Advances in WLED/RGB LED Drivers

Application Information
Advances in WLED/RGB LED Drivers
Abstract
The increasing use of LEDs for backlighting TFT LCD panels
in automotive navigation/infotainment, computer monitors,
LCD TVs, and notebook computer screens, has generated
a demand for optimization of power regulation techniques
for those LEDs. The potential savings in system design and
increase in overall performance can have a growing impact
on customer acceptance as these applications spread across
a wide variety of market segments such as: Automotive,
Consumer, Industrial, Medical, and others. This application
note discusses an important trend in the successful implementation of these LED arrays—the increasing integration
of control functionality in sophisticated optimized control
devices. The Allegro™ A8501 LED driver is examined as
an example. It offers 2 MHz, 4 channel × 100 mA WLED/
RGB LED driving, with add-on features such as Output
Disconnect, and integrating a boost converter, 40 V DMOS
switch with load dump (for automotive) and overvoltage
protection (OVP).
Integration Level
An important consideration for the rapid development of
complex power management designs is the integration of
multiple features in the control electronics. Not only does this
save in PCB footprint area and peripheral parts matching and
qualification, but it also allows the extensive reuse of these
critical components in modular design elements, improving
the designs of derivative applications.
The Allegro A8501 demonstrates superior integration of
an extensive set of complementary features. The primary
features of interest include the following:
▪ 600 kHz to 2.2 MHz switching frequency—ability to operate above the AM band in Automotive applications
▪ Internal bias supply for single-supply operation (VIN = 8 to 21 V)
▪ Ability to ride through transient input voltages as low as 6.8 V
and as high as 40 V
▪ Boost converter with integrated 40 V DMOS switch and
OVP–load-dump protection
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Figure 1. The A8501 is provided in a thin industrystandard 28-pin TSSOP package, with an exposed
pad for enhanced thermal dissipation.
▪ 3.5 μA shutdown current—limits battery drain
▪ Active current sharing between LED strings for
0.8% current matching and 0.7% accuracy
▪ Drive up to 9 series LEDs in 4 parallel strings, 36 LEDs
maximum (Vf = 3.5 V, If = 100 mA)
▪ LED sinks rated for 100 mA each (400 mA total)
▪ PWM dimming with LED PWM duty cycle control
▪ 4000:1 dimming range
▪ Extensive fault mode protection schemes:
▫ Shorted LED protection against misconnected loads—
with true output disconnect
▫ Open LED disconnect protects against LED failures
▫ External thermistor sensing to limit LED temperature
▫ Output overvoltage protection (OVP): 19.5 V default
can be adjusted as high as 38 V
▫ Open Schottky and open OVP resistor protection
against external component failure
▫ Input under- and overvoltage protection (UVLO and
OVLO) against VIN variation
▫ Boost current limit, output short circuit limit, overtemperature protection (OTP), and soft start
through an integrated output disconnect switch. An optional
external thermistor can be used to limit LED current based on
panel temperature.
Introduction
The Allegro A8501 is a multioutput WLED/RGB driver for
backlighting medium-size displays. It integrates a boost converter
and four 100 mA current sinks. LED channels can be tied
together for up to 400 mA sink capability. It can work from a
single power supply of 8 to 21 V and withstand transients as
low as 6.8 V and up to 40 V. The boost converter is a constant
frequency, current-mode converter.
The device is supplied in a surface mount, 28-pin TSSOP
package (suffix LP), with exposed thermal pad for enhanced
thermal dissipation. It is lead (Pb) free, with a leadframe plating
choice of 100% matte-tin (suffix T) or tin-bismuth (suffix B).
Operating frequency can be set to 2 MHz in order to avoid
interference with the AM radio band in Automotive applications.
The integrated boost DMOS switch is rated for 40 V at 3.6 A.
PWM dimming allows LED currents to be controlled at up to a
1000:1 ratio. Additional 4:1 dimming can be achieved by using
the DIM pin.
Applications include:
▪ GPS navigation systems
▪ Automotive infotainment
▪ Back-up camera displays
▪ Cluster backlighting
▪ Portable DVD players
▪ Industrial LCD displays
The device provides protection against output connector shorts
SW SW SW
VIN
BIAS
Regulator
Bias Supply
CAP
Overvoltage
Comparators
Internal
Supply
FSET
OVP
Charge
Pump
Boost
OUT
Overcurrent
Comparators
OSC
+
–
PGND
Feedback
Control
COMP
SEL1
Device
Control
SEL2
EN
Current Sinks
Open LED Detect
and Disconnect
OVP Fault
2.46 V
LED2
Shorted LED Detect
100 kΩ
VTO
LED1
÷2
VTI
1.23 V
Minimum
Select
LED Current
Reference
LED3
÷4
LED4
References
+
–
100 kΩ
ISET
AGND
PGND
PGND
PGND
LGND
DGND
DIM
PAD
Figure 2. Functional block diagram of the device.
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Power Limiting at Start-up
Many of the new applications, such as notebook computers, are
often operated in a battery-powered mode. Even in the case of
devices, such as televisions that are intended to be operated from
the utility mains, environment-friendly standards are driving
designers to reduce power consumption wherever possible. In the
A8501, these requirements are met by a combination of soft start
and compensation techniques.
D1
VBATT
VOUT
SW SW SW
COUT
OVP
At startup, the output capacitor, COUT in figure 3, is discharged
and the device enters soft start. The boost current is limited to
0.6 A and all active LEDx pins sink 1/20 of the set current until all
the enabled LEDx pins reach 0.75 V. When the device comes out
of soft start, the boost current and the LEDx pin currents are set
to normal.
Figure 3. The output capacitor, COUT, controls the soft start threshold.
The output capacitor charges to voltage required to supply full
LEDx currents within a few cycles. Once VOUT reaches the
required level, LEDx current toggles between 0 and 100% in
response to PWM signals. Soft start behavior on evaluation
boards is shown in figure 4.
A
C1
C2
BC
D
E
F
VBAT
Symbol
C1
C2
C3
C4
t
IBAT
VOUT
Parameter
VBAT
IBAT
VOUT
IOUT
time
Units/Division
10 V
500 mA
20 V
500 mA
5 ms
C3
C4
IOUT
A
BC
D
t
E
F
A.
VBAT voltage slowly increased with EN held high.
A–B. Input bulk capacitor CBAT and boost output capacitor COUT are charged to
VUVLO .
B.
VBAT reaches VUVLO, and enables device through soft start.
B–C. During soft start period, boost switch peak current is limited to 600 mA and
LED current to 1/20 of desired level. Narrow current spike at B is due to
parasitic capacitance from OUT to ground and CBIAS. COMP pin is help low
during soft start.
D.
After VOUT reaches a level such that all LED pins > 0.75 V, the device
comes out of soft start.
C–E. After initial rise of VOUT , the capacitor CCOMP starts charging slowly (CCOMP
not shown).
E.
VCOMP reaches desired level for stable operation.
F.
Driver device and LEDs reach thermal steady state.
Figure 4. The A8501 has options for controlling start-up. One method is to use soft start controlled by a rising
VBAT . In this example, VBAT = 12 V, IOUT = 400 mA. 4 channels are enabled, each with 6 series LEDs.
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Power Management in Normal Operation
While power regulation at start-up is important, continuing the
control seamlessly into normal operating modes is critical. The
A8501 uses an innovative power management scheme that regulates based on current. The maximum LED current through the
LEDs, referred as the 100% current, can be set externally. This is
done by connecting a resistor, RISET, between the ISET pin and
analog (power) ground.
While the device can provide up to 100 mA per channel, the
100% current level is set to a reference current level, IISET ,
according to the following formula:
IISET = 1.235 / RISET
where IISET is in mA and RISET is in kΩ.
This current is multiplied internally with a gain of 960, and mirrored on all enabled LED pins. The function is shown in figure 5.
LED Current versus 1/ RISET
100
100
90
90
80
80
70
70
60
60
ILED (mA)
ILED (mA)
LED Current versus RISET
50
40
50
40
30
30
20
20
10
10
0
0
0
10
20
30
40
50
60
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
1/RISET (RISET in kΩ)
RISET (kΩ)
Figure 5. Characterization of reference current ISET. The current through the LEDs is controlled by the value
of the resistor, RISET, connected to the ISET pin.
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Dimming LED Output in Normal Operation
From a customer acceptance standpoint, the ability to control
light output levels has an immediate impact on satisfaction. Backlighting levels must be adjustable to ambient light conditions and
to the displayed contents, in order to adapt to user viewing capabilities and preferences. In addition, power-conservation mode
when under battery power is essential for extended portable use.
Such diverse requirements can be optimized by providing different approaches to the dimming function. With the A8501, the
LED current can be reduced from the 100% current level by three
alternative dimming methods:
• PWM dimming using the EN pin. PWM dimming is performed
by applying an external PWM signal on the EN pin. When
the EN pin is pulled high, the device turns on and all enabled
LEDs sink 100% current. For optimal accuracy, the external
PWM signal should be in the range 100 to 300 Hz. There is
a slight delay between the PWM signal and the LED current,
which causes an offset. To compensate for the offset, a small
turn-on delay should be added to the PWM signal. When EN is
pulled low, the boost converter and LED sinks are turned off.
The compensation (COMP) pin is floated, and critical internal circuits are kept active. If EN is pulled low for more than
tPWML , the device enters shutdown mode and clears all internal
fault registers. As an example, for a 2 MHz clock, the maximum
PWM low period while avoiding shutdown is 65 ms. The PWM
dimming function is illustrated in figure 6.
• Analog dimming using the DIM pin. When the DIM pin is
pulled low, the LED sinks draw 100 % current; when the pin is
pulled high, the LED current level drops to 25%.
• Analog dimming using the VTI pin. External DC voltage can
be applied to the VTI pin to control LED current. LED current
varies as a function of voltage on the VTI pin.
100
90
80
ILED (mA)
70
60
50
40
30
PWM
100 Hz
200 Hz
20
10
0
0
20
40
60
PWM Duty Cycle (%)
80
100
Figure 6. LED current versus PWM duty cycle. PWM is one of
the three methods available in the device for LED dimming.
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Protection Functions
In previous sections of this application note, the essential function of power level management was described, both for powering-up and for normal operation. The industry-leading driver
ICs, however, also offer circuit management functions. In this
category, the most important are the system protection functions.
The A8501 offers several useful protection features, described in
this section.
LED Open Detect When any LED string opens, the boost
circuit increases the output voltage until it reaches the overvolt-
age protection level. The OVP event causes any LED string that
is not in regulation to be locked-out from regulating the loop. By
removing the open LED from controlling the boost, the output
voltage returns to normal operating voltage. This behavior is
shown in figure 7.
Every OVP event retests all LED strings. An EN low signal does
not reset the LED string regulation lock unless it shuts down the
device (exceeds tPWML). The locked-out LED pins always attempt
to sink desired current regardless of lock-out state.
VBAT
C1
Symbol
C1
C2
C3
C4
t
VOUT
C2
VLED1
Parameter
VBAT
VOUT
VLED1
IOUT
time
Units/Division
10 V
20 V
1V
500 mA
100 μs
LED string #1 disconnected. VOUT
increases to OVP level, and LED string
#1 is removed from regulation. The rest
of the LED strings continue to function
normally.
C3
IOUT
C4
t
VBAT
C1
VOUT
C2
Symbol
C1
C2
C3
C4
t
VLED1
Parameter
VBAT
VOUT
VLED1
IOUT
time
Units/Division
10 V
20 V
1V
500 mA
100 μs
C3
All four LED strings disconnected
simultaneously. VOUT increases to
OVP level, and all LED strings are
removed from regulation.
IOUT
C4
t
Figure 7. Output LED open protection. VBAT = 12 V, ILED = 100 mA per LED string, EN = high.
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LED Short Detect Any LED pin that has a voltage exceeding VLEDSC will force the device to disable the boost circuit and
LEDx outputs until EN shuts down the device (EN low exceeds
tPWML). This protects the LEDx pins from potentially hazardous voltages when multiple LEDs are shorted in one string. This
function is exemplified in figure 8.
VCAP
Symbol
C1
C2
C3
t
VOUT
IOUT
C1
Parameter
IOUT
VCAP
VOUT
time
Units/Division
200 mA
5V
5V
1 μs
(LED Short Detect activated,
causing a latched shutdown)
C2
C3
t
Figure 8. Output LED open protection. This example shows a VOUT to LED1 short. VBAT = 12 V,
ILED = 100 mA per LED string, EN = high; 4 channels enabled, 8 series LEDs each.
Overvoltage Protection The device has overvoltage protection (OVP) and open Schottky diode protection.
The OVP has a default level of 19.5 V and can be increased up to
38 V by the selection of an external resistor, as shown in figure 9.
When the current though OVP pin exceeds 200 μA, the OVP
comparator goes low. When VOUT falls and current through the
OVP pin drops below 165 μA, the OVP is released, as shown in
figures 10 and 11.
D1
VBATT
VOUT
SW SW SW
COUT
ROVP
Latch
–
+
1.23 V
OVP
18 V
–
+
OVP
Disable
1.23 V
Figure 9. Overvoltage protection (OVP) circuit.
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VOVP
ILED
C2
C1
t
Symbol
C1
C2
t
Parameter
VOVP
ILED
time
Units/Division
10 V
50 mA
100 μs
Figure 10. Output overvoltage protection (OVP) operation.
VOUT
Symbol
C1
C2
C3
t
IOUT
Parameter
IOUT
VSW
VOUT
time
Units/Division
200 mA
10 V
5V
20 μs
VSW
C1
(Secondary OVP activated,
causing a latched shutdown)
C2
C3
t
Figure 11. Open Schottky diode (D1 in figure 9) disconnect.
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and also some output capacitance (such as ESD capacitors) is
present, a clamping diode on the output must be used. This diode
will prevent the output from momentarily going negative during
a short circuit condition. The diode must be chosen such that its
reverse breakdown voltage is higher than normal operating voltage and its reverse current leakage is small.
Overcurrent Protection The boost switch is protected with
pulse-by-pulse current limiting at 3.6 A. The output disconnect
switch protects against output overcurrent. At 1 A typical, the
A8501 disables. This process is detailed in figure 12.
In some instances, when the LEDs are connected by long wires
A
VCAP
VOUT
B
C
D
E
F
G
30 V
t
5V
VCOMP
t
1A
IOUT
t
30 V
VSW
t
5V
VEN
t
A
B
A. Overcurrent on disconnect switch is detected and disconnect
switch latches off. Boost is turned off when >3 V is detected
across the disconnect switch. LEDs stop sinking current
because there is insufficient voltage across them.
B. COMP pin reaches lockout level. LEDs are internally turned
off and the COMP pin is discharged.
C. COMP pin reaches ground voltage, LEDs are internally turned
on, in soft start mode, and boost is put into soft start mode.
Boost and LEDs remain off because VOUT is still at ground
C
D
E
F
G
potential due to the disconnect switch being latched off.
D. User turns off EN.
E. The device shuts down when EN is off for more than 131,072
clock cycles. If any other fault conditions were present prior
to shutdown, such as: open LED, TSD, shorted LED, or
secondary OVP, these are now cleared and the part is ready
to be re-enabled.
F. User re-enables operation. Device enters soft start mode.
G. Soft start mode finished.
Figure 12. Disconnect switch overcurrent fault timing diagram.
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Automotive Input Voltage Transient Concerns A major
concern in automotive applications is input voltage transients.
These fast transients can cause LED backlighting to flicker.
The dimming of the LED during an input voltage transient can
introduce a distraction to the viewer when the ambient light is
low intensity. To address this concern, the system design engineer
can add more output capacitance. This supplies the necessary
energy while the switching converter recovers from the transient
response. This solution is the simplest. However, in most cases
the amount of capacitance required is quite large and physical
space in automotive design is at a premium. The Allegro A8501
can survive such transients with very little capacitance, typically
two 4.7 μF capacitors will suffice even at 1% PWM duty cycle.
Figure 13 provides examples of harsh input voltage transients
while the A8501 is running at various PWM duty cycles at
approximately 270 Hz.
VIN
VOUT
C1,C2
IOUT
Symbol
Parameter
Units/Division
C1
C2
C3
t
VIN
VOUT
IOUT
time
5V
10 V
100 mA
1 ms
A. Falling edge at 10% duty cycle.
C3
t
VIN
VOUT
C1,C2
IOUT
Symbol
Parameter
Units/Division
C1
C2
C3
t
VIN
VOUT
IOUT
time
5V
10 V
100 mA
1 ms
B. Falling edge at 1% duty cycle.
C3
t
VOUT
VIN
C1,C2
IOUT
Symbol
Parameter
Units/Division
C1
C2
C3
t
VIN
VOUT
IOUT
time
5V
10 V
100 mA
1 ms
C. Rising edge at 1% duty cycle.
C3
t
Figure 13. A8501 response to harsh supply voltage transients. These examples demonstrate that the
LED current, IOUT (green trace), varies very little when the transition occurs.
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Thermal Derating Thermal derating can be achieved by connecting an NTC thermistor between VTI and ground, as shown in
figure 14. When the device is enabled and VTI > 1.1 V, 100% current for the LEDs is controlled by the ISET and DIM pins. This is
represented by the solid blue curves in figure 15. When VTI falls
below 1.1 V, VISET starts to follow VTI , resulting in ILEDX varying
proportionately with VTI represented by the overlap of the dotted
and solid curves.
The proportion of ILED to VTI , when LED current is controlled
through the VTI pin, is calculated as:
IILEDx = 960 × VTI / RISET
where ILEDx is the LEDx pin current in mA, and RISET is in kΩ.
There is a hysteresis built into the VTI pin circuit, so while VTI
is decreasing, there is a delay before proportional change begins
if VTI pin voltage starts above 1.1 V, as shown by the solid blue
curves in figure 15. When VTI starts below 1.1 V, or falls below
1.1 V during operation and then starts increasing again VISET will
follow VTI until the voltage reaches 1.23 V as shown by the redand-white dotted curves in figure 15.
ILED versus VTI
at TA = 125°C
100
90
80
ILED (mA)
70
60
50
40
VTI Decreasing
30
VTI Increasing
20
10
0
0
0.2
0.4
0.6
0.8
V TI (V)
1
1.2
1.4
I LED versus VTI
at TA = 25°C
100
90
80
RVC
NTC
VTI
2.46 V
÷2
1.23 V
Minimum
Select
–t°
+
LED Current
Reference
ILED (mA)
70
VTO
–
60
50
40
VTI Decreasing
30
VTI Increasing
20
10
ISET
0
RISET
0
0.2
0.4
0.6
0.8
1
1.2
1.4
V TI (V)
ILED versus VTI
at TA = –40°C
Figure 14. Thermal derating reference circuit.
100
90
80
ILED (mA)
70
60
50
40
VTI Decreasing
30
VTI Increasing
20
10
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
V TI (V)
Figure 15. LEDx current versus VTI .
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Typical Application Layouts
This package is easily designed into various customer applications, which is a particular benefit with advanced, multi-featured
products such as the A8501 that can be configured in many ways
to adapt to different application requirements. Some examples are
VBAT
8 to 21 V
CBAT
4.7 μF
50 V
D1
L1
10 μH
CIN
VIN
ROVP
SW SW SW
FSET
BIAS
SEL1
VTO
RFSET
25.5 kΩ
VBAT
8 to 21 V
L1
10 μH
SW SW SW
OVP CAP
OUT
FSET
BIAS
VTI
ISET
AGND PGND PGND PGND LGND DGND
B. Typical circuit for analog dimming with external DC voltage
CC
1 μF
50 V
VBAT
8 to 16 V
CBAT
4.7 μF
50 V
L1
10 μH
CIN
PAD
RFSET
25.5 kΩ
NC
LED1
LED2
LED3
LED4
CCOMP
1 μF
10 V
CBIAS
0.1 μF
10 V
SW SW SW
DIM
BIAS
SEL2
VTO
RISET
24.3 kΩ
C. Typical circuit with ESD capacitors across LEDs (CPx ≤10 nF), with
thermal derating
OVP
FSET
PAD
COUT
4.7 μF
50 V
CAP
OUT
COMP
VTI
RISET
24.3 kΩ
ROVP
RFSET
25.5 kΩ
NC
SEL1
CP1 CP2 CP3 CP4
ISET
AGND PGND PGND PGND LGND DGND
D1
L2
10 μH
EN
DIM
RVC
LED1
LED2
LED3
LED4
VTI
VIN
COMP
SEL1
VTO
RNTC
–t°
COUT
4.7 μF
50 V
ROVP
EN
SEL2
RFSET
25.5 kΩ
NC
PAD
SEL2
RISET
24.3 kΩ
D1
CBIAS
0.1 μF
10 V
FSET
BIAS
VTO
A. Typical circuit for driving 2 LED strings at up to 35 V at 200 mA per LED
string, with thermal derating
CCOMP
1 μF
10 V
OVP CAP
OUT
DIM
DAC
ISET
AGND PGND PGND PGND LGND DGND
VIN
SW SW SW
SEL1
VTI
CIN
COUT
4.7 μF
50 V
ROVP
COMP
CBIAS
0.1 μF
10 V
RISET
12.4 kΩ
CBAT
4.7 μF
50 V
L1
10 μH
CIN
CCOMP
1 μF
10 V
LED1
LED2
LED3
LED4
RVC
RNTC
–t°
D1
CBAT
4.7 μF
50 V
VIN
NC
PAD
SEL2
VBAT
8 to 21 V
EN
COMP
DIM
CBIAS
0.1 μF
10 V
The example device is provided in a 28-pin standard TSSOP
package. On the mounting side of the package, an exposed pad
allows direct thermal conduction to the PCB (see figure 17).
OVP CAP
OUT
EN
CCOMP
1 μF
10 V
COUT
4.7 μF
50 V
provided in figure 16.
LED1
LED2
LED3
LED4
ISET
AGND PGND PGND PGND LGND DGND
D. Typical circuit as SEPIC converter (SEPIC converters can provide
output voltage higher or lower than the input voltage; this topology can be
used if the required output voltage level is within application input voltage
range)
Figure 16. Typical application circuits.
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Summary
This application note has examined some of the important aspects
of designs for the LED backlighting arrays that are gaining so
much market share. The important characteristics include power
management, fault management, and additional value-added features that can be incorporated into the newer thermally-enhanced
packages.
The A8501 is a multioutput WLED/RGB driver for medium-size
LCD backlighting. It integrates a current-mode boost converter
with internal power switch and four current sinks. Each individual current sink is capable of 100 mA, and channels can be combined for up to 400 mA total. It can work from a single power
supply of 8 to 21 V, with the ability to ride through transient input
voltages as low as 6.8 V and as high as 40 V, and has extensive
fault mode protection schemes. It is targeted for use in OEM or
aftermarket in-cabin telematics, as well as consumer and industrial LCD display backlighting.
0.45
9.70 ±0.10
28
+0.05
0.15 –0.06
0.65
28
4° ±4
1.65
B
3.00
4.40 ±0.10
6.40 ±0.20
3.00
6.10
0.60 ±0.15
A
1
(1.00)
2
5.00
0.25
28X
SEATING
PLANE
0.10 C
+0.05
0.25 –0.06
0.65
C
SEATING PLANE
GAUGE PLANE
1 2
5.00
C
PCB Layout Reference View
1.20 MAX
0.10 MAX
For reference only
(reference JEDEC MO-153 AET)
Dimensions in millimeters
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
A Terminal #1 mark area
B Exposed thermal pad (bottom surface)
C Reference land pattern layout (reference IPC7351 SOP65P640X120-29CM);
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)
Figure 17. Package outline and reference solder pad layout for the 28-pin
TSSOP package with thermal pad.
Article published in EDN Asia, October 2010. Reprinted with permission.
Portions not copyrighted by EDN, Copyright ©2010-2013 Allegro MicroSystems, LLC
The information contained in this document does not constitute any representation, warranty, assurance, guaranty, or inducement by Allegro to the
customer with respect to the subject matter of this document. The information being provided does not guarantee that a process based on this information will be reliable, or that Allegro has explored all of the possible failure modes. It is the customer’s responsibility to do sufficient qualification
testing of the final product to insure that it is reliable and meets all design requirements.
296074-AN
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
13
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