DESCRIPTION
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.
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 pulse-
width 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.
ALT80800-DS, Rev. 1
MCO-0000344
FEATURES AND BENEFITS
• 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
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
PACKAGE:
Not to scale
ALT80800
Figure 1: ALT80800 Typical Application Circuit
APPLICATIONS:
Automotive lighting
• Daytime running lights
• Front and rear fog lights
•Turn/stoplights
• Map light
• Dimmable interior lights
16-Pin eTSSOP (suffix LP)
November 13, 2018
LED+
GND
VIN
CIN
PWM
L1
GND
ADIM
VCC
FFn
External PWM
dimming signal
External analog
dimming signal
FDSET
BOOT
CSH
CSL
SGND
EN
VCCIN
TON
PWM
ALT80800
ADIM
VCC
FFn
SW
VIN
RON
CBIAS
CBOOT
CLED
RSENSE
PGND
VIN
GND
VCC
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
2
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
SELECTION GUIDE
Part Number Package Packing
ALT80800KLPATR 16-pin TSSOP with exposed thermal pad 4000 pieces per 13-inch reel
ABSOLUTE MAXIMUM RATINGS
Characteristic Symbol Notes Rating Unit
Supply Voltage VIN, VVCCIN –0.3 to 60 V
Bootstrap Drive Voltage VBOOT –0.3 to VIN + 8 V
Switching Voltage VSW
Continuous –0.3 to VIN + 0.3 V
Pulsed, t < 50 ns –1 to VIN + 3 V
EN Voltage VEN –0.3 to VIN + 0.3 V
Current Sense Voltages VCSH, VCSL V
Linear Regulator Terminal VCC
–0.3 to 7
V
ADIM pin, TON pin VADIM, VTON V
FDSET Voltages VFDSET V
FFn and PWM Voltages VFFn, VPWM V
Maximum Junction Temperature TJ(max) 150 °C
Storage Temperature Tstg –55 to 150 °C
THERMAL CHARACTERISTICS*: May require derating at maximum conditions; see application section for optimization
Characteristic Symbol Test Conditions* Value Unit
Package Thermal Resistance
(Junction to Ambient) RθJA On 4-layer PCB based on JEDEC standard 34 °C/W
Package Thermal Resistance
(Junction to Pad) RθJP 2 °C/W
*Additional thermal information available on the Allegro™ website.
SPECIFICATIONS
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 20
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
3
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
TSSOP-16 (LP) Pinout Diagram
PGND
PGND
VIN
EN
PWM
FDSET
ADIM
TON
SW
BOOT
VCCIN
FFn
VCC
SGND
CSL
CSH
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
PAD
Terminal List Table
Number Name Function
1, 2 PGND Power ground terminal.
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.
8TON 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 Signal ground terminal.
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.
PINOUT DIAGRAM AND TERMINAL LIST TABLE
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
4
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Figure 2: Functional Block Diagram
LED+
VIN
C
IN
PWM
C
BOOT
R
ON
L1
C
BIAS
RSENSE
SW
BOOT
PGND
CSH
CSL
VIN
TON
PWM
VCC
C
LED
VIN
On-Time
Select
Buck
Converter
Duty Cycle
Control
V
OUT
V
OUT
LDO
Internal 5.0 V bias
V
REF
(0 – 200 mV)
i_LED
reference
ADIM
ADIM
On-Time
VIN
Fault
Handling
FFn
FFn
V
CC
LED
Current
Gate
Driver
Differential
Amp
R
ADJ
FDSET
VCCIN
SGND
Up to 14 mA
external load
REF1
-
+
VCSREG
EN
UVLO
V
IN
LED Open
Fault
LED Short
Fault
V
OUT
Other Faults
17 ms
Boot
Charger
ALT80800
V
CC
C
BIAS
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
5
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
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 Symbol Test Conditions Min. Typ. Max. Unit
Input Supply Voltage VIN 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
VIN Pin Supply Current IIN
VCSH – VCSL = 0.5 V, VEN = VIH_EN,
VPWM = VIH_PWM, RON = 402 kΩ – 5 – mA
VIN Pin Shutdown Current IINSD VEN = VIL_EN – 1 10 µA
Output Current Sense Common Mode
Voltage (measured at CSL pin) [1] VOUT VIN = 55 V, fSW = 500 kHz, iLED = 0.5 A 2.65 – 50 V
Buck Switch Current Limit Threshold ISWLIM 2.5 3.25 4.0 A
Buck High-Side Switch On-Resistance RDSON(HS) VBOOT = VIN + 4.3 V, TJ = 25°C, ISW = 0.5 A – 0.2 0.32 Ω
Buck Low-Side Switch On-Resistance RDSON(LS) TJ = 25°C, ISW = 0.5 A – 0.15 0.24 Ω
BOOT Undervoltage Lockout Threshold VBOOTUV VBOOT to VSW increasing 3.1 3.4 3.7 V
BOOT Undervoltage Lockout Hysteresis VBOTUVHYS VBOOT to VSW decreasing – 750 – mV
Switching Minimum Off-Time tOFFmin VCSH – VCSL = 0 V – 100 125 ns
Switching Minimum On-Time tONmin – 65 90 ns
Selected On-Time tON VIN = 12 V, VOUT = 6 V, RON = 42.2 kΩ 200 – 300 ns
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
REGULATION COMPARATOR AND ERROR AMPLIFIER
Load Current Sense Regulation
Threshold at 100%
[3] VCSREG
VCSH – VCSL decreasing, SW turns on, ADIM
tied to VCC 194 200 206 mV
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
INTERNAL LINEAR REGULATOR
VCC Regulated Output VCC 0 mA < ICC < 14 mA, VVCCIN > 6 V 4.85 5.0 5.15 V
VCC Dropout Voltage VLDO
Measure VVCCIN – VCC: VVCCIN = 4.8 V,
ICC = 14 mA – 0.3 0.55 V
VCC Current Limit iVCCLIM VCC ≥ 4.35 V 20 – – mA
VCC Undervoltage Lockout VCCUVLO Rising 3.65 3.9 4.05 V
VCCUVLOHYS Hysteresis 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
EN INPUT
Maximum IC Turn Off Delay tOFFDelay
Measured while PWM dimming signal applied at
EN keeping low and exceeding tOFFDelay results
in shutdown
10 17 – ms
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…
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
6
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Characteristics Symbol Test Conditions Min. Typ. Max. Unit
ANALOG DIMMING INPUT
Input Voltage for 100% LED Current VADIMH VCSH – VCSL = VCSREG 2.1 – – V
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
FAULT
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
LED Open-Fault Enable Reference VREF1 2.352 2.4 2.448 V
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% –
LED Open Fault Current Hysteresis [1] VCS_OPEN_HYS
VCSREG = 200 mV start falling (PWM duty =
max), VADIM = VCC, VFDSET = VCC
(6 mV)
3%
(12 mV)
6%
(18 mV)
9% –
Fault Deglitch Timer tFDG 35 50 65 µs
Fault Mask Timer 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
FFn Rising Time [1] tRISE
The transition time FFn pin takes from Low
to High – – 10 µs
FFn Falling Time [1] tFALL
The transition time FFn pin takes from High
to Low – – 10 µs
Cool Down Timer for Fault Retry tRETRY – 1 – ms
THERMAL SHUTDOWN
Thermal Shutdown Threshold
[1] TSD 150 165 180 °C
Thermal Shutdown Hysteresis TSDHYS – 25 – °C
[1] Determined by design and characterization. Not production tested.
[2] Guaranteed by design, HS and LS switches are interlocked, as illustrated below:
tdead
tLS_ONmin
SW
Low Side VGS
tdead
tOFFmin
tdead ≈
(t
OFFMIN
– t
LS_ONmin
) / 2
[3] 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.
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
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
7
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
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 bright-
ness. The LED current level is easily programmable by selection
of an external sense resistor, with a value determined as follows:
iLED = VCSREG / RSENSE
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:
CSH
CSL Radj RSENSE
VCSREG
+
VSENSE
–
iLED
iCSH
iCSL
– +
iCSL × Radj
VCSREG = iLED × RSENSE + iCSL × Radj
Therefore
iLED = (VCSREG – iCSL × Radj) / RSENSE
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 bright-
ness calibration, or for thermal foldback protection. See Analog
Dimming section for details.
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.
C
BOOT
SW
BOOT
GND
VIN
Floating
Gate Driver
Integrated
Switch
SW
L
i_L
Rsc
CLED
VOUT
Boot
Charger
ALT80800
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 5: Normal SW waveform with SR conguration
when VIN ≈ VOUT: VIN = 5.4 V, VOUT = 5.14 V
(2 white LEDs)
FUNCTIONAL DESCRIPTION
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
8
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Switching Frequency
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:
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.
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
0 100 200 300 400 500 600 700 800 900 1000
fSW (MHz)
RON (kΩ)
2.4
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
resistor is not necessary if EN is driven from a logic input.
The PWM pin is a logic input pin and is internally pulled up to
VCC through a resistor.
EN pin and PWM pin function as illustrated below:
EN pin 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 send-
ing 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.
In PWM dimming operation and when VIN is above 40 V, a
10kΩresistorisneededtobeinparallelwitha0.047µFoutput
capacitoracrosstheLEDstringtofacilitateBOOTchargingdur-
ing PWM dimming OFF period.
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
9
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
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 fre-
quency) 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 deter-
mined 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 on-
time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.
ANALOG DIMMING
In addition to PWM dimming, the ALT80800 also provides an
analog dimming feature. When V
ADIM is over 2.0 V, the LED cur-
rent is at 100% level (as defined by the SENSE resistor). When
V
ADIM is below 2 V, the LED current decreases linearly down to
20% at V
ADIM = 0.4 V. This is shown in the following figure:
ADIM pin
voltage
2 V
200 mV
±6 mV
(100%)
0
0.4 V
40 mV
100 mV
1 V
VCSREG
Figure 7: ADIM Pin Voltage Controls SENSE Reference
Voltage (hence LED current)
It is possible to pull ADIM pin below 0.4 V to achieve lower
than 20% analog dimming. However, if the average LED cur-
rent 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:
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.
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.
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:
NTC
R1
R
P
R
S
VCC
ADIM
Figure 8: Using an External NTC Thermistor
to Implement Thermal Foldback
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.
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
10
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
OUTPUT VOLTAGE AND DUTY CYCLE
The figure below provides simplified equations for approximat-
ing 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.
VIN
VOUT
SW
GND
MOS
CIN
L
iLRSENSE
VSW
iL
t
t
VIN
0
tON
Period, t
–VD
tOFF
iRIPPLE
D
Figure 9: Simplied Waveforms for a Buck Converter
During SW on-time:
iRIPPLE = (VIN – VOUT) / L × tON = (VIN – VOUT) / L × t × D
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, iAV G is the average current through inductor and equal to
LED current.
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
11
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
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 switch-
ing to regulate, but the current accuracy will suffer and not be
guaranteed.
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.
0
2
4
6
8
10
12
14
16
18
20
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
VOUT (V)
Frequency (MHz)
VOUT(max) (V)
VOUT(min) (V)
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 cur-
rent in regulation.
If the LED string is completely shorted (VOUT = 0 V), the con-
troller 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. Assum-
ing 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.
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
12
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
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 volt-
age 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).
1 ms
Hiccup
Mode
FFn
FFn
VCC
FDSET
SGND
LED Open
LED Short
V
IN
REF1
-
+
Duty
MaxDuty
-
+
TON Resistor Open /Short,
R
SENSE
Open/Short,
Inductor Open/Short,
Overcurrent
1.5 V
+
-
VOUT
25% iLED
+
-
VCSREG
ADIM
0.264 V
-
+
Figure 11a: Simplied Faults Block Diagram
FDSET
PWM
VCSREG
REF1
(or ADIM
@ 264 mV)
25% × iLED
FFn Flag
LED OPEN FAULT LED OPEN FAULT
Figure 11b: LED Open Fault Timing Diagram
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
13
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Table 1: LED Open Fault Truth Table
FDSET ADIM FFnnLED Open Fault Event? PWM FFnn+1
VCSREG < 25% × iLED Max Duty
High High x No Open Fault 1
High High x Yes, Open Fault 0
Low x 1 x x 1
x Low 1 x x 1
Low x 0 Yes, Open Fault x 0
x Low 0 Yes, Open Fault x 0
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 out-
put 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 continu-
ously. 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.
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
14
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
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
Fault
Short
Removed
Short
Fault
Short
Removed
Short
Fault
t
FDG
t
MASK
PWM
FFn
Figure 12a: LED Short Fault Timing Diagram Overview
tMASK
Open
Fault
Open
Removed
Open
Fault
Open
Removed
Open
Fault
tFDG t
MASK
PWM
FFn
SW
~1 ms Hiccup period
~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
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
15
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Case 1: LED Open/Short Event is outside
Mask Timer at PWM = H
Case 2: LED Open/Short Event is within
Mask Timer at PWM = H
Mask Timer
Fault Event
Deglitch
Timer
PWM
Fault
Event
Fault Flag
LED Open/Short
No LED Fault
Mask Timer
Fault Event
Mask Timer
PWM
Fault
Event
Fault Flag
LED Open/Short
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
Fault Event
PWM
Fault Flag
Mask Timer
Fault
Event
LED Open/Short
Mask Timer
No LED Fault
Mask Timer
Fault Event
PWM
Fault Flag
Fault
Event
Mask Timer
LED Open/Short
Case 5: LED Open/Short Removed at PWM = L
a) LED Short Removed at PWM = L b) LED Open Removed at PWM = L
Mask Timer
Fault Removed
PWM
Fault Flag
Short
Event
Mask Timer
No LED Short
Mask Timer
Fault Removed
Fault Flag
Open
Event
Current to
regulation timer: *
PWM
No LED Open
Case 6: LED Open/Short Removed outside Mask Timer at PWM = H
a) LED Short Removed outside
Mask Timer at PWM = H
b) LED Open Removed outside
Mask Timer at PWM = H
Mask Timer
Fault Removed
PWM
Fault Flag
Short
Event
No LED Short
LED Short
Mask Timer
Fault Removed
Current to
regulation timer *
PWM
Fault Flag
Open
Event
LED Open
Hiccup period:
~1 ms
No LED Open
Case 7: LED Open/Short Removed within Mask Timer at PWM = H
a) LED Short Removed within
Mask Timer at PWM = H
b) LED Open Removed within
Mask Timer at PWM=H
Mask Timer
Fault Removed
PWM
Fault Flag
Short
Event
No LED Short
LED Short
Mask Timer
Fault Removed
Current to
regulation Timer *
PWM
Fault Flag
Open
Event
LED Open
No LED Open
* Current to regulation timer is 256 switching cycles.
ThebasictimingconfigurationsaredetailedbelowforLEDOpen/Shortfaults:
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
16
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
SYSTEM FAILURE DETECTION AND PROTECTION DEMONSTRATION
IC-Level Failure Modes
Protected against
- Any pin open
- Any pin shorted to ground
- Adjacent pin-to-pin short
C4 open
or short
R1 = open
or short
L1 = open
or short
C5 = open
or short
R
SENSE
open
or short
C3 open
or short
LED string open
or short to GND
C1,C2 = open or short
System-Level Failure Modes
Protected against open/short fault for
all external components, including:
- LED string
- Sense resistor
- Inductor
- Input/output caps, etc.
V
IN
LED+
GND
EN
GND
SW
BOOT
GND
CSH
CSL
VIN
TON
EN
PWM
VCC
ALT80800
PWM
C1 C2
R1
C5
C3
C4
L1
R
SENSE
SGND
PGND
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? ALT80800 Response
Inductor open Dim light from LED Yes [1] 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.
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 Ye s [1] Enter into Hiccup mode with about 1 ms retry period.
LED Strings shorted [2]
(Either LED shorted to GND or
Output cap shorted to GND) < 1.5 V
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)
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 VIN is below preset mask threshold, ADIM is below 0.264 V or PWM dimming pulse width is
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.
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
17
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
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.
L1 RSENSE
CSH
D2
SW
VIN
D1
CLED
VOUT
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.
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
18
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
COMPONENT SELECTIONS
The inductor is often the most critical component in a buck con-
verter. Follow the procedure below to derive the correct param-
eters 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.
In PWM dimming operation and when VIN is above 40 V, it is
suggested to use a 0.047 µF output capacitor, as described in
Enable and Dimming section.
ADDITIONAL NOTES ON RIPPLE CURRENT
• For consistent switching frequency, it is recommended to
choose the inductor and switching frequency to ensure the induc-
tor ripple current percentage is at least 10% over normal operat-
ing 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 cur-
rent × 1.2 in this case). However, higher ripple current percentage
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 induc-
tor ripple current remains high while the LED ripple current is
greatly reduced.
• The effectiveness of this filter capacitor depends on many fac-
tors,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 require-
ment.
Figure 15: Minimum switching frequency vs. LED current,
given di󰀨erent inductance used
(VIN = 12 V, VOUT = 6 V, ripple current = 20%)
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
19
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
VIN
GND
L1
RSENSE
LED+
iRIPPLE
VIN
GND
L1
RSENSE
LED+
iRIPPLE
iRIPPLE
Without output capacitor:
The same inductor ripple current flows through
sense resistor and LED string.
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
E󰀨ects of Output Capacitor on LED Ripple Current
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
20
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
APPLICATION CIRCUIT DIAGRAMS
Figure 17: Application Circuit Example for ALT80800
(LED current = 1 A, 500 kHz)
LED+
V
IN
PWM
C
BOOT
R
ON
L1
2.2 µF
R
SENSE
ADIM C
LED
External PWM
dimming signal
BOOT
FFn
CSH
CSL
TON
PWM
ADIM
VCC
ALT80800
GND
SW
VIN
VCCIN
EN
V
IN
FDSET
V
IN
FFn
0.2 Ω
178 kΩ
33 µH
0.47 µF
0.1 µF
187 kΩ
187 kΩ
100 kΩ
10 kΩ
0.1 µF
+
33 µF 4.7 µF
SGND
PGND
V
CC
V
CC
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
21
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
APPLICATION CIRCUIT DIAGRAMS (continued)
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).
C
BOOT
SW
BOOT
GND
VIN
SW
L2
i_L2
Rsc2
C
LED2
V
OUT
C
BOOT
SW
BOOT
GND
VIN
SW
L1
i_L1
Rsc1
C
LED1
ADIM
ADIM
Input
Voltage
ADIM
Voltage
CSH1 CSL1
CSH2 CSL2
ALT80800
ALT80800
FFn
FFn
5 V
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
22
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Package LP, 16-Pin TSSOP with Exposed Thermal Pad
A
1.20 MAX
0.15
0.00
0.30
0.19
0.20
0.09
8º
0º
0.60 ±0.15
1.00 REF
C
SEATING
PLANE
C0.10
16X
0.65 BSC
0.25 BSC
21
16
5.00 ±0.10
4.40 ±0.10 6.40 ±0.20
GAUGE PLANE
SEATING PLANE
A
B
B
C
D
Exposed thermal pad (bottom surface); dimensions may vary with device
6.10
0.65
0.45
1.70
3.00
3.00
16
21
1
C
D
Branded Face
3 NOM
3 NOM
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
PCB Layout Reference View
Terminal #1 mark area
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)
Branding scale and appearance at supplier discretion
Standard Branding Reference View
YYWW
NNNNNNN
LLLL
= Device part number
= Supplier emblem
= Last two digits of year of manufacture
= Week of manufacture
= Characters 5-8 of lot number
N
Y
W
L
PACKAGE OUTLINE DRAWINGS
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
23
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
For the latest version of this document, visit our website:
www.allegromicro.com
Revision History
Number Date Description
– December 7, 2017 Initial release
1 November 13, 2018 Updated Enable and Dimming section (page 8) and Output Filter Capacitor section (page 18).
Copyright ©2018, Allegro MicroSystems, LLC
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permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that
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