ALT80800
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
FEATURES AND BENEFITS
DESCRIPTION
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The ALT80800 is a synchronous buck switching regulator
that provides constant-current output to drive high-power
LEDs. It integrates both high-side and low-side N-channel
DMOS switches for DC-to-DC step-down conversion. A true
average current is output using a cycle-by-cycle, controlled
on-time method.
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•
•
•
•
•
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•
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AEC-Q100 qualified
Supply voltage 4.5 to 55 V
2.0 A maximum output over operating temperature range
Integrated high-side and low-side MOSFETs:
200 mΩ / 150 mΩTYP
True average output current control
Internal control loop compensation
Integrated 5 V, 14 mA LDO regulator for peripheral circuits
Dimming via PWM pin or EN pin down to 0.1% at 200 Hz
Analog dimming (ADIM pin) for brightness calibration
and thermal foldback
Low-power shutdown (1 µA typical)
Cycle-by-cycle current limit
Active low fault flag output
LED open fault mask setting for low VIN operation
Undervoltage lockout (UVLO) and thermal shutdown
protection
Switching frequency dithering for improved EMC
Robust protection against:
□□ Adjacent pin-to-pin short
□□ Pin-to-ground short
□□ Component open/short faults
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 PWM pin while EN is
enabled. Alternatively, applying a PWM signal at the EN pin
while PWM pin is high can enable “chopped battery” PWM
dimming for legacy control modules.
Furthermore, an Analog Dimming input (ADIM pin) can be
used, for example, to calibrate the LED current or implement
thermal foldback in conjunction with external NTC thermistor.
The ALT80800 is provided in a 16-pin TSSOP (suffix LP),
with exposed pad for enhanced thermal dissipation.
APPLICATIONS:
PACKAGE:
16-Pin eTSSOP (suffix LP)
Automotive lighting
• Daytime running lights
• Front and rear fog lights
• Turn/stop lights
• Map light
• Dimmable interior lights
Not to scale
VIN
CIN
VIN
GND
ALT80800
EN
VCCIN
External PWM
dimming signal
External analog
dimming signal
TON
RON
PWM
SW
CBOOT
ADIM
VCC
LED+
CSH
CSL
VCC
FFn
VIN
VCC
CBIAS
SGND
PGND
RSENSE
BOOT
PWM
ADIM
L1
FFn
CLED
GND
FDSET
GND
Figure 1: ALT80800 Typical Application Circuit
ALT80800-DS
MCO-0000344
December 7, 2017
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
SELECTION GUIDE
Part Number
ALT80800KLPATR
Package
Packing
16-pin TSSOP with exposed thermal pad
4000 pieces per 13-inch reel
SPECIFICATIONS
ABSOLUTE MAXIMUM RATINGS
Characteristic
Supply Voltage
Bootstrap Drive Voltage
Switching Voltage
EN Voltage
Current Sense Voltages
Linear Regulator Terminal
ADIM pin, TON pin
FDSET Voltages
FFn and PWM Voltages
Maximum Junction Temperature
Storage Temperature
Symbol
Notes
VIN, VVCCIN
VBOOT
VSW
Continuous
Pulsed, t < 50 ns
VEN
Rating
Unit
–0.3 to 60
V
–0.3 to VIN + 8
V
–0.3 to VIN + 0.3
V
–1 to VIN + 3
V
–0.3 to VIN + 0.3
VCSH, VCSL
V
V
VCC
V
VADIM, VTON
V
–0.3 to 7
VFDSET
VFFn, VPWM
V
V
TJ(max)
150
°C
Tstg
–55 to 150
°C
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*
On 4-layer PCB based on JEDEC standard
Value
Unit
34
°C/W
2
°C/W
*Additional thermal information available on the Allegro™ website.
Table of Contents
Features and Benefits 1
Description 1
Applications 1
Package 1
Typical Application Circuit 1
Selection Guide 2
Specifications 2
Absolute Maximum Ratings 2
Thermal Characteristics 2
Pinout Diagrams and Terminal List Tables 3
Functional Block Diagrams 4
Electrical Characteristics 5
Functional Description 7
Application Circuit Diagrams 19
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
PINOUT DIAGRAM AND TERMINAL LIST TABLE
TSSOP-16 (LP) Pinout Diagram
PGND 1
16 SW
PGND 2
15 BOOT
VIN 3
14 VCCIN
EN 4
13 FFn
PWM 5
FDSET 6
PAD
12 VCC
11 SGND
ADIM 7
10 CSL
TON 8
9 CSH
Terminal List Table
Number
Name
Function
1, 2
PGND
3
VIN
Supply input voltage for power stage.
4
EN
Enable pin for internal LDO regulator and whole IC. EN pin can
also be used as PWM dimming when keeping PWM pin High.
5
PWM
Logic input for PWM dimming: when PWM = LOW, LED is off; if
PWM = High and at the same time EN is enabled, LED is ON.
6
FDSET
FDSET pin to set the LED Open fault mask threshold. Connect
to a voltage divider formed between VIN and PGND. When VIN
is low, resulting in FDSET below the internal reference, LED
Open Fault detection will be masked.
7
ADIM
Analog dimming control voltage input. If not used for analog
dimming, tie ADIM to 5 V or VCC; if used for analog dimming,
keep ADIM less than 2.5 V.
8
TON
Regulator on-time setting resistor terminal. Connect a resistor
between TON pin and SGND to set the switching frequency.
9
CSH
Current Sense (positive end) feedback input for LED current.
10
CSL
Current Sense (negative end) feedback input for LED current.
11
SGND
12
VCC
Internal IC bias regulator output. Connect at least 1 µF MLCC to
PGND. Can be used to supply up to 14 mA for external load.
13
FFn
Open-drain output which is pulled low in case of fault. Connect
through an external pull-up resistor to the desired logic level.
14
VCCIN
It is recommended to connect VCCIN to VIN to bias the internal
LDO regulator.
15
BOOT
High-side gate driver bootstrap terminal; a 0.47uF capacitor is
recommended between BOOT and SW.
16
SW
Switched output terminal. The output inductor should be
connected to this pin.
–
PAD
Exposed pad for enhanced thermal dissipation; connect to
ground.
Power ground terminal.
Signal ground terminal.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
3
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
VIN
VIN
CIN
VOUT
TON
Up to 14 mA
external load
CBOOT
17 ms
VCC
LDO
CBIAS
PWM
PWM
ADIM
ADIM
i_LED
reference
L1
PGND
VOUT
LED+
LED
Current
UVLO
SW
RSENSE
Buck
Converter
Duty Cycle
Control
Internal 5.0 V bias
VCC
VIN
Gate
Driver
On-Time
EN
BOOT
Boot
Charger
On-Time
Select
RON
VCCIN
VIN
CSH
Differential
VCSREG
Amp
CSL
RADJ
CLED
ALT80800
VREF
(0 – 200 mV)
VIN
Other Faults
FDSET
+
REF1
VOUT
LED Open
Fault
LED Short
Fault
VCC
FFn
Fault
Handling
FFn
SGND
Figure 2: Functional Block Diagram
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
4
ALT80800
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ELECTRICAL CHARACTERISTICS: Valid at VIN = 12 V, VOUT = 6 V, TJ = –40°C to 125°C, typical values at TJ = 25°C,
unless otherwise noted
Characteristics
Input Supply Voltage
Symbol
Test Conditions
VIN
Min.
Typ.
Max.
Unit
4.5
–
55
V
VIN Undervoltage Lockout Threshold
VUVLO(ON)
VIN increasing, VIN = VVCCIN, ICC = 0 mA
–
–
4.3
V
VIN Undervoltage Lockout Hysteresis
VUVLO(HYS)
VIN decreasing, VIN = VVCCIN, ICC = 0 mA
100
–
300
mV
VCSH – VCSL = 0.5 V, VEN = VIH_EN,
VPWM = VIH_PWM, RON = 402 kΩ
–
5
–
mA
–
1
10
µA
2.65
–
50
V
2.5
3.25
4.0
A
–
0.2
0.32
Ω
–
0.15
0.24
Ω
3.1
3.4
3.7
V
VIN Pin Supply Current
IIN
VIN Pin Shutdown Current
IINSD
VEN = VIL_EN
Output Current Sense Common Mode
Voltage (measured at CSL pin) [1]
VOUT
VIN = 55 V, fSW = 500 kHz, iLED = 0.5 A
Buck Switch Current Limit Threshold
ISWLIM
Buck High-Side Switch On-Resistance
RDSON(HS)
VBOOT = VIN + 4.3 V, TJ = 25°C, ISW = 0.5 A
Buck Low-Side Switch On-Resistance
RDSON(LS)
TJ = 25°C, ISW = 0.5 A
BOOT Undervoltage Lockout Threshold
VBOOTUV
VBOOT to VSW increasing
BOOT Undervoltage Lockout Hysteresis
VBOTUVHYS
VBOOT to VSW decreasing
–
750
–
mV
VCSH – VCSL = 0 V
–
100
125
ns
–
65
90
ns
200
–
300
ns
Switching Minimum Off-Time
tOFFmin
Switching Minimum On-Time
tONmin
Selected On-Time
tON
VIN = 12 V, VOUT = 6 V, RON = 42.2 kΩ
Low-Side Switching Minimum On-Time [2]
tLS_ONmin
–
60
90
ns
tON Dithering Range
fSW_DITH
RON = 42.2 kΩ
–
±5%
–
–
Dithering Modulation Frequency
fSW_MOD
RON = 42.2 kΩ
–
12.5
–
kHz
194
200
206
mV
REGULATION COMPARATOR AND ERROR AMPLIFIER
Load Current Sense Regulation
Threshold at 100% [3]
VCSREG
VCSH – VCSL decreasing, SW turns on, ADIM
tied to VCC
CSH Input Sense Current [4]
ICSH
VCSH – VCSL = 0.2 V
–
–250
–
µA
CSL Input Sense Current
ICSL
VCSH – VCSL = 0.2 V
50
75
100
µA
VCC
0 mA < ICC < 14 mA, VVCCIN > 6 V
4.85
5.0
5.15
V
VLDO
Measure VVCCIN – VCC: VVCCIN = 4.8 V,
ICC = 14 mA
–
0.3
0.55
V
INTERNAL LINEAR REGULATOR
VCC Regulated Output
VCC Dropout Voltage
VCC Current Limit
VCC Undervoltage Lockout
iVCCLIM
VCCUVLO
VCC ≥ 4.35 V
Rising
VCCUVLOHYS Hysteresis
20
–
–
mA
3.65
3.9
4.05
V
175
225
275
mV
PWM INPUT
Logic High Voltage
VIH_PWM
VEN increasing
1.8
–
–
V
Logic Low Voltage
VIL_PWM
VEN decreasing
–
–
1.2
V
PWM Pin Pull-Up Resistance
RPWMPU
VCC = 5 V
–
100
–
kΩ
tOFFDelay
Measured while PWM dimming signal applied at
EN keeping low and exceeding tOFFDelay results
in shutdown
10
17
–
ms
EN INPUT
Maximum IC Turn Off Delay
Logic High Voltage
VIH_EN
EN increasing
1.8
–
–
V
Logic Low Voltage
VIL_EN
EN decreasing
–
–
0.4
V
Continued on the next page…
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
5
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
ELECTRICAL CHARACTERISTICS (continued): Valid at VIN = 12 V, VOUT = 6 V, TJ = –40°C to 125°C, typical values at
TJ = 25°C, unless otherwise noted
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
2.1
–
–
V
ANALOG DIMMING INPUT
Input Voltage for 100% LED Current
VADIMH
Regulation Threshold at 50% Analog
Dimming
VCSREG50
VADIM = 1.0 V
–
100
–
mV
Regulaton Threshold at 20% Analog
Dimming
VCSREG20
VADIM = 0.4 V
38.4
40
41.4
mV
LED Open/Short Detect Condition
ADIM Range
VADIM rising
244
264
284
mV
LED Short Fault Output Voltage Low
Threshold
VOUT falling
1.3
1.5
1.7
V
2.352
2.4
2.448
V
VCSH – VCSL = VCSREG
FAULT
LED Open-Fault Enable Reference
VREF1
LED Open Fault Current Threshold
VCS_OPEN
VCSREG = 200 mV start falling (PWM duty =
max), VADIM = VCC, VFDSET = VCC
(20 mV)
10%
(50 mV)
25%
(80 mV)
40%
–
VCS_OPEN_HYS
VCSREG = 200 mV start falling (PWM duty =
max), VADIM = VCC, VFDSET = VCC
(6 mV)
3%
(12 mV)
6%
(18 mV)
9%
–
35
50
65
µs
LED Open Fault Current Hysteresis [1]
Fault Deglitch Timer
Fault Mask Timer
tFDG
tMASK
70
100
130
µs
FFn Pull-Down Voltage
VFAULT(PD)
Fault condition asserted, pull-up current = 1 mA
–
–
0.4
V
FFn Pin Leakage Current
IFAULT(LKG)
Fault condition cleared, pull-up to 5 V
–
–
1
µA
–
–
10
µs
–
–
10
µs
tRETRY
–
1
–
ms
TSD
150
165
180
°C
TSDHYS
–
25
–
°C
FFn Rising Time [1]
tRISE
The transition time FFn pin takes from Low
to High
FFn Falling Time [1]
tFALL
The transition time FFn pin takes from High
to Low
Cool Down Timer for Fault Retry
THERMAL SHUTDOWN
Thermal Shutdown Threshold [1]
Thermal Shutdown Hysteresis
[1]
[2]
Determined by design and characterization. Not production tested.
Guaranteed by design, HS and LS switches are interlocked, as illustrated below:
SW
tOFFmin
tdead ≈
(tOFFMIN – tLS_ONmin) / 2
tLS_ONmin
Low Side VGS
tdead
tdead
In test mode, a ramp signal is applied between CSH and CSL pins to determine the VCSH – VCSL regulation threshold voltage. In actual application,
the average VCSH – VCSL voltage is regulated at VCSREG regardless of ripple voltage.
[4] Negative current is defined as coming out of (sourcing) the specified device pin or node.
[3]
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
FUNCTIONAL DESCRIPTION
The ALT80800 is a synchronous buck regulator designed for
driving a high-current LED string. It uses 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:
Synchronous Regulation
The ALT80800 integrates an N-channel DMOS as the low-side
switch to implement synchronous regulation for LED drivers, as
shown in Figure 4.
iLED = VCSREG / RSENSE
VIN
where VCSREG = VCSH – VCSL = 0.2 V typical.
If necessary, a resistor can be inserted in series with the CSL pin
to fine-tune the LED current, as shown below:
iCSH
iLED
CSH
VCSREG
CSL
iCSL
Radj RSENSE
+
VSENSE
–
–
+
iCSL × Radj
VCSREG = iLED × RSENSE + iCSL × Radj
Therefore
iLED = (VCSREG – iCSL × Radj) / RSENSE
Boot
Charger
BOOT
CBOOT
L
SW
Floating
Gate Driver
SW
GND
Rsc
VOUT
i_L
CLED
Integrated
ALT80800 Switch
Figure 4: Synchronous Buck LED Driver
The Synchronous configuration can effectively pull down SW
to ground by forcing the low-side synchronous switch on even
with small inductor current, as shown in Figure 5. Therefore, the
BOOT capacitor can be charged normally every switch cycle to
ensure the normal operation of buck LED drivers.
Figure 3: How To Fine-Tune LED Current Using Radj
For example, with a desired LED current of 1.4 A, the required
RSENSE = 0.2 V / 1.4 A = 0.143 Ω. But the closest power resistor
available is 0.13 Ω. Therefore, the difference is
Radj × iCSL = 0.2 V – 1.4 A × 0.13 Ω = 0.018 V
where iCSL = 75 µA typical
Radj = 0.018 V / 75 µA = 240 Ω
The LED current is further modulated by the ADIM (Analog
Dimming) pin voltage. This feature can be used for LED brightness calibration, or for thermal foldback protection. See Analog
Dimming section for details.
Figure 5: Normal SW waveform with SR configuration
when VIN ≈ VOUT: VIN = 5.4 V, VOUT = 5.14 V
(2 white LEDs)
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
7
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
Switching Frequency
resistor is not necessary if EN is driven from a logic input.
The ALT80800 operates in fixed on-time mode during switching.
The on-time (and hence switching frequency) is programmed
using an external resistor connected between the TON pin and
ground, as given by the following equation:
The PWM pin is a logic input pin and is internally pulled up to
VCC through a resistor.
EN pin
tON = k × (RON + RINT ) × ( VOUT / VIN )
fSW = 1 / [ k × (RON + RINT )]
where k = 0.0127, with fSW in MHz, tON in µs, and RON and RINT
(internal resistance, 3 kΩ) in kΩ.
2.4
2.2
2.0
1.8
fSW (MHz)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
100
200
300
400
500
600
700
800
EN pin and PWM pin function as illustrated below:
900
1000
RON (kΩ)
Figure 6: Switching Frequency vs. TON resistance
To minimize the peaks of switching frequency harmonics in EMC
measurement, a dithering feature is implemented. The dithering
range is internally set at ±5%. The actual switching frequency is
swept linearly between 0.95 × fSW and 1.05 × fSW, where fSW is
the programmed switching frequency. The rate of modulation for
fSW is fixed internally at 12.5 kHz.
ENABLE AND DIMMING
The ALT80800 is activated when a logic high signal is applied to
the EN (enable) pin and VIN = VVCCIN is above UVLO threshold
4.3 V. The buck converter ramps up the LED current to a target
level set by RSENSE when PWM pin = High.
The EN pin is high-voltage tolerant and can be directly connected
to a power supply. However, if VEN is higher than the VIN voltage
at any time, a series resistor (10 kΩ) is required to limit the current
flowing into the EN pin. This resistor is helpful in preventing EN
from damage in case of reverse-battery connection. This series
PWM pin
VCC
LED
High
Low
ON
OFF
High
High/Open
ON
ON
Low
x
Shutdown
When the EN pin is forced from high to low, the LED current is
turned off, but the IC remains in standby mode for up to at least
10 ms. If EN goes high again within this period, the LED current
is turned on immediately if PWM pin is high. If EN pin is low for
more than tOFFDelay, 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
150 μs. This startup delay is not present during PWM operation.
Active dimming of the LED is achieved with 2 options: by sending a PWM (pulse-width modulation) signal to the EN pin (while
PWM = High), or by sending a dimming PWM signal to the
PWM pin (while EN is enabled) as illustrated in the table above.
The resulting LED brightness is proportional to the duty cycle of
the applied PWM signal. A practical range for PWM dimming
frequency is between 100 Hz (period = 10 ms) and 2 kHz.
If the PWM dimming signal at PWM pin is low when the EN pin
is high, the LED will be off immediately and IC is alive waiting
for next PWM pulse. The internal LDO is still on and can provide
bias to the internal and external circuits.
PWM DIMMING RATIO
The brightness of the LED string can be changed by adjusting the
PWM duty cycle at the EN pin as follows:
Dimming ratio = PWM on-time / PWM period
For example, by selecting a PWM period of 5 ms (200 Hz PWM frequency) and a PWM on-time of 5 µs, a dimming ratio of 0.1% can
be achieved. This is sometimes referred to as “1000:1 dimming.”
In an actual application, the minimum dimming ratio is determined by various system parameters, including: VIN , VOUT ,
inductance, LED current, switching frequency, PWM frequency,
and fault flag usage. The device is easily capable of PWM ontime as short as 5 µs; however, if fault flag for open/short LED
detection is required, it should be above 130 µs due to the fault
mask timer.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
8
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
ANALOG DIMMING
In addition to PWM dimming, the ALT80800 also provides an
analog dimming feature. When VADIM is over 2.0 V, the LED current is at 100% level (as defined by the SENSE resistor). When
VADIM is below 2 V, the LED current decreases linearly down to
20% at VADIM = 0.4 V. This is shown in the following figure:
200 mV
±6 mV
(100%)
VCSREG
ADIM is tied to 5 V (or VCC) if never used for analog dimming,
or always less than 2.5 V when used for analog dimming. For
long term reliability, or extended period with extreme temperature
condition, it is better to keep ADIM always less than 2.5 V.
OUTPUT VOLTAGE AND DUTY CYCLE
The figure below provides simplified equations for approximating output voltage. The output voltage of a buck converter is
approximately given as:
VOUT ≈ VIN × D , D = tON / (tON + tOFF)
where D is the duty cycle.
100 mV
VIN
ADIM pin
voltage
40 mV
0
0.4 V
1V
MOS
CIN
2V
Figure 7: ADIM Pin Voltage Controls SENSE Reference
Voltage (hence LED current)
L
SW
It is possible to pull ADIM pin below 0.4 V to achieve lower
than 20% analog dimming. However, if the average LED current
determined by ADIM becomes too low and is below half the
inductor current ripple, negative current will flow through the
inductor. To prevent such cases from happening, it is suggested
that ADIM voltage should meet the condition below:
iL
RSENSE
VOUT
D
GND
ADIM > RSENSE / 0.2 × (VIN – VOUT) / L × D × T
where D is duty cycle, D ≈ VOUT / VIN, T is switching period,
T = 1 / fSW, L is the inductance.
VSW
For example, when RSENSE = 0.2 Ω, RON = 178 kΩ, L = 33 µH,
VIN = 12 V, VOUT = 5.2 V, ADIM voltage should be above 0.21 V,
i.e. 11% level, to avoid negative inductor current.
VIN
ADIM pin can be used in conjunction with PWM dimming to
provide wider LED dimming range over 1000:1. In addition, the IC
can provide thermal foldback protection by using an external NTC
(negative temperature coefficient) thermistor, as shown below:
–VD
t
0
iL
VCC
RS
NTC
iRIPPLE
R1
t
ADIM
RP
tON
tOFF
Period, t
Figure 8: Using an External NTC Thermistor
to Implement Thermal Foldback
Figure 9: Simplified Waveforms for a Buck Converter
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
9
ALT80800
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
During SW on-time:
20
18
iRIPPLE = (VIN – VOUT) / L × tON = (VIN – VOUT) / L × t × D
16
VOUT (V)
where D = tON / t.
During SW off-time:
iRIPPLE = VOUT / L × tOFF = VOUT / L × t × (1 – D)
Simplified equation for output voltage:
VOUT = VIN × D
More precisely:
VOUT = (VIN – iAVG × RDSON(HS)) × D – (1 – D) × RDSON(LS) × iAVG
– (DCR + RSENSE) × iAVG
where DCR is the internal resistance of the inductor, RSENSE is
the current sensing resistance, RDSON(HS) is the on-resistance
of high-side switch, RDSON(LS) is the on-resistance of low-side
switch, iAVG is the average current through inductor and equal to
LED current.
MINIMUM AND MAXIMUM OUTPUT VOLTAGES
For a given input voltage, the maximum output voltage depends
on the switching frequency and minimum tOFF . For example, if
tOFF(min) = 100 ns and fSW = 1 MHz, then the maximum duty
cycle is 90%. So for an 18 V input, the maximum output is
approximately 16.2 V (based on the simplified equation of VOUT
= VIN × D). This means up to 5 LEDs can be operated in series,
assuming Vf = 3.3 V or less for each LED.
The minimum output voltage depends on minimum tON and
switching frequency. For example, if the minimum tON = 65 ns
and fSW = 1 MHz, then the minimum duty cycle is 6.5%. That
means with VIN = 18 V, the theoretical minimum VOUT is just
1.2 V. However, the internal current sense amplifier is designed to
guarantee the current accuracy down to VOUT = 2.65 V. When the
output voltage is lower than 2.65 V, the regulator keeps switching to regulate, but the current accuracy will suffer and not be
guaranteed.
14
12
VOUT(max) (V)
10
VOUT(min) (V)
8
6
4
2
0
0
0.2
0.4
0.6
0.8
1
1.2
Frequency (MHz)
1.4
1.6
1.8
2
Figure 10: Minimum and Maximum Output Voltage vs.
Switching Freqency
(VIN = 18 V, minimum tON = 90 ns and tOFF = 100 ns)
If the required output voltage is lower than that permitted by the
minimum tON , the controller will automatically extend the tOFF to
maintain the correct duty cycle. This means that the switching
frequency will drop lower when necessary to keep the LED current in regulation.
If the LED string is completely shorted (VOUT = 0 V), the controller will continue to switch at minimum tON and will not enter
into Hiccup mode.
THERMAL BUDGETING
The ALT80800 is capable of supplying a 2 A current through
its high-side switch. However, depending on the duty cycle, the
conduction loss in the high-side switch may cause the package to
overheat. Therefore care must be taken to ensure the total power
loss of package is within budget. For example, if the maximum
temperature rise allowed is ∆T = 60°C at the device case surface,
then the maximum power dissipation of the IC is 1.75 W. Assuming the maximum RDSON(HS) = 0.32 Ω, RDSON(LS) = 0.24 Ω, and
a duty cycle of 70%, then the maximum LED current is limited to
2 A approximately.
To a lesser degree, the output voltage is also affected by other
factors such as LED current, on-resistance of the high-side
switch, and DCR of the inductor.
As a general rule, switching at lower frequencies allows a wider
range of VOUT , and hence more flexible LED configurations.
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10
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
FAULT HANDLING
The ALT80800 is designed to handle the following faults:
• Pin-to-ground short
• Pin-to-neighboring pin short
• Pin open
• External component open or short
• Output short to ground
LED OPEN/OUTPUT SHORT FAULTS
Referring to Fault Function block diagram below, LED Open Fault
is masked when VIN is below the pre-set adjustable threshold at
FDSET pin or ADIM is below 264 mV. When FDSET is below
REF1 or ADIM is below 264 mV with asserting fault flag (FFn =
Low), the fault flag keeps asserted if open LED fault exists. Only
when FDSET is above REF1 and ADIM is above 264 mV, then
the Open fault will be detected by checking current sensing voltage VCSREG and duty cycle. LED Open fault will force regulator
into Hiccup mode and assert fault flag, and then fault flag remains
asserted during the remaining hiccup mode periods. Once LED
open fault disappears, fault flag goes high after hiccup mode period
when PWM is high. (refer to Figure 11 and Table 1).
VIN
FDSET
TON Resistor Open /Short,
RSENSE Open/Short,
Inductor Open/Short ,
Overcurrent
+
-
REF1
ADIM +
-
LED Open
1 ms
Hiccup
Mode
VCC
0.264 V
VCSREG -
FFn
+
25% i LED
Duty
FFn
+
-
SGND
MaxDuty
VOUT -
LED Short
+
1.5 V
Figure 11a: Simplified Faults Block Diagram
FDSET
(or ADIM
@ 264 mV)
REF1
PWM
VCSREG
25% × i LED
LED OPEN FAULT
LED OPEN FAULT
FFn Flag
Figure 11b: LED Open Fault Timing Diagram
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11
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
Table 1: LED Open Fault Truth Table
LED Open Fault Event?
FDSET
ADIM
FFnn
High
High
x
No Open Fault
High
High
x
Yes, Open Fault
Low
x
1
x
VCSREG < 25% × iLED
Max Duty
PWM
FFnn+1
1
0
x
1
x
Low
1
x
x
1
Low
x
0
Yes, Open Fault
x
0
x
0
x
Low
0
Yes, Open Fault
Low
x
0
No Open Fault
1
x
Low
0
No Open Fault
1
FDSET High means FDSET > REF1; FDSET Low means FDSET < REF1;
ADIM High means ADIM > 264 mV; ADIM Low means ADIM < 264 mV
When output Short fault (such as LED shorted to ground or output capacitor shorted to ground) occurs, FFn will be flagged as
VOUT drops below 1.5 V and ADIM voltage is above 264 mV; but
regulator will not enter into Hiccup mode and will work continuously. When short is removed, ALT80800 will return to normal
operation.
When an LED Open/Short fault occurs, the Fault pin will be
flagged if the fault remains active after a deglitch period (tFDG).
A mask timer (tMASK) is also introduced whenever PWM signal
goes from Low to High. During this mask time, faults will not be
detected, so the fault will not be detected when the PWM pulse
width is less than this mask time. When PWM goes low, fault
flag is latched. Fault flag will keep prior state when PWM is
Low.
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12
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
The Fault deglitch time is fixed; and the Fault mask time is also
fixed (refer to Electrical Characteristics table). The LED Open/
Short Fault timing diagrams are illustrated below:
t MASK
Short
Removed
Short
Fault
PWM
t FDG
Short
Fault
Short
Removed
Short
Fault
t MASK
FFn
Figure 12a: LED Short Fault Timing Diagram Overview
t MASK
Open
Fault
Open
Removed
Open
Removed
Open
Fault
Open
Fault
PWM
t FDG
t MASK
~1 ms Hiccup period
SW
FFn
~1 ms Hiccup
Current to
regulation timer
~1 ms Hiccup
~1 ms Hiccup
Current to
regulation timer
Figure 12b: LED Open Fault Timing Diagram Overview
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13
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
The basic timing configurations are detailed below for LED Open/Short faults:
Case 1: LED Open/Short Event is outside
Mask Timer at PWM = H
PWM
Case 2: LED Open/Short Event is within
Mask Timer at PWM = H
PWM
Mask Timer
Fault Event
Fault No LED Fault
Event
Fault Flag
Fault Event
Fault
Event
LED Open/Short
Fault Flag
LED Open/Short
Deglitch
Timer
Mask Timer
Case 3: LED Open/Short Event is close to
PWM ↓ at PWM = H
Case 4: LED Open/Short Event is at PWM = L
Mask Timer
PWM
Mask Timer
Fault
Event
Mask Timer
PWM
Fault Event
Fault Event
Fault
Event
No LED Fault
Mask Timer
LED Open/Short
LED Open/Short
Fault Flag
Fault Flag
Mask Timer
Mask Timer
Case 5: LED Open/Short Removed at PWM = L
a) LED Short Removed at PWM = L
b) LED Open Removed at PWM = L
PWM
PWM
Mask Timer
Fault Removed
Short
Event
Mask Timer
Fault Removed
Open
Event
No LED Short
Fault Flag
Mask Timer
No LED Open
Current to
regulation timer: *
Fault Flag
Case 6: LED Open/Short Removed outside Mask Timer at PWM = H
a) LED Short Removed outside
b) LED Open Removed outside
Mask Timer at PWM = H
Mask Timer at PWM = H
PWM
Short
Event
Mask Timer
Fault Removed
No LED Short
LED Short
Fault Flag
PWM
Open
Event
Fault Flag
Mask Timer
Fault Removed
No LED Open
LED Open
Hiccup period:
~1 ms
Current to
regulation timer *
Case 7: LED Open/Short Removed within Mask Timer at PWM = H
a) LED Short Removed within
b) LED Open Removed within
Mask Timer at PWM = H
Mask Timer at PWM=H
PWM
Short
Event
Mask Timer
Fault Removed
No LED Short
LED Short
Fault Flag
PWM
Open
Event
Fault Flag
Mask Timer
Fault Removed
No LED Open
LED Open
Current to
regulation Timer *
* Current to regulation timer is 256 switching cycles.
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14
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
VIN
SYSTEM FAILURE DETECTION AND PROTECTION DEMONSTRATION
C1,C2 = open or short
C1
C2
GND
R1 = open
or short
ALT80800
R1
EN
PWM
C5 = open
or short
VIN
SW
TON
BOOT
EN
CSH
PWM
CSL
VCC
C4 open
or short
L1 = open
or short
RSENSE open
or short
C4
LED+
C3
C3 open
or short
SGND GND
PGND
C5
IC-Level Failure Modes
Protected against
- Any pin open
- Any pin shorted to ground
- Adjacent pin-to-pin short
RSENSE
L1
LED string open
or short to GND
GND
System-Level Failure Modes
Protected against open/short fault for
all external components, including:
- LED string
- Sense resistor
- Inductor
- Input/output caps, etc.
Figure 13: Demonstration of various possible fault cases in an application circuit
Table 2: System Failure Mode Table (partial)
Failure Mode
Symptom Observed
FAULT flag
asserted?
Inductor open
Dim light from LED
Yes [1]
Inductor shorted
Dim light from LED
Yes
Current spike trips SW OCP and turns off switching, entering into
Hiccup mode with about 1 ms retry period.
Sense resistor open
Dim light from LED
Yes
High differential sense voltage causes IC to shut off switching,
entering into Hiccup mode with about 1 ms retry period.
Sense resistor shorted
Dim light from LED
Yes
Triggers SW OCP fault, entering into Hiccup mode with about 1 ms
retry period.
LED string open [1]
No light from LED
Yes [1]
LED Strings shorted [2]
Dim light from LED
Yes
Continues switching at minimum TON; regulator will not enter into
Hiccup mode.
Output cap open
Normal light from LED
No
Normal operation (since IC only monitors inductor current)
(Either LED shorted to GND or
Output cap shorted to GND) < 1.5 V
ALT80800 Response
When VIN is below preset FDSET setting, regulator switches at
maximum duty cycle; when VIN is above FDSET setting, enters into
Hiccup mode with 1 ms retry period.
Enter into Hiccup mode with about 1 ms retry period.
Boot capacitor open
Dim light from LED
Yes
IC attempts to switch but can’t fully turn on SW.
Boot capacitor shorted
No light from LED
No
IC detects undervoltage fault across BOOT capacitor and will not
start switching.
TON resistor open
Dim light from LED
Yes
Enter into Hiccup mode with about 1 ms retry period.
TON resistor shorted
Dim light from LED
Yes
Enter into Hiccup mode with about 1 ms retry period.
[1] For LED Open Fault, fault flag will not be asserted when V is below preset mask threshold, ADIM is below 0.264 V or PWM dimming pulse width is
IN
below fault mask timer.
[2] For
LED Short Fault, fault flag will not be asserted when ADIM is below 0.264 V or PWM dimming pulse width is below fault mask timer.
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15
ALT80800
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
CLAMP DIODES FOR LED OPEN/SHORT PROTECTION
Refer to Figure 14. It is recommended to add clamp diode D1 to
provide LED short protection when VIN is above 40 V; if VIN is
below 40 V, D1 is not needed. Diode D2 is needed to clamp the
overshoot from L-C resonance due to LED Open fault when VIN
is above 45 V; when VIN is below 45 V, D2 is not required.
VIN
L1
SW
D2
VOUT
RSENSE
CSH
CLED
D1
Figure 14: Clamp Diode D1 for LED Short Protection when VIN is above 40 V.
Clamp Diode D2 for LED Open Protection when VIN is above 45 V.
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16
ALT80800
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
COMPONENT SELECTIONS
The inductor is often the most critical component in a buck converter. Follow the procedure below to derive the correct parameters for the inductor:
1. Determine the saturation current of the inductor. This can be
done by simply adding 20% to the average LED current:
iSAT ≥ iLED × 1.2.
2. Determine the ripple current amplitude (peak-to-peak value). As
a general rule, ripple current should be kept between 10% and
30% of the average LED current:
0.1 < iRIPPLE(pk-pk) / iLED < 0.3.
3. Calculate the inductance based on the following equations:
L = (VIN – VOUT ) × D × t / iRIPPLE , and
D = VOUT / VIN ,
where
D is the duty cycle, and
t is the period 1/ fSW.
OUTPUT FILTER CAPACITOR
The ALT80800 is designed to operate in current regulation mode.
Therefore it does not require a large output capacitor to stabilize
the output voltage. This results in lower cost and smaller PCB
area. In fact, having a large output capacitor is not recommended.
In most applications, however, it is beneficial to add a small filter
capacitor (around 0.1 μF) across the LED string. This capacitor
serves as a filter to eliminate switching spikes seen by the LED
string. This is very important in reducing EMI noises, and may
also help in ESD testing.
affects the accuracy of LED current, and limits the minimum
current that can be regulated when using ADIM.
• In general, allowing a higher ripple current percentage enables
lower-inductance inductors to be used, which results in smaller
size and lower cost.
• If lower ripple current is required for the LED string, one
solution is to add a small capacitor (such as 1 to 2.2 μF) across
the LED string from LED+ to ground. In this case, the inductor ripple current remains high while the LED ripple current is
greatly reduced.
• The effectiveness of this filter capacitor depends on many factors, such as: switching frequency, inductors used, PCB layout,
LED voltage and current, and so forth.
• The addition of this capacitor introduces a longer delay in LED
current during PWM dimming operation. Therefore the accuracy
of average LED current is reduced at short PWM on-time.
INDUCTOR SELECTION CHART
The chart in the figure below summarizes the relationship
between LED current, switching frequency, and inductor value.
Based on this chart: assuming LED current = 1 A and L = 22 μH,
then minimum fSW = 0.68 MHz in order to keep the ripple current
at 20% or lower. If the switching frequency is lower, then a larger
inductance must be used to meet the same ripple current requirement.
ADDITIONAL NOTES ON RIPPLE CURRENT
• For consistent switching frequency, it is recommended to
choose the inductor and switching frequency to ensure the inductor ripple current percentage is at least 10% over normal operating voltage range (ripple current is lowest at lowest VIN).
• If ripple current is less than 10%, the switching frequency may
jitter due to insufficient ripple voltage across CSH and CSL pins.
However, the average LED current is still regulated.
• For best accuracy in LED current regulation, a low current
ripple of less than 20% is required.
• There is no hard limit on the highest ripple current percentage
allowed. A 40% ripple current is still acceptable, as long as both
the inductor and LEDs can handle the peak current (average current × 1.2 in this case). However, higher ripple current percentage
Figure 15: Minimum switching frequency vs. LED current,
given different inductance used
(VIN = 12 V, VOUT = 6 V, ripple current = 20%)
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17
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
Effects of Output Capacitor on LED Ripple Current
VIN
VIN
L1
iRIPPLE
RSENSE
L1
iRIPPLE
RSENSE
LED+
LED+
iRIPPLE
GND
Without output capacitor:
The same inductor ripple current flows through
sense resistor and LED string.
GND
With a small capacitor across LED string:
Ripple current through LED string is reducted, while
ripple voltage across RSENSE remains high.
Figure 16: Using an Output Filter Capacitor to Reduce
Ripple Current in LED String
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18
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
APPLICATION CIRCUIT DIAGRAMS
VIN
33 µF
+
ALT80800
4.7 µF
VIN
RON
TON
178 kΩ
PWM
ADIM
External PWM
dimming signal
VCC
2.2 µF
SW
BOOT
PWM
CSH
ADIM
CSL
VCCIN
RSENSE
0.2 Ω
0.47 µF
LED+
CLED
0.1 µF
VCC
VCC
VIN
L1 33 µH
CBOOT
10 kΩ
FFn
FFn
EN
0.1 µF
187 kΩ
GND
SGND
PGND
FDSET
100 kΩ
VIN
187 kΩ
Figure 17: Application Circuit Example for ALT80800
(LED current = 1 A, 500 kHz)
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19
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
APPLICATION CIRCUIT DIAGRAMS (continued)
Input
Voltage
VIN
5V
BOOT
ADIM
Voltage
CBOOT
L1
SW
SW
ADIM
Rsc1
i_L1
CSH1 CSL1
CLED1
GND
FFn
VOUT
ALT80800
VIN
BOOT
CBOOT
SW
ADIM
SW
GND
L2
Rsc2
i_L2
CSH2 CSL2
CLED2
FFn
ALT80800
Figure 18: Using 2 (or more) ALT80800 in parallel to drive the same LED string.
Total LED current is the sum of currents from each LED driver.
(Note: each LED driver shares the same VIN and ADIM as illustrated).
Allegro MicroSystems, LLC
115 Northeast Cutoff
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1.508.853.5000; www.allegromicro.com
20
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
PACKAGE OUTLINE DRAWINGS
For Reference Only – Not for Tooling Use
(Reference MO-153 ABT)
Dimensions in millimeters. NOT TO SCALE
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
0.65
0.45
8º
0º
5.00 ±0.10
16
16
0.20
0.09
1.70
B
3 NOM 4.40 ±0.10
3.00
6.40 ±0.20
A
6.10
0.60 ±0.15
1.00 REF
1
2
3 NOM
1
0.25 BSC
2
Branded Face
3.00
SEATING PLANE
C
16X
0.10
SEATING
PLANE
C
0.30
0.19
GAUGE PLANE
C
PCB Layout Reference View
1.20 MAX
0.65 BSC
NNNNNNN
YYWW
LLLL
0.15
0.00
A
Terminal #1 mark area
B
Exposed thermal pad (bottom surface); dimensions may vary with device
C
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)
D
1
D Standard Branding Reference View
N = Device part number
= Supplier emblem
Y = Last two digits of year of manufacture
W = Week of manufacture
L = Characters 5-8 of lot number
Branding scale and appearance at supplier discretion
Package LP, 16-Pin TSSOP with Exposed Thermal Pad
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
21
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
Revision History
Number
Date
Description
–
December 7, 2017
Initial release
Copyright ©2017, Allegro MicroSystems, LLC
Allegro MicroSystems, LLC reserves the right to make, from time to time, such departures from the detail specifications as may be required to
permit 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 any devices or systems, including but not limited to life support devices or systems, in which a failure of
Allegro’s product can reasonably be expected to cause bodily harm.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC 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
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22
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