simple
on-off control or for PWM brightness control using a suitable
externally generated PWM signal. The LED current can be
reduced in a single step by reducing the voltage between the
IREF pin and GND to less than 1 V.
SG Gate drive for external logic-level MOSFET low-side switch
that connects the inductor to ground.
The pin functions and circuit operation are described in detail in
the following sections.
SP, SN Sense amplifier connections for switch current limit
sense resistor, RSS
LP Positive sense amplifier connection for LED current limit
sense resistor, RSL
LN Negative sense amplifier connection for LED current limit
.
Pin Functions
VIN Supply to the control circuit. A bypass capacitor must be
connected between this pin and GND.
.
GND Ground reference connection. This pin should be connected sense resistor, RSL. The voltage at LN also determines whether
directly to the negative supply.
the boost or buck-boost mode is configured.
EN Logic input to enable operation. Can be used as direct PWM
VREG Compensation capacitor for internal 5 V regulator.
input. Chip enters low power sleep mode when low for longer
LA Anode reference connection to LEDs. Using an external resis-
tor divider with the same ratio as the number of LEDs provides
a measurement of the voltage across all LEDs in the load. This
than the disable time, tDIS
.
FF1 Fault Flag output and isolation control. Open drain current
sink output, when high impedance indicates detection of a critical is compared to the voltage on the LF pin to provide shorted LED
circuit fault. An external pull-up resistor should be connected to a detection. In addition, it is compared against voltage references to
suitable logic supply for simple logic fault flag operation or to the provide open circuit or shorted LED string detection.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
8
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
known as discontinuous mode operation, and results in some low
frequency ripple. The average LED current, however, remains
regulated down to zero. In discontinuous mode, when the induc-
tor current drops to zero, the voltage at the drain of the external
MOSFET rings, due to the resonant LC circuit formed by the
inductor, and the switch and diode capacitance. This ringing is
low frequency and is not harmful.
LF Single diode forward voltage reference input. Measures the
forward voltage of the first LED. This value is used as a reference
against the voltage on the LA pin to detect possible shorted LEDs
in the LED string.
Circuit Operation
Converter A constant frequency, current mode control scheme
is used to regulate the current through the LEDs. There are two
control loops within the regulator. The inner loop formed by the
amplifier, AS (see the Functional Block Diagram for AS, AC, AE,
and AL), comparator, AC, and the RS bistable, controls the induc-
tor current as measured through the switch by the switch sense
Switch Current Limit The switch current is measured by the
switch sense resistor, RSS , and the switch sense amplifier, AS
(see the Functional Block Diagram). The input limit of the sense
amplifier, VIDS, and the maximum switch current, ISMAX , define
the maximum value of the sense resistor as:
resistor, RSS
.
RSS = VIDS / ISMAX
(1)
The outer loop including the amplifier, AL, and the integrating
error amplifier, AE, controls the average LED current by provid-
ing a setpoint reference for the inner loop.
This defines the maximum measurable value of the switch (and
inductor) current.
The maximum switch current is modulated by the on-time of the
switch. An internal slope compensation signal is subtracted from
the voltage sense signal to produce a peak sense voltage which
effectively defines the current limit. This signal is applied at a
rate of –16 mV/ꢀs starting with no contribution (t=0ꢀs) at the
beginning of each switching cycle. Figure 1 illustrates how the
peak sense voltage (typical values) changes over a period of 3 ꢀs.
The LED current is measured by the LED sense resistor, RSL
and compared to the internal reference current to produce an
,
integrated error signal at the output of AE. This error signal sets
the average amount of energy required from the inductor by the
LEDs. The average inductor energy transferred to the LEDs is
defined by the average inductor current as determined by the
inner control loop.
For example, the maximum current (typical) through the switch
at t= 1.5ꢀs (D=50%) would be 145 mV/RSS , however, if the
switch remained on for a further 1 ꢀs, the maximum current
The inner loop establishes the average inductor current by
controlling the peak switch current on a cycle-by-cycle basis.
Because the relationship between peak current and average cur-
rent is non-linear, depending on the duty cycle, the reference
level for the peak switch current is modified by a slope generator.
This compensation reduces the peak switch current measurement
by a small amount as the duty cycle increases (refer to figure 1).
The slope compensation also removes the instability inherent in a
fixed frequency current control scheme.
through the switch would be 129 mV/RSS
.
200
150
100
50
The control loops work together as follows: the switch current,
sensed by the switch current sense resistor, RSS , is compared
to the LED current error signal. As the LED current increases
the output of AE will reduce, reducing the peak switch current
and thus the current delivered to the LEDs. As the LED current
decreases the output of AE increases, increasing the peak switch
current and thus increasing the current delivered to the LEDs.
0
0
1
2
3
Under some conditions, especially when the LED current is set to
a low value, the energy required in the inductor may result in the
inductor current dropping to zero for part of each cycle. This is
Period (ꢀs)
Figure 1. Slope compensation for peak switch current control.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
9
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
Provided this is low, then the complete circuit may remain con-
nected to the power supply under all conditions. Note that the
disable time is derived from the oscillator period by a ratio of
32,768:1, so any variation in the oscillator frequency will change
the disable time. For the default switching frequency of 350kHz,
this means the disable time would be:
LED Current Level The LED current is determined by a
combination of the LED sense resistor, RSL, the LED current
threshold voltage, VIDL, and the voltage between the IREF pin
and GND (VIREF).
The 100% current level, when the IREF pin is connected to
VREG, is defined as:
tDIS = 32,768 × (1/350×103 ) = 94 ms
(4)
ILED(max) = VIDL / RSL
(2)
Oscillator The main oscillator may be configured as a clock
source or it may be driven by an external clock signal. The oscil-
lator is designed to run between 100 and 700 kHz.
If VIREF is less than 1 V then the 100% current level is defined as:
LED(max) = VIREF /(10 × RSL (3)
I
)
This feature provides direct analog dimming using a voltage from
0 to 1 V. This can be used for a number of different functions:
When the oscillator is configured as a clock source, the frequency
is controlled by a single external resistor, ROSC (kΩ), between the
OSC pin and the GND pin. The oscillator frequency is approxi-
mately:
• To provide intensity matching between modules or groups of
LEDs in critical display or backlighting applications.
fOSC = 21700 / ROSC
(kHz)
(5)
• To provide a soft start, by connecting a capacitor from IREF to
GND and a resistor from IREF to VREG, or one-step dimming
by use of a single logic control.
Figure 2 shows the resulting fOSC for various values of ROSC
If the OSC pin is connected to VREG or GND, the oscillator
frequency will be set internally to approximately 350 kHz.
.
• To reduce the LED current during cold-crank conditions, thus
avoiding overstressing the power components
When an external clock source is used to drive the OSC pin, it
can synchronize a number of A6268s operating together. This
ensures that only a single fundamental frequency is detectable
on the supply line, thus simplifying the design of any required
EMC filter. The disadvantage of using a single external clock
source is that all controllers will be switching current from the
supply at the same time. However, this effect may be reduced,
and the EMC performance may be further enhanced, by using
LED Brightness: PWM Dimming LED brightness can
be controlled by changing the current, which affects the light
intensity. However in some applications, for example with amber
LEDs, this will have some effect on the color of the LEDs.
In these cases it is more desirable to control the brightness by
switching the fixed LED current with a pulse width modulated
signal. This allows the LED brightness to be set with little effect
on the LED color and intensity and allows direct digital control
of the LED brightness.
700
600
500
400
300
200
100
A PWM signal can be applied to the EN input to enable PWM
dimming. The period of this signal should be less than the
minimum disable time, tDIS . During PWM dimming, the A6268
switches the LED current between 100% and 0% of the full cur-
rent. Note that during PWM dimming, the gate drive is disabled
when EN is low. The rate of change of the LED current is also
limited, to reduce any large variations in the input current.
Sleep Mode If EN is held low for longer than the disable time,
tDIS , then the A6268 will shut down and put all sections into a
low-power sleep mode. In this mode the bias current is typically
less than 4 ꢀA. In the buck-boost configuration the only leakage
path remaining will be the path through the MOSFET.
30
50
70
90
110 130 150 170 190 210
(kꢁ)
External Resistor Value, R
OSC
Figure 2. Internal oscillator frequency when set by ROSC
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A6268
Automotive High Current LED Controller
the CKOUT pin of another A6268 as the external clock source.
In this case the switching point of each subsequent A6268 in the
chain will be delayed from that of the previous A6268, and the
current pulses will be spread across the oscillator period.
If either FF1 is pulled low (due to an output short or overcur-
rent condition), or FF2 is pulled low when an open LED fault is
indicated, then the output disable will be overridden.
At start-up or during pulse width modulation of the Enable pin, a
fault blank period, tFB, occurs before the fault detection circuitry
becomes active. This period allows steady state conditions to
be established before fault monitoring takes place. Note that the
fault blank period is derived from the oscillator period by a ratio
of 1024:1, so any variation in the oscillator frequency will change
the fault blank period. For the default switching frequency of
350 kHz, this means the fault blank period would be:
Diagnostics
The circuit includes several diagnostic and safety functions to
assist in ensuring safe operation of the LEDs, the A6268, and the
external components. When any fault is detected, one or both of
the fault flag outputs, FF1 and FF2, will be inactive (high imped-
ance, open drain) until the fault is removed. The action taken by
the A6268 when a fault occurs is defined in table 1. To be able to
monitor the state of FF1 and FF2, add a suitable external pull-up
resistor.
t
FB = 1024 × (1 / 350 × 103) = 3 ms
(6)
Note that no fault blanking is applied to the following faults:
open LED, LED sense resistor negative overcurrent, VIN under-
voltage, or VREG undervoltage.
The A6268 will continue to drive the LEDs under most fault con-
ditions and will only disable the drive to the LEDs when a high
voltage hazard is present or the external components are likely
to be over-stressed. For output short circuits, LED sense resis-
tor overcurrents, LED sense resistor negative overcurrents, or
shorted LED string, the fault status is latched until a power cycle
occurs, or by pulling EN low for a time greater than the disable
time (> tDIS). In all other cases the drive will be re-established
when the hazard is removed.
VIN Undervoltage If the voltage at VIN drops below the
specified turn-off voltage, VINUV, the gate drive output, SG, will
be driven low and both fault flags, FF1 and FF2, will be high
impedance. VIN must rise above the turn-on threshold, VINUV
+
∆VINUV, before the A6268 can start up.
VREG Undervoltage If the voltage at VREG, VREG , drops
below the specified turnoff voltage, VREGUV, the gate drive
output, SG, will be driven low and both fault flags, FF1 and
FF2, will be high impedance. VREG must rise above the turn-on
threshold, VREGUV + ΔVREGUV, before the output circuits are
activated. This ensures that the external FET is operating in its
fully enhanced state and avoids permanent damage to the FET,
caused by overheating.
Table 1. Fault Table
Pin
Fault
Action
Latched
FF1 FF2
No Fault
L
Z
Z
L
L
L
L
Z
L
Z
Z
Z
Z
Z
Z
L
No Action
Disable*
Disable*
No Action
Disable*
No Action
No Action
Disable*
–
VIN Undervoltage
VREG Undervoltage
Overtemperature
Open LED
No
No
No
Yes
No
No
Yes
The VREG Regulator is designed to operate with a typical maxi-
mum load current of 15 mA. The majority of the VREG load will
be determined by the total gate charge of the external MOSFET.
The VREG pin can be also be used as a pull-up supply for the
fault flag outputs. The current required for this function has to be
considered in the overall load calculation. Note that if FF1 is used
for driving a series protection MOSFET then only FF2 is pulled
up to the VREG supply.
Shorted LED
LED Undercurrent
Output Short
LED Sense Resistor
Overcurrent
Z
L
Disable*
Yes
Overtemperature Warning If the chip temperature exceeds
the overtemperature threshold, TJF , fault flag FF2 will be high
impedance. No action will be taken by the A6268 to limit the
chip temperature. An external control circuit must take action
to avoid permanent damage to the A6268 and/or the LEDs. The
temperature will continue to be monitored and the fault flags will
LED Sense Resistor
Negative Overcurrent
Z
Z
L
L
Disable*
Disable*
Yes
Yes
Shorted LED String
*SG low, MOSFET off
L = active pull-down, Z = inactive, open drain
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A6268
Automotive High Current LED Controller
be deactivated when the temperature drops below the recovery
threshold provided by the specified hysteresis.
time-out period would be:
OTO = 32,768 × (1/350×103 ) = 94 ms
t
(10)
LED Diagnostics The voltage with respect to ground at the
three pins LP, LF, and LA, namely VLP, VLF , and VLA, determine
the status of the LEDs in the load. These voltages provide two
differential voltage measurements:
When the gate drive output is re-enabled at the end of the open
fault time-out period, the output is again monitored for an open
circuit. If the open circuit is still present, then the fault will again
be flagged and the switch drive disabled. This cycle will con-
tinue, as long as the open circuit condition is present.
• the voltage across a single reference LED:
V
LED = VLF – VLP
(7)
Shorted LED – A short circuit on one or more LEDs is detected
when:
• the ratio of the voltage across all LEDs in a single string:
• for the first (reference) LED:
VSTR = VLA – VLP
(8)
VSTR > VLED + VSCOR
• for other than the first (reference) LED:
VLED > VSTR + VSCO
(11)
(12)
These measurements are used to determine if there is an open
circuit, if one or more LEDs are shorted, if the output is shorted,
or if there is a short across the LED string.
where VSTR and VLED are as defined above, VSCO is the nonrefer-
ence LED short offset voltage, and VSCOR is the reference LED
short offset voltage. VSCO and VSCOR are defined in the Electrical
Characteristics table.
The voltage, VSTR , is derived from the voltage across all LEDs in
the string, by an external resistor divider with a ratio equal to the
quantity of LEDs in the string. To minimize the effects of the bias
currents introducing an offset voltage, it is recommended that the
resistor between LP and LA should be approximately 560 Ω.
When a short is present, the fault flag FF2 is high impedance, but
the regulator continues to operate and drives the remaining LEDs
with the correct regulated current. FF2 will remain high imped-
ance while the short circuit condition is present.
So for example, if eight LEDs were used, the ratio required
would be an eighth, therefore the resistor connected between LA
and the anode end of the LED string would be 3.9 kΩ;
560/[560 + 3900] = 1/8.
A short circuit on one or more LEDs will not cause a hazard
because the output is current-controlled. If one LED fails and
becomes a short circuit, then the remaining LEDs will continue
to be lit with the same current through, and voltage across,
each LED.
Open LED–An open circuit is evaluated when:
VSTR > VOP
(9)
where VOP is the LED open circuit voltage defined in the Electri-
cal Characteristics table.
Note—Accuracy: The output status monitor relies on all the
LEDs in the load having a similar forward voltage drop. Where
possible all the LEDs forming the load for a single controller
should be taken from the same voltage bin. The selection of
LEDs from the same bin is more critical when higher numbers of
LEDs are used in a single string. With only two or three LEDs a
wider variation in forward voltage is acceptable.
Because the output is current-controlled it is possible for an open
circuit on the output to cause extremely high voltages to be pres-
ent. Therefore, to prevent any hazardous voltages or damage to
the circuits, the gate drive output, SG, is immediately driven low
when an open circuit is detected. After an open circuit fault has
been detected, FF2 will become high impedance, and the open
circuit fault state will remain until the open fault time-out period,
LED Undercurrent – Under some circuit conditions, particu-
larly during a low input voltage condition, it is possible that there
could be insufficient drive to maintain the current to the LEDs
at the required level. If the voltage across the LED current sense
resistor, RSS , falls below the target sense voltage, VIDL, by an
amount that is more than the LED undercurrent voltage differ-
t
OTO, expires.
Note that the open fault time-out period is derived from the oscil-
lator period by a ratio of 32,768:1, so any variation in the oscilla-
tor frequency will change the open fault time-out period. For the
default switching frequency of 350 kHz, this means the open fault ence, VUCL, the A6268 will indicate an LED undercurrent condi-
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A6268
Automotive High Current LED Controller
tion by setting FF2 to high impedance. However, the A6268 will
continue to drive the output. When the output again reaches the
required current level, FF2 will go low.
Shorted LED Stack – A short circuit across the LED stack, is
detected when:
VSTR < VSCL
(13)
Output Short – An output short can consist of the LP, LN, or LF
terminals of the LED string being shorted, either to the battery
terminal or to ground. An output short is detected when both a
shorted LED and an LED undercurrent condition occur (defined
previously). If the above two conditions occur, the A6268 will set
FF1 high impedance. Note that at start-up, or during pulse width
modulation of the Enable pin, a fault blank period, tFB , occurs
before this fault detection circuitry becomes active.
If the above condition occurs, the A6268 will indicate an LED
overcurrent by setting FF1 high impedance. Note that at startup,
or during pulse width modulation of the Enable pin, a fault
blank period, tFB , occurs before this fault detection circuitry
becomes active.
Fault Flag One If any shorted condition occurs, including:
output short, LED sense resistor overcurrent, LED sense resistor
negative overcurrent, or shorted LED string, the A6268 will stop
the switching action by pulling SG low. The fault flag FF1 will
go high impedance and should be pulled up to the supply with
suitable external pull-up resistors to indicate the fault. Any of the
aforementioned faults will be latched and will only be cleared by
cycling the power, or by pulling EN low for a time greater than
the disable time (> tDIS).
LED Sense Resistor Overcurrent – Under some circuit fault
conditions, for example in boost mode, if the cathode connec-
tion is pulled to VBAT, the control loop can no longer control
the LED current to the target level. If the voltage across the LED
sense resistor, RSS , increases above the target sense voltage,
VIDL, by an amount that is more than the overcurrent voltage
difference, VOVCL, the A6268 will indicate a LED overcurrent by
setting FF1 high impedance.
The FF1 output can also be used with pull-up resistors and a
P-channel MOSFET in the supply, to isolate the switching ele-
ments and the load from the supply. This MOSFET should be
connected, as shown in figure 3, with the source connected to the
supply and the drain connected to the inductor of the converter.
Note that even if FF1 drives a supply isolation FET, the sense
resistor may still be damaged because it is effectively between
VBAT and GND. Note that at startup, or during pulse width
modulation of the Enable pin, a fault blank period, tFB , occurs
before this fault detection circuitry becomes active.
Two pull-up resistors are used to limit the voltage across the gate-
source junction during high input voltages or load dump condi-
tions. If the battery voltage is restricted, one resistor across the
gate-source junction can be used. The FF1 provides a sink current
of typically 1.3 mA.
Also in boost mode, if a “soft” short is applied across an LED
string, causing the string voltage to be less than the input voltage,
the control loop may not control as described previously and FF1
will be set. Alternatively, a soft short may cause a shorted LED
string, as described in the section Shorted LED String. The actual
detection of a soft short, whether by shorted LED string or LED
sense resistor overcurrent detection, will depend on the actual
application setup.
This circuit can be used to avoid most hazardous conditions and
protect the circuit components from over-stress. Note that under
extreme cases, the circuit cannot protect against certain fault
conditions as described in the following section.
LED Sense Resistor Negative Overcurrent – Under some
circuit fault conditions, for example in buck-boost mode, if the
cathode connection is pulled to GND, current will flow through
the sense resistor, RSS , in the opposite direction. If the voltage
across the sense resistor exceeds the negative overcurrent thresh-
old, VNOCL, the A6268 will set FF1 high impedance.
RSL (buck boost)
VBAT
Note that if FF1 does not drive a supply isolation FET, the sense
resistor may be damaged. Also, note that the fault blank period
will not be applied if this fault is present at startup, or during
pulse width modulation of the Enable pin, because it is always
regarded as a non-standard condition.
To VIN
To FF1
Figure 3. Example of a supply isolation MOSFET
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A6268
Automotive High Current LED Controller
the input source supply recovers, the A6268 will automatically
restart and the process will be repeated.
Protection Not Provided
Boost Mode
Buck-Boost Mode
• The cathode end of the LED string is shorted to VBAT –
Although an LED sense resistor overcurrent fault is produced,
causing an FF1 flag, the LED sense resistor, RSS, is effectively
between VBAT and ground. Depending on either the current
limit of the source supply, or the input fuse rating, the fault cur-
rent may damage the LED sense resistor.
• The cathode end of the LED string is connected to VBAT –
The short circuit impedance effectively appears in parallel with
the series protection MOSFET and the LED sense resistor, RSS.
This will tend to reduce the effective impedance of the LED
sense resistor and correspondingly increase the LED current.
• The anode connection is shorted to ground during startup –
A fault current determined by the current limit of the source
supply (VBAT), or a value less than the input fuse rating, will
flow through the series protection FET, inductor, and recircula-
tion diode. If the source supply can supply the fault current for
the duration of the fault blank period, tBK , then an FF1 flag will
occur. Otherwise, either the source supply voltage will fold back
and an input voltage UVLO will occur, disabling the A6268,
or the input fuse will blow, causing an open circuit. Assuming
the input source supply recovers, the A6268 will automatically
restart and the process will be repeated.
• The LF node is shorted to VBAT – Although a LED sense
resistor overcurrent fault is produced, causing an FF1 flag, the
LED sense resistor, RSS, and the reference LED are effectively
between VBAT and ground. Depending on the current limit of
the source supply, or the input fuse rating, the fault current may
damage the LED sense resistor and/or the reference LED.
• The cathode connection is shorted to ground – A fault current
determined by the impedance of the shorting link (now effec-
tively the LED sense resistor, RSS) flows through the power
circuit. The fault current will either be limited by the maximum
switch current sense, VIDS, or if the source supply cannot main-
tain this current, the source supply will either foldback, or if the
current exceeds the input fuse rating, the fuse will blow, causing
an open circuit.
To ensure the A6268 inputs (LP and LN) are not damaged dur-
ing any of the above faults, it is necessary to add differential
resistors between each of the LED sense resistor connections
and the respective connection to the A6268. In the case of buck
boost mode, one resistor is used on each connection. In the case
of boost mode, only one resistor is required between the LN
input and the cathode connection of the sense resistor. Refer to
the circuit diagrams on page 3. These resistor values should be
approximately 150 Ω.
• The anode connection is shorted to ground during startup –
A fault current determined by the current limit of the source
supply (VBAT), or a value less than the input fuse rating, will
flow through the series protection FET, inductor, and recircula-
tion diode. If the source supply can supply the fault current for
the duration of the fault blank period, tBK , then an FF1 flag will
occur. Otherwise, either the source supply voltage will fold back If an output short is detected but it is necessary to keep the output
and an input voltage UVLO will occur, disabling the A6268,
or the input fuse will blow, causing an open circuit. Assuming
active, the FF1 output can be pulled low. This will override the
output disable but will not clear the fault.
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A6268
Automotive High Current LED Controller
Application Information
In boost mode, the power loss in the current sense resistor is
Component Selection
worse at the lowest input voltage:
External component selection is critical to the successful appli-
cation of the LED driver. Although the inductor, the switching
MOSFET, and the output capacitor are the most critical elements,
the specification of the rectifying diode and sense resistors should
also be carefully considered.
PLOSS = (VLED / VIN(min)) × RLS × I2
(16)
LED
In buck-boost mode, the power loss in the current sense resistor is
worse at the lowest input voltage:
PLOSS = ([VIN + VLED] / VIN) × RLS × I2
(17)
The starting point for component selection is to define the maxi-
mum LED current, the voltage across the LEDs, and the input
operating voltage range. This then allows the average inductor
current under worst case conditions to be calculated.
LED
The power rating of the sense resistor should exceed the above
rating at the maximum temperature.
The resistors should be of a low inductance construction. Surface
mount chip resistors are usually the most suitable, however, axial
or radial leaded resistors can be used provided that the lead length
is kept to a minimum.
The inductor value is then selected based on the acceptable
inductor ripple current. The amount of ripple current will then
determine the maximum inductor current under worst case condi-
tions. From this current the switch current sense resistor can be
calculated.
Inductor Selection Selecting the correct inductance is a
balance between choosing a value that is small enough to help
reduce size and cost, but high enough to ensure that the inductor
current ripple is kept to an acceptable level. A reasonable target
for the ripple current is 20% of the maximum average current.
LED Current Sense Resistor (RLS) If the voltage at the
IREF pin, VIREF, is greater than 1 V, or if IREF is tied to VREG,
then the value of the LED current sense resistor, RLS , can be
calculated from:
The inductor current equations differ slightly depending on
whether the A6268 is configured as a boost or as a buck-boost
converter.
RLS = VIDL / ILED(max)
(14)
where VIDL is the differential voltage across the LED current
sense amplifier and ILED(max) is the maximum LED current.
• In a boost converter configuration:
If VIREF is less than 1 V, then the value of the LED current sense
resistor can be calculated from:
▫ The maximum average inductor current is approximately:
IL(av)(max) = ILED(max) × VLED / VIN(min)
▫ The inductor current ripple is approximately:
(18)
(19)
(20)
RLS = VIREF / (10 × ILED(max))
(15)
The typical value for VIDL is 100 mV. Examples of various sense
resistor values are given in table 2.
I
LRIP = VIN × (VLED – VIN) / (fOSC × L × VLED
▫ The inductor value is therefore:
L = VIN ×(VLED –VIN) / (fOSC ×ILRIP×VLED
• In a buck-boost configuration:
)
Table 2. Sense Resistor Values
)
ILED(max)
(mA)
RLS
(mΩ)
▫ The maximum average inductor current is approximately:
350
700
286
143
100
IL(av)(max) = ILED(max)×(VIN(min)+VLED)/VIN(min) (21)
▫ The inductor current ripple is approximately:
1000
ILRIP = VIN × VLED / (fOSC × L × [VIN + VLED])
(22)
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A6268
Automotive High Current LED Controller
▫ The inductor value is therefore:
However, as the current rating increases, the diode capacitance
also increases so the optimum selection is usually the lowest cur-
L = VIN ×VLED / (fOSC ×ILRIP×[VIN+ VLED ]) (23)
rent rating above the required maximum, in this case IL(PK)
.
where:
VLED is the voltage across the LED string,
VIN is the supply voltage,
VIN(min) is the minimum supply voltage,
L is the inductor value, and
fOSC is the oscillator frequency.
Switch Current Sense Resistor (RSS) Neither the absolute
value of the switch current nor the accuracy of the measurement
is important, because the regulator will continuously adjust the
switch current, within a closed loop, to provide sufficient energy
for the output. For maximum accuracy the switch sense resistor
value should be chosen to maximize the differential signal seen
by the sense amplifier. The input limit of the sense amplifier,
With an internal oscillator frequency of 350 kHz, the value of the
inductor for most cases will be between 20 and 50 ꢀH.
V
IDS, and the maximum switch current, IS(max), therefore define
the maximum value of the sense resistor as:
The maximum inductor current can then be calculated as:
RSS = VIDS / IS(max)
(25)
I
L(PK) = IL(av)(max) + (IRIP / 2)
(24)
where IS(max) is the maximum switch current and should be set
above the maximum inductor current, IL(PK)
This defines the minimum peak switch current as set by the
switch current sense resistor.
.
This represents the maximum measurable value of the switch
(and inductor) current; however, the peak switch current will
always be less than this, set by the control circuit, depending on
the input voltage and the required load conditions. Because the
switch current control is within a closed loop, it is possible to
reduce the value of the sense resistor to reduce its power dissipa-
tion. However this will reduce the accuracy of the regulated LED
current.
The current rating for the inductor should be greater, by some
margin, than the peak value above. When selecting an inductor
from manufacturers datasheets, there are two current levels usu-
ally defined, the smallest value being the figure to work with:
• Saturation level, where the inductance value typically drops by
10%, or
• Temperature rise, where the part experiences a certain rise in
temperature at full rated current. This parameter can be defined
between a 20°C and 50°C rise in temperature. It is important to
understand how manufacturers define the maximum operating
temperature, because this can often incorporate the self-heating
temperature rise.
In boost mode, the power loss in the switch sense resistor is
worse at the lowest input voltage:
P
LOSS = RSS ×I2LED × (VLED [VLED – VIN(min)]/V 2IN(min)) (26)
In buck-boost mode, the power loss in the switch sense resistor is
worse at the lowest input voltage:
In most cases the limiting current is usually the saturation value.
To improve efficiency, the inductor should also have low winding
resistance, typically < 50 mΩ, and the core material will usually
be ferrite, with low losses at the oscillator frequency.
PLOSS = RSS ×I2LED × (VLED /VIN(min))(VLED + VIN(min)) (27)
The power rating of the sense resistor should exceed the above
rating at the maximum temperature.
Recommended inductor manufacturers/series are:
• Coilcraft/ MSS1278T
External Switch MOSFET A logic-level N-channel MOSFET
is used as the switch for the DC-to-DC converter. In the boost
configuration the voltage at the drain of the MOSFET is equal
to the maximum voltage across the string of LEDs. In the
buck-boost configuration the output voltage is referenced to
the positive supply. This means that the voltage at the drain of
the MOSFET will reach a voltage equal to the sum of the LED
voltage and the supply voltage. Under load dump conditions,
up to 90 V may be present on this node. In this case the external
• TDK/ SLF12575 type H
Diode The diode should have a low forward voltage, to reduce
conduction losses, and a low capacitance, to reduce switching
losses. Schottky diodes can provide both these features if care-
fully selected. The forward voltage drop is a natural advantage
for Schottky diodes and reduces as the current rating increases.
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A6268
Automotive High Current LED Controller
MOSFET should therefore be rated at greater than 100 V.
Normally this large ripple current, in conjunction with the
requirement for a larger capacitance value for stability, would
dictate the use of large electrolytic capacitors. However in this
case stability is not a consideration, and the capacitor value can
be low, allowing the use of ceramic capacitors.
The peak switch current is defined by the maximum inductor cur-
rent, IL(PK) . However in most cases the MOSFET will be chosen
by selecting low on-resistance, which usually results in a current
rating of several times the required peak current.
To minimize self-heating effects and voltage ripple, the equiva-
lent series resistance (ESR), and the equivalent series inductance
(ESL) should be kept as low as possible. This can be achieved by
multilayer ceramic chip (MLCC) capacitors. To reduce perfor-
mance variation over temperature, low drift types such as X7R
and X5R should be used.
In addition to minimizing cost, the choice of MOSFET should
consider both the on-resistance and the total gate charge. The
total gate charge will determine the average current required from
the internal regulator and thus the power dissipation.
When the input voltage, VIN , reduces below the 5 V regulator
drop out level, the gate drive voltage will correspondingly reduce.
The level that this occurs at will depend on the average cur-
rent required for the gate charge. This level will typically occur
with an input voltage of around 5.3 V. The effect of a reduced
gate drive voltage may be an increase in the on-resistance of the
switching MOSFET.
The value of the output capacitor will typically be about 10 ꢀF
and it should be rated above the maximum voltage defined by the
series output LEDs.
Reverse Supply Protection Protection for the A6268 is
provided by an external low current diode between the supply
and the VIN pin, as shown in the Functional Block Diagrams
section. The isolation MOSFET shown in figure 3 is only able to
provide isolation when the supply polarity is correct. However,
with an additional P-channel MOSFET, it is also possible to
provide reverse battery protection to the switching elements and
the LEDs. The additional FET should be connected, as shown in
figure 4, with the drain to the supply and the source to the source
connection of the original isolation MOSFET.
Output Capacitor There are several points to consider when
selecting the output capacitor.
Unlike some switch-mode regulators, the value of the output
capacitor in this case is not critical for output stability. The
capacitor value is only limited by the required maximum ripple
voltage.
Due to the switching topology used, the ripple current for
this circuit is high because the output capacitor provides the
LED current when the switch is active. The capacitor is then
In the complete circuit, consideration should be given to limiting
the maximum gate-source voltage of the FET. If the supply volt-
recharged each time the inductor passes energy to the output. The age is likely to exceed 20 V, then either: a Zener clamp must be
ripple current on the output capacitor will be equal to the peak
inductor current.
added in parallel with the gate-source resistor to prevent damage
to the FET, or a second resistor added as shown in figure 3.
VBAT
To VIN
To FF1
Figure 4. Example of a supply isolation MOSFET
Allegro MicroSystems, Inc.
115 Northeast Cutoff
17
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
Package LP 16-Pin TSSOP with Exposed Thermal Pad
0.45
0.65
5.00±0.10
8º
0º
16
16
1.70
0.20
0.09
B
6.10
3.00
3 NOM 4.40±0.10 6.40±0.20
0.60 ±0.15
1.00 REF
A
1
2
3 NOM
1
2
0.25 BSC
Branded Face
SEATING PLANE
GAUGE PLANE
3.00
PCB Layout Reference View
C
16X
SEATING
PLANE
0.10
C
C
0.30
0.19
For Reference Only; not for tooling use (reference MO-153 ABT)
Dimensions in millimeters
1.20 MAX
0.65 BSC
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
0.15
0.00
Terminal #1 mark area
A
B
C
Exposed thermal pad (bottom surface); dimensions may vary with device
Reference land pattern layout (reference IPC7351
SOP65P640X110-17M);
All pads a minimum of 0.20 mm from all adjacent pads; adjust as
necessary to meet application process requirements and PCB layout
tolerances; when mounting on a multilayer PCB, thermal vias at the
exposed thermal pad land can improve thermal dissipation (reference
EIA/JEDEC Standard JESD51-5)
Allegro MicroSystems, Inc.
115 Northeast Cutoff
18
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A6268
Automotive High Current LED Controller
Revision History
Revision
Revision Date
Description of Revision
Update application information
Rev. 1
July 26, 2012
Copyright ©2012, Allegro MicroSystems, Inc.
Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to per-
mit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the
information being relied upon is current.
Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the
failure of that life support device or system, or to affect the safety or effectiveness of that device or system.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use;
nor for any infringement of patents or other rights of third parties which may result from its use.
For the latest version of this document, visit our website:
www.allegromicro.com
Allegro MicroSystems, Inc.
115 Northeast Cutoff
19
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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