LM3492HCMHX/NOPB [TI]
具有升压转换器和电流调节器的高对比度 2 通道独立可调光 LED 驱动器 | PWP | 20 | -40 to 125;
| 型号： | LM3492HCMHX/NOPB |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | 具有升压转换器和电流调节器的高对比度 2 通道独立可调光 LED 驱动器 | PWP | 20 | -40 to 125 升压转换器 驱动 光电二极管 接口集成电路 显示驱动器 调节器 驱动程序和接口 |
| 文件： | 总17页 (文件大小：317K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
March 28, 2012
LM3492HC/LM3492HCQ
Two-Channel Individual Dimmable LED Driver with Boost
Converter and Fast Current Regulator
General Description
Features
The LM3492HC integrates a boost converter and a two-chan-
nel current regulator to implement a high efficient and cost
effective LED driver for driving two individually dimmable LED
strings with a maximum power of 15W and an output voltage
of up to 65V. The boost converter employs a proprietary Pro-
jected-On-Time control method to give a fast transient re-
sponse with no compensation required, and a nearly constant
switching frequency programmable from 200 kHz to 1 MHz.
The application circuit is stable with ceramic capacitors and
produces no audible noise on dimming. The programmable
peak current limit and soft-start features reduce current
surges at startup, and an integrated 190 mΩ, 3.9A N-Channel
MOSFET switch minimizes the solution size. The fast slew
rate current regulator allows high frequency and narrow pulse
width dimming signals to achieve a very high contrast ratio of
10000:1. The LED current is programmable from 50 mA to
250 mA by a single resistor.
Boost Converter:
LM3492HCQ is an Automotive Grade Product that is AEC
Q100 grade 1 qualified
■
Very wide input voltage ranged from 4.5V-65V
■
■
■
■
Programmable soft-start
No loop compensation required
Stable with ceramic and other low ESR capacitors with no
audible noise
Nearly constant switching frequency programmable from
200 kHz to 1 MHz
■
Current Regulators:
Programmable LED current from 50 mA to 250 mA
■
■
■
10000:1 contrast ratio, 300 ns minimum pulse width
Two individual dimmable LED strings up to 65V, total 15W
(typically 28 LEDs @ 150 mA)
To maximize the efficiency, Dynamic Headroom Control
(DHC) automatically adjusts the output voltage to a minimum.
DHC also facilitates a single BOM for different number of LED
in a string, which is required for backlight panels of different
size, thereby reducing overall development time and cost.
The LM3492HC comes with a versatile COMM pin which
serves as a bi-directional I/O pin interfacing with an external
MCU for the following functions: power-good, over-tempera-
ture, IOUT over- and under-voltage indications, switching
frequency tuning, and channel 1 disabling. Other supervisory
functions of the LM3492HC include precise enable, VCC un-
der-voltage lock-out, current regulator over-power protection,
and thermal shutdown protection. The LM3492HC is avail-
able in the thermally enhanced eTSSOP-20 package.
Dynamic Headroom Control maximizes efficiency
■
■
■
Over-Power protection
±3% current accuracy
Supervisory Functions:
Precision enable
■
■
■
■
COMM I/O pin for diagnostic and commands
Thermal shutdown protection
Thermally enhanced eTSSOP-20 package
Applications
Ultra-high contrast ratio 6.5”-10” LCD display backlight up
to 28 LEDs
■
Automotive or marine GPS display
■
© 2012 Texas Instruments Incorporated
301705 SNVS797
www.ti.com
Typical Application
30170528
Connection Diagram
30170502
Top View
20-Lead Plastic eTSSOP (MXA20A)
Ordering Information
Order Number
Package Type
NSC Package
Drawing
Supplied As
Feature
LM3492HCMH
LM3492HCMHX
LM3492HCQMH
73 Units per Anti-Static Tube
2500 Units on Tape and Reel
73 Units per Anti-Static Tube
Exposed Pad
TSSOP-20
MXA20A
AEC-Q100 Grade 1
qualified. Automotive
Grade Production Flow*
LM3492HCQMHX
2500 Units on Tape and Reel
*Automotive Grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, including defect detection methodologies.
Reliability qualification is compliant with the requirements and temperature grades defined in the AEC-Q100 standard. Automotive grade products are identified
with the letter Q. For more information go to http://www.national.com/automotive.
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2
Pin Descriptions
Pin
Name
Description
Application Information
Internally pull-up. Connect to a voltage higher than 1.63V to provide precision
enable for the device.
1
EN
Enable
2
3, 4
5
VIN
SW
Input Supply Voltage
Switch Node
Supply pin to the device. Input range is 4.5V to 65V.
Internally connected to the drain of the integrated MOSFET.
Sense the output voltage for nearly constant switching frequency control.
VOUT
Output Voltage Sense
An external resistor from the VOUT pin to this pin sets the switching
frequency.
6
RT
Frequency Control
The output voltage is connected to this pin through a feedback resistor
divider for output voltage regulation. The dynamic range of this pin is from
1.05V to 2.0V.
7
FB
Output Voltage Feedback
8
9
GND
Analog Ground
Signal Ground
Input of the current regulator of channel 2. The regulated current is
programmable (refer to the IREF pin).
IOUT2
Current Regulator Input of Channel 2
Input of the current regulator of channel 1. The regulated current is
programmable (refer to the IREF pin).
10
11
12
13
14
15
IOUT1
CDHC
IREF
Current Regulator Input of Channel 1
Dynamic Headroom Control
An external capacitor connected to this pin sets the DHC sensitivity. At
startup, a 120 µA internal current source charges an external capacitor to
provide a soft-start function.
Current Setting of the Current
Regulator
An external resistor connected from this pin to ground programs the
regulated current of the current regulator of channels 1 and 2.
This pin is open drain for various indications (power-good, over-temperature,
COMM Bi-directional Logic Communication IOUT over- and under-voltage) and command sending (switching frequency
tuning and channel 1 disabling).
Current regulator ground. Must be connected to the GND pin for normal
operation. The LGND and GND pins are not internally connected.
LGND
Ground of the Current Regulator
Control the on/off of the current regulator of channel 1. This pin is internally
pulled low by a 5 µA current. This pin also serves as a clock signal for latching
input/output data of the COMM pin.
DIM1/CLK Dimming Control of Channel 1
Control the on/off of the current regulator of channel 2. This pin is internally
pulled low by a 5 µA current.
16
17, 18
19
DIM2
PGND
VCC
Dimming Control of Channel 2
Power Ground
Integrated MOSFET ground. Must be connected to the GND pin for normal
operation. The PGND and GND pins are not internally connected.
Nominally regulated to 5.5V. Connect a capacitor of larger than 0.47 µF
between the VCC and GND pins.
LDO Regulator Output
Connect an external resistor from the ILIM pin to the VCC pin reduces peak
current limit. Connect the ILIM pin to the ground to obtain the maximum
current limit.
20
ILIM
DAP
Peak Current Limit Adjust
Exposed Pad
DAP
Thermal connection pad. Connect to a ground plane.
3
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ESD Rating (Note 2)
Human Body Model
Storage Temperature Range
JunctionTemperature (TJ)
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the Texas Instruments Sales Office/
Distributors for availability and specifications.
±2kV
−65°C to +150°C
150°C
VIN, RT, VOUT to GND
SW to GND
SW to GND (Transient)
ILIM to GND
FB to GND
COMM, DIM1, DIM2, to GND
−0.3V to 67V
−0.3V to 67V
−2V (<100 ns)
−0.3V to 1V
−0.3V to 5V
−0.3V to 6V
Operating Ratings (Note 1)
Supply Voltage (VIN)
4.5V to 65V
−40°C to +125°C
32.7°C/W
Operation Temperature Range (TA)
Thermal Resistance (θJA) (Note 3)
Electrical Characteristics Specification with standard type are for TA = TJ = +25°C only; limits in boldface type
apply over the full Operating Junction Temperature (TJ) range. Minimum and Maximum are guaranteed through test, design or
statistical correlation. Typical values represent the most likely parametric norm at TJ = +25°C, and are provided for reference
purposes only. Unless otherwise stated the following conditions apply: VIN = 12V.
Symbol
Parameter
Conditions
Min
Typ Max Units
Start-Up Regulator, VCC
VCC
VCC pin output voltage
CVCC = 0.47 µF, no load
ICC = 2 mA
4.7
4.7
5.5
5.5
6.3
6.3
V
V
V
VCC-UVLO
VCC pin under-voltage lockout threshold VCC increasing
3.56 3.78 4.00
(UVLO)
VCC-UVLO-HYS
VCC pin UVLO hysteresis
IIN operating current
VCC decreasing
No switching, VFB = 0V
VEN = 0V
310
mV
mA
µA
IIN
3.6
30
30
5.2
95
IIN-SD
IVCC
IIN operating current, Device shutdown
VCC pin current limit (Note 4)
VCC = 0V
18
mA
VCC pin output voltage when supplied by
VOUT
VCC-VOUT
VIN = Open, ICC = 1 mA, VOUT = 18V
3.5
4.1
4.7
V
Enable Input
VEN
EN pin input threshold
VEN rising
VEN falling
VEN = 0V
1.55 1.63 1.71
V
VEN-HYS
EN pin threshold hysteresis
Enable Pull-up Current at shutdown
194
2
mV
µA
µA
IEN-SHUT
IEN-OPER
Enable Pull-up Current during operation VEN = 2V
40
Current Regulator
VIREF
IREF pin voltage
VIN = 4.5V to 65V
1.231 1.256 1.281
0.160 0.225 0.290
0.38 0.48 0.58
0.81 0.99 1.17
0.81 1.21 1.44
V
V
VDHC50
VIOUT under DHC at IOUT = 50 mA
VIOUT under DHC at IOUT = 100 mA
VIOUT under DHC at IOUT = 200 mA
VIOUT under DHC at IOUT = 250 mA
RIREF = 25 kΩ
VDHC100
VDHC200
VDHC250
V
RIREF = 12.5 kΩ
V
RIREF = 6.25 kΩ
V
RIREF = 5 kΩ
47.5
46.5
97
50
50
52.5
53.5
103
104
206
208
mA
mA
mA
mA
mA
mA
RIREF = 25 kΩ, VIOUT = VDHC50
RIREF = 25 kΩ, VIOUT = VDHC50
RIREF = 12.5 kΩ, VIOUT = VDHC100
RIREF = 12.5 kΩ, VIOUT = VDHC100
RIREF = 6.25 kΩ, VIOUT = VDHC200
RIREF = 6.25 kΩ, VIOUT = VDHC200
RIREF = 5 kΩ, VIOUT = VDHC250
RIREF = 5 kΩ, VIOUT = VDHC250
VDIM = 0, VIOUT = 65V
Current Output under DHC at VIOUT
VDHC50
=
=
=
=
IOUT50
100
100
200
200
Current Output under DHC at VIOUT
VDHC100
IOUT100
IOUT200
IOUT250
96
194
192
Current Output under DHC at VIOUT
VDHC200
241.3 250 258.8 mA
Current Output under DHC at VIOUT
VDHC250
238
250
262
5
mA
µA
V
IOUTOFF
Leakage at Maximum Work Voltage
Minimum Work Voltage, 50 mA
Minimum Work Voltage, 100 mA
Minimum Work Voltage, 200 mA
VIOUT50-MIN
VIOUT100-MIN
VIOUT200-MIN
0.1
0.2
0.4
0.15
0.35
0.65
RIREF = 25 kΩ, IOUT = 0.98 x IOUT50
RIREF = 12.5 kΩ, IOUT = 0.98 x IOUT100
RIREF = 6.25 kΩ, IOUT = 0.98 x IOUT200
V
V
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Symbol
Parameter
Conditions
Min
Typ Max Units
VIOUT250-MIN
Minimum Work Voltage, 250 mA
0.5
0.82
V
V
V
RIREF = 5 kΩ, IOUT = 0.98 x IOUT250
VDIM-HIGH
DIM Voltage HIGH
DIM Voltage LOW
1.17
VDIM-LOW
0.7
Boost Converter
VCDHC = 1.6V, VFB = 3V, VIOUT = 0V, DIM
= High
60
56
µA
µA
ICDHC-SRC
CDHC pin source current
VCDHC = 1.6V, VFB = 3V, VIOUT = 3V, DIM
= High
ICDHC-SINK
ICDHC-PULLUP
ICL-MAX
CDHC pin sink current
DIM = Low, VCDHC = 2.3V, VFB = 3V
10
200
3.9
500
4.5
nA
A
CDHC pin pull-up current
Integrated MOSFET peak current limit
threshold
3.3
ICL-HALF
Half integrated MOSFET peak current limit
threshold
2.0
A
RILIM = 11 kΩ
RDS(on)
Integrated MOSFET RDS(on)
Power-Good FB pin threshold
FB pin over-voltage protection threshold
FB pin OVP hysteresis
ISW = 500 mA
0.19 0.43
Ω
V
VFBTH-PWRGD
2.25
VFB rising, VCDHC = 4V
2.64 2.76 2.88
V
VFB-OVP
IFB
VFB falling
0.1 0.215 0.323
V
Feedback pin input current
VFB = 3V
1
1460
800
µA
ns
VIN = 12V, VOUT = 65V, RRT = 300 kΩ
VIN = 24V, VOUT = 32.5V, RRT = 300 kΩ
VIN = 12V, VOUT = 65V, RRT = 100 kΩ
VIN = 24V, VOUT = 32.5V, RRT = 100 kΩ
ns
ns
ns
ns
ton
ON timer pulse width
550
350
ton-min-ILIM
ON timer minimum pulse width at current
limit
145
toff
OFF timer pulse width
145
6.7
350
7.8
ns
COMM PIN
COMM goes LOW during VIOUT rising,
other VIOUT = 1.2V
VIOUT-OV
IOUT pin over-voltage threshold
5.6
V
VCOMM-LOW
ILEAK-FAULT
Thermal Protection
TOTM
COMM pin at LOW
5 mA into COMM
VCOMM = 5V
0.7
V
COMM pin Open Leakage
5
µA
Over-temperature indication
TJ rising
TJ falling
TJ rising
135
15
°C
°C
°C
°C
TOTM-HYS
TSD
Over-temperature indication hysteresis
Thermal shutdown temperature
165
20
TSD-HYS
Thermal shutdown temperature hysteresis TJ falling
Note 1: Absolute Maximum Ratings are limits which damage to the device may occur. Operating ratings are conditions under which operation of the device is
intended to be functional. For guaranteed specifications and test conditions, see the electrical characteristics. Thermal shutdown might occur within ambient
operating temperature range as junction temperature rises above TSD level, customer should refer to efficiency data and thermal resistance data to estimate the
junction temperature to ambient temperature delta.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Note 3: The θJA is measured on a 4-layer standard JEDEC thermal test board with 12 vias, no air flow and 1W power dissipation. Thermal shutdown will occur
if the junction temperature exceeds 165°C. The maximum power dissipation is a function of TJ(MAX), θJA and TA. The maximum allowable power dissipation at any
ambient temperature is PD = (TJ(MAX) – TA) /θJA
.
Note 4: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading.
5
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Typical Performance Characteristics
Unless otherwise specified the following conditions apply: TJ = 25°C, VIN = 12V with configuration in typical application circuit for
ILED = 250 mA shown in this datasheet.
Quiescent Current, IIN vs VIN
VCC vs IVCC
30170511
30170513
30170515
30170512
30170514
30170516
VCC vs VIN
Switching Frequency, fSW vs VIN
ILED Regulation vs Temperature
RDS(on) vs Temperature
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Efficiency vs VIN (ILED = 0.25A)
100
ILED Regulation vs VIN (ILED = 0.25A)
1.00
0.75
95
90
85
80
75
70
-40°C
0.50
0.25
25°C
0.00
25°C
-40°C
-0.25
-0.50
-0.75
-1.00
125°C
125°C
20
10
15
20
25
10
15
25
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
30170532
30170533
Power Up (ILED = 0.25A)
Enable Transient (ILED = 0.25A)
30170534
30170535
Steady State Operation (ILED = 0.25A)
LED 50% Dimming
(ILED = 0.25A, Dimming frequency = 200Hz)
30170536
30170537
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1000:1 LED Dimming
(ILED = 0.25A, Dimming frequency = 200Hz)
10000:1 LED Dimming
(ILED = 0.25A, Dimming frequency = 200Hz)
30170539
30170538
Simplified Functional Block Diagram
30170503
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8
(MCUs). The LM3492HC will not latch off and continue to op-
erate in the presence of the indications. Other useful features
include thermal shutdown, VCC under-voltage lock-out, and
precision enable. The LM3492HC is available in the thermally
enhanced eTSSOP-20 package.
Overview
The LM3492HC integrates a boost converter and a two-chan-
nel current regulator to implement a high efficient and cost
effective LED driver for driving two individually dimmable LED
strings with a maximum power of 15W and an output voltage
of up to 65V. The boost converter provides power for the LED
strings, and the current regulator controls the dimming of the
LED strings individually. The LM3492HC integrates an N-
channel MOSFET switch and a two-channel current regulator
in order to minimize the component count and solution size.
LDO Regulator
A 5.5V LDO regulator is integrated in the LM3492HC. For
stability, an external capacitor CVCC of more than 0.47 µF
should be connected between the VCC and GND pins. The
current limit of the LDO is typically 30 mA. It can be used to
pull-up the open-drain COMM pin with an external resistor,
and inject current to the ILIM pin to adjust the current limit of
the integrated MOSFET. When the voltage on the VCC pin
(VCC) is higher than the under-voltage lock-out (UVLO)
threshold of 3.78V, the LM3492HC is enabled and the CDHC
pin sources a current to charge up an external capacitor
CCDHC to provide a soft-start function.
The boost converter of the LM3492HC employs a Projected
On-Time (POT) control method to determine the on-time of
the MOSFET with respect to the input and output voltages
and an external resistor RRT. During the on-period, the boost
inductor is charged up, and the output capacitor is discharged
to provide power to the output. A cycle-by-cycle current limit
(which is 3.9A typically and programmable by an external re-
sistor) is imposed on the MOSFET for protection. After the on-
period, the MOSFET is turned off such that the boost inductor
is discharged. The next on-period is started when the voltage
of the FB pin is dropped below a threshold which is deter-
mined by Dynamic Headroom Control (DHC) and is ranged
from 1.05V to 2.0V (DHC affects the threshold only when the
DIM1 and/or DIM2 pins are high). The boost converter under
POT control can maintain the switching frequency nearly con-
stant so that the switching frequency depends on only RRT
Figure 1. Also, POT control requires no compensation circuit
and gives a fast transient response of the output voltage.
Enable and Disable
To enable the LM3492HC, the voltage on the EN pin (VEN
)
must be higher than an enable threshold of typically 1.63V. If
VEN is lower than 1.43V, the LM3492HC is shutdown. In this
case, the LDO regulator is turned off and the CDHC pin is
internally grounded. The EN pin is internally pulled up. After
enable, the EN pin is pulled up by a 40 µA current source. If
the EN pin is connected to low such that the LM3492HC is
shutdown, the pull-up current is reduced to 2 µA. These take
the advantages that the LM3492HC can effectively avoid
false disabling by noise during operation, and minimize power
consumption during shutdown. The enable threshold is pre-
cise such that it can be used to implement an UVLO function
for the input voltage as shown in Figure 2. The input voltage
can be connected to the EN pin through a resistor divider
consists of REN1 and REN2. This can ensure that the
LM3492HC is operated after the input voltage reaches a min-
imum require value VIN(EN), which can be calculated by the
following equation:
VIN(EN) = 1.63V(1 + REN1/ REN2
)
(1)
A zener diode DEN should be placed between the EN and
GND pins to keep VEN below its absolute maximum caused
by the increase of the input voltage.
30170525
FIGURE 1. Switching Frequency
The two-channel current regulator of the LM3492HC is fast
response so that it can allow very high contrast ratio of
10000:1. The two channels are dimmable individually. Chan-
nel 1 of the current regulator can be disabled by a digital
command send through the COMM pin. In this case, the DIM1
pin can serve only as a clock signal for the data flow of the
COMM pin. The power dissipated by the current regulator is
adaptively minimized by Dynamic Headroom Control in order
to maximize efficiency.
30170504
FIGURE 2. Input Voltage UVLO Implemented by Precision
Enable
Once the EN pin is pulled low, the LM3492HC will perform the
following functions: reset IOUT over- and under-voltage indi-
cations and the corresponding COMM bit pattern, resume the
switching frequency tuning to the normal frequency, and re-
sume channel 1 of the current regulator if it is disabled. Pulling
the EN pin low for a short period of about 200 ns can achieve
The LM3492HC can be applied in numerous applications like
automotive LCD backlight panels. It can operate efficiently for
inputs as high as 65V. Diagnostic functions including power
good indication, over-temperature indication, IOUT over- and
under-voltage indications facilitate the interface of the
LM3492HC application circuit with external micro-processors
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the above functions with nearly no effect on the operation of
the boost converter and the current regulator.
During startup, the voltage of the CDHC pin is rised from 0V
to 2.25V at a speed depends on CCDHC. This makes the volt-
age of the FB pin as well as the output voltage ramps up in a
controlled manner, and effectively a soft-start function is im-
plemented. The soft-start can be programmable by the
Current Limit
The current limit ICL of the integrated MOSFET of the
LM3492HC provides a cycle-by-cycle current limit for protec-
tion. It can be decreased by injecting a small signal current
IILIM into the ILIM pin, and the relationship between ICL and
IILIM is
CCDHC
.
An internal switch grounds the CDHC pin if any of the follow-
ing cases happens: (i) VCC is below the VCC UVLO threshold;
(ii) a thermal shutdown occurs; or (iii) the EN pin is pulled low.
The CDHC pin cannot be connected to the ground externally.
ICL = ICL(MAX) – 4290 IILIM
(2)
where ICL(MAX) is the maximum current limit. Its typical value
is 3.9A. As shown in Figure 3, IILIM can be provided by con-
necting a resistor RILIM from the VCC pin to the ILIM pin. The
typical voltage on the ILIM pin is 0.7V. To obtain the maximum
current limit, connect the ILIM pin to the ground.
Current Regulator
The LM3492HC integrates a two-channel current regulator for
controlling the current of two LED strings. The two LED strings
are dimmable individually by dimming signals applied to the
DIM1 and DIM2 pins for LED strings 1 and 2, which are con-
nected from the VOUT pin to the IOUT1 and IOUT2 pins. The
DIM1 and DIM2 pins are internally pulled low. The lowest
contrast ratio is 10000:1. The finest pulse width of the dim-
ming signal for the DIM1 and DIM2 pins is 300 ns.
The current of an LED string (ILED) is programmable from 50
mA to 250 mA by an external resistor RIREF connecting from
the IREF pin to the ground. The relationship between ILED and
RIREF is shown in Figure 4. The two channels of the current
regulator can work in parallel for only one LED string by con-
necting the IOUT1 and IOUT2 pins together to provide an LED
current of up to 500 mA. In this case, the DIM1 and DIM2 pins
should also be connected together.
30170505
FIGURE 3. Programmable Current Limit
250
200
150
100
50
Thermal Protection
Thermal protection is implemented by an internal thermal
shutdown circuit, which activates at 165°C (typically) to dis-
able the LM3492HC. In this case, the LDO regulator is turned
off and the CDHC pin is internally grounded. Thermal protec-
tion helps prevent catastrophic failures from accidental device
overheating. When the junction temperature of the
LM3492HC falls back below 145°C (typical hysteresis = 20°
C), the LM3492HC resumes normal operation.
Dynamic Headroom Control, Over-
ridding, and Soft-start
0
5
10
15
(kΩ)
20
25
Dynamic headroom control (DHC) is implemented in the
LM3492HC to adjust the output voltage VOUT of the boost
converter in order to reduce the power loss of the current reg-
ulator to maximize efficiency. Let VLED,n and VIOUT,n be the
forward voltage of an LED string connecting to the IOUTn pin
and the voltage of the IOUTn pin, where n is 1, 2 for channels
1, 2 of the current regulator. Since VLED,n is normally decreas-
ing gradually (in terms of minutes) owing to the rise of LED
die temperature during operation, DHC adjust VOUT by ad-
justing a threshold which is reflected on the voltage of the FB
pin with reference to VIOUT,n, which is the difference between
VOUT and VLED,n. The sensitivity of DHC, which is the re-
sponse time on adjusting VOUT, is set by CCDHC. If CCDHC is
R
IREF
30170531
FIGURE 4. ILED vs RIREF
small, VOUT is more sensitive to the variation of VLED,n
.
DHC over-ridding can be implemented by internal pull-up or
external pull-up (by connecting the CDHC and VCC pins with
a resistor, e.g. 10 MΩ). In this case, the voltage of the CDHC
pin will rise over 2.5V, and the voltage of the FB pin will rise
until over-voltage protection is hit. Since the pull-up is weak,
DHC over-ridding will occur only at low contrast ratio (e.g.
<1%).
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30170506
FIGURE 6. Output Voltage Feedback Circuit
Over-Voltage Protection
When VFB is higher than the FB pin over-voltage protection
(OVP) threshold VFB-OVP (typically 2.76V and maximum
2.88V), the on-period of the integrated MOSFET stop imme-
diately, and the MOSFET keeps off until VFB falls back below
below 2.545V (typical hysteresis 0.215V).
30170527
FIGURE 5. Over-power Protection
An alternative method to implement OVP is to directly monitor
VOUT instead of VFB. An external circuit as shown in Figure
7 is required. Current is injected to the ILIM pin to drive the
LM3492HC to the current limit mode once VOUT is higher than
the avalanche voltage of the zener diode DOVP plus 0.7V, the
typical voltage on the ILIM pin. In this case, a maximum limit
If the voltage on the IOUTn (n = 1, 2) pin is over 24V when
channel n is on, the regulated current of channel n will be
reduced linearly if the voltage further increases (as shown in
Figure 5). The regulated current of another channel is not af-
fected. This over-power protection feature avoids damaging
the current regulator owing to the shorting of many LEDs in
one string.
of VOUT is imposed. However, at the maximum limit of VOUT
,
VFB should be higher than 2.25V to avoid affecting the startup
of the LM3492HC.
Output Voltage Feedback
The output voltage is fed back to the FB pin through a feed-
back circuit consists of RFB1, RFB2, and CFB as shown in Figure
6. The value of CFB is recommended to be 10 pF. in order to
help feed the AC component of the output voltage back. The
DC component of the output voltage is fed back by RFB1 and
RFB2. The voltage of the FB pin VFB can be adjusted by DHC.
When VFB reaches VFB-OVP, the maximum output voltage of
the boost converter VOUT(MAX) reaches its maximum, and it is
calculated as follows:
30170530
FIGURE 7. External OVP circuit
VOUT(MAX) = 2.88V (1 + RFB1/ RFB2
)
(3)
Bi-Directional Communication Pin
Under DHC, the output voltage should be maintained at a
nominal voltage but not the maximum. The nominal output
voltage (VOUT(NOM)) is
The COMM pin of the LM3492HC is an open-drain bi-direc-
tional I/O pin for interfacing with an external MCU for the
following functions: power-good indication, over-temperature
indication, IOUT over- and under-voltage indications, switch-
ing frequency tuning, and channel 1 disabling. Except the
power good indication and the over-temperature alert, all data
flow through the COMM pin is serial and is latched by the
falling edge of the signal applying to the DIM1 pin, even when
channel 1 of the current regulator is disabled. If the DIM1 pin
stays only low or only high, either by an external circuit or
letting it open and pull low internally, data flow will not occur.
Figure 8 and Figure 9 show a timing diagram of reading and
writing a bit from and to the LM3492HC through the COMM
pin.
VOUT(NOM) = max (VLED,n + VIOUT,n), n = 1, 2
(4)
where VLED,n is the forward voltage of LED string n and
VIOUT,n is the voltage of the IOUTn pin, where n is 1, 2 for
channels 1, 2 of the current regulator). The minimum value of
VIOUT,n is about 5Ω x ILED. It is recommended that the nominal
voltage of the FB pin (VFB(NOM)) is between 1.05V to 2V.
Hence, the equation relating VOUT(MAX), VOUT(NOM), and VFB
(NOM) is as follows:
VOUT(MAX) = VOUT(NOM) x 2.88V / VFB(NOM)
(5)
The COMM pin should be pull-up by an MCU I/O pin which
has pull-up capability, or an external resistor RCOMM to the
VCC pin. Otherwise, the voltage of the COMM pin will remain
at zero. The rise time of the output signal of the COMM pin
depends on the pull-up power. If the rise time is long
(RCOMM is too large or pull-up power from the connecting MCU
I/O pin is too weak), data may be ready after a longer duration
after the falling edge. In this case, a longer delay between the
falling edge latching and the (input or output) bit is required.
11
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1) can be read from the COMM pin. The LM3492HC will not
latch off and continue to operate in the presence of the IOUT
under-voltage indication.
IOUT OVER-VOLTAGE INDICATION
The LM3492HC gives an IOUTn (n = 1, 2) over-voltage indi-
cation if the voltage of the IOUTn pin when DIMn is high is
higher than a threshold of typically 6.5V, and the condition
lasts for 508 consecutively dimming signals applied on the
DIMn pin. The IOUT over-voltage indication can be regarded
as a short fault of the LED string n except the following 2 cas-
es: i) powering up the LM3492HC at a very low dimming ratio
such that VOUT maintains at a maximum and DHC is not fast
enough to reduce VOUT; ii) under DHC over-ridding. A bit pat-
tern (refer to table 1) can be read from the COMM pin. The
LM3492HC will not latch off and continue to operate in the
presence of the IOUT over-voltage indication.
30170507
FIGURE 8. Read from the COMM Pin
TABLE 1. COMM Bit Patterns
Indication
COMM Bit Pattern
IOUT1 over-voltage
indication
0001
IOUT2 over-voltage
indication
0011
0101
0111
IOUT1 under-voltage
indication
IOUT2 under-voltage
indication
COMM BIT PATTERN
Table 1 summarized all COMM bit patterns of IOUT over- and
under-voltage indications. A bit pattern can be read from the
COMM pin continuously even the condition causing IOUT
over- and under-voltage disappeared. An existing COMM bit
pattern will be clear if one of the following condition occurs:
30170508
FIGURE 9. Write to the COMM Pin
POWER-GOOD INDICATION
i) the LM3492HC is shutdown;
ii) the LM3492HC is disabled by pulling the EN pin low;
iii) the over-temperature indication is appearing.
Upon startup, the COMM pin reads low. The output voltage
of the boost converter of the LM3492HC will rise until the volt-
age on the FB pin (VFB) reaches 2.25V, when the COMM pin
reads high to indicate power-good. The power-good indica-
tion is independent of the signal applied on the DIM1 pin.
To clear the COMM bit pattern without affecting the operation
of the boost converter and the current regulator, a reset signal
which pulls the EN pin low for about 200 ns can be applied.
In this case, the COMM pin will not give bit pattern any more
unless a condition causing IOUT over- or under-voltage ap-
pears and lasts for 508 consecutively dimming signals.
OVER-TEMPERATURE INDICATION
If an external MCU is used to read the COMM bit pattern, it is
recommended that a reset signal should be send to clear the
COMM bit pattern after the bit pattern is read. This can avoid
that the existing COMM bit pattern is overwritten by another
pattern appeared in a later time.
If the junction temperature of the LM3492HC reaches 135°C,
the COMM pin reads low, showing an over-temperature indi-
cation. External MCU should consider to turn off or reduce the
brightness of the LED strings in order to prevent over-tem-
perature. The over-temperature indication is independent of
the signal applied on the DIM1 pin. The COMM pin reads high
if the junction temperature falls below 120°C. The LM3492HC
will not latch off and continue to operate in the presence of
the over-temperature indication.
In case of over-temperature, the COMM pin will be pulled low
to give an over-temperature indication disregard of any exist-
ing COMM bit pattern. After that the over-temperature indi-
cation disappears, the COMM bit pattern appeared before the
over-temperature indication will appear again.
IOUT UNDER-VOLTAGE INDICATION
SWITCHING FREQUENCY TUNING
The LM3492HC gives an IOUTn (n = 1, 2) under-voltage in-
dication if the voltage of the IOUTn pin when DIMn is high is
lower than its minimum required voltage which can regulate
ILED, and the voltage of the CDHC pin reaches its maximum,
and these conditions last for 508 consecutively dimming sig-
nals applied on the DIMn pin. This means that the current of
the LED string n does not reach its regulated value. In most
case, the IOUT under-voltage indication can be regarded as
an open fault of the LED string n. A bit pattern (refer to table
After power good, the switching frequency (fSW) of the
LM3492HC can be tuned down 20% or 40%, or resume nor-
mal by writing commands (refer to table 2) to the COMM pin.
This helps avoid interfering some sensitive devices, for ex-
ample radios, working nearby the LM3492HC. Upon reset,
fSW of the LM3492HC will resume normal by default. If the
over-temperature indication or any COMM bit pattern has al-
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12
ready presented, no command can be written to the
LM3492HC.
D1: The selection of the boost diode D1 depends on two fac-
tors. The first factor is the reverse voltage, which equals to
VOUT for a boost converter. The second factor is the peak
diode current at the steady state, which equals to the peak
inductor current as shown in (10). In this example, a 100V 3A
schottky diode is selected.
CHANNEL 1 DISABLE
After power good, channel 1 of the current regulator can be
disabled by writing a command (refer to table 2) to the COMM
pin. If LED string 1 is malfunctioning, channel 1 can be dis-
abled and the signal applied on the DIM1 pin can serve as
only a clock signal for the data flow of the COMM pin. Channel
1 is by default enabled after reset. If the over-temperature in-
dication or any COMM bit pattern has already presented, no
command can be written to the LM3492HC.
CIN and COUT: The function of the input capacitor CIN and the
output capacitor COUT is to reduce the input and output volt-
age ripples. Experimentation is usually necessary to deter-
mine their value. The rated DC voltage of capacitors used
should be higher than the maximum DC voltage applied. Ow-
ing to the concern of product lifetime, ceramic capacitors are
recommended. But ceramic capacitors with high rated DC
voltage and high capacitance are rare in general. Multiple ca-
pacitors connecting in parallel can be used for CIN and
COUT. In this example, two 10 µF ceramic capacitor are used
TABLE 2. Commands
Command
Command Bit Pattern
1111011101110111
1111000100010001
1111001100110011
1111010101010101
fSW resume normal
fSW tune down by 20%
fSW tune down by 40%
Channel 1 disable
for CIN, and two 2.2 µF ceramic capacitor are used for COUT
.
CVCC: The capacitor on the VCC pin provides noise filtering
and stabilizes the LDO regulator. It also prevents false trig-
gering of the VCC UVLO. CVCC is recommended to be a 1 µF
good quality and low ESR ceramic capacitor.
CCDHC: The capacitor at the CDHC pin not only affects the
sensitivity of the DHC but also determines the soft-start time
tSS, i.e. the time for the output voltage to rise until power good.
tSS is determined from the following equation:
Application Information
EXTERNAL COMPONENTS
The following procedures are to design an LED driver using
the LM3492HC with an input voltage ranged from 10V to 24V
and two LED strings consists of 10 LEDs each with a forward
voltage of 3V for each LED when running at 250 mA. The
output power is 15W. The switching frequency fSW is designed
to be 300 kHz.
In this example, CCDHC is recommended to be a 0.47 µF good
quality and low ESR ceramic capacitor.
RFB1, RFB2, and CFB: The nominal voltage of the LED string
with 10 LEDs is 30V, and the minimum voltage of the IOUTn
pin (n = 1, 2) is 1.25V when ILED is 250 mA. As a result, VOUT
RRT and RIREF: The resistors RRT and RIREF set the switching
frequency fSW of the boost converter and the LED current
ILED respectively. From Figure 1, if fSW is 300 kHz, RRT is se-
lected to be 499 kΩ. From Figure 4, if ILED is 250 mA, RIREF
is selected to be 4.99 kΩ.
RCOMM: Since the COMM pin is open drain, a resistor
RCOMM of 52.3 kΩ is used to connect the VCC and COMM
pins to act as a pull-up function.
is 31.25V. Design VOUT(MAX) to be 50V. From (5), VFB
(NOM)
is about 1.8V, which falls in the recommended operation
(NOM)
range from 1.05V to 2V. Also, design RFB2 to be 16.2 kΩ. From
(3), RFB1 is calculated to be 265.1 kΩ, and a standard resistor
value of 261 kΩ is selected. CFB is selected to be 10 pF as
recommended.
L1: The main parameter affected by the inductor is the peak
to peak inductor current ripple (ILR). To maintain a continuous
conduction mode (CCM) operation, the average inductor cur-
rent IL1 should be larger than half of ILR. For a boost converter,
IL1 equals to the input current IIN. Hence,
PC Board Layout
The layout of the printed circuit board is critical in order to
optimize the performance of the LM3492HC application cir-
cuit. In general, external components should be placed as
close to the LM3492HC and each other as possible in order
to make copper traces short and direct. In particular, compo-
nents of the boost converter CIN, L1, D1, COUT, and the
LM3492HC should be closed. Also, the output feedback ca-
IIN = (VOUT(NOM) x 2ILED ) / VIN
(6)
Also,
ton = (1 – VIN/VOUT) / fSW
L1 = (VIN x ton) / 2IIN
(7)
(8)
pacitor CFB should be closed to the output capacitor COUT
.
If VIN is maximum, which is 24V in this example, and only one
LED string is turned on (since the two channels of the
LM3492HC are individually dimmable), IIN is minimum. From
(6)-(8), it can be calculated that IIN(MIN), ton, and L1 are 0.326A,
0.77 µs, and 28.5µH. On the other hand, from (6), IIN is max-
imum when VIN is minimum, which is 10V in this example, and
the two LED strings are turned on together. Hence IIN(MAX) is
1.56A. Then, ILR is
The ground plane connecting the GND, PGND, and LGND
pins and the exposed pad of the LM3492HC and the ground
connection of the CIN and COUT should be placed on the same
copper layer.
Good heat dissipation helps optimize the performance of the
LM3492HC. The ground plane should be used to connect the
exposed pad of the LM3492HC, which is internally connected
to the LM3492HC die substrate. The area of the ground plane
should be extended as much as possible on the same copper
layer around the LM3492HC. Using numerous vias beneath
the exposed pad to dissipate heat of the LM3492HC to an-
other copper layer is also a good practice.
ILR = (VIN x ton) / L1
(9)
From (7), ton is 2.27 µs. From (9), ILR is 0.80A. The steady
state peak inductor current IL1(PEAK) is
IL1(PEAK) = IL1 + ILR / 2
(10)
As a result, IL1(PEAK) is 1.96A. A standard value of 27 µH is
selected for L1, and its saturation current is larger than 1.96A.
13
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30170529
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14
Physical Dimensions inches (millimeters) unless otherwise noted
20-Lead Plastic eTSSOP Package
NS Package Number MXA20A
15
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Notes
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