LM3432SQX [NSC]
6-Channel Current Regulator for LED Backlight Application; 6通道电流调节器用于LED背光应用
| 型号： | LM3432SQX |
| 厂家： | 
National Semiconductor |
| 描述： | 6-Channel Current Regulator for LED Backlight Application |
| 文件： | 总18页 (文件大小：405K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
May 22, 2008
LM3432/LM3432B
6-Channel Current Regulator for LED Backlight Application
General Description
Features
The LM3432/LM3432B are 6-channel high voltage current
regulators which provide a simple solution for LED backlight
applications. These devices incorporate six individual current
regulator channels to give accurate driving current for each
LED string. The string-to-string tolerance is kept within ±2.0%.
Additionally, the Dynamic Headroom Control output can com-
municate with a LM3430 boost regulator to adjust the LED
supply voltage to the lowest level needed to keep the string
current in regulation, yielding optimal overall system efficien-
cy.
Dynamic Headroom Control (DHC) output to maximize
■
efficiency when used in conjunction with National
Semiconductor's LM3430 Boost Controller for LED
Backlighting
Current sinking adjustable up to 40mA in each string
■
■
■
■
■
■
■
Fast current switching slew rate, tr = 60ns typical
Wide dimming ratio, up to 4000:1 with fDIM = 500Hz
High LED driving voltage up to 80V
±2.0% current matching between strings
The 6-channel current sink can be adjusted from 15mA to
40mA by an external resistor. Their output drivers can with-
stand up to 80V. Digital PWM or analog voltage signal can be
used to control the duty cycle of all the channels. With a fast
current switching slew rate, tr = 60ns typical, accurate current
control and wide dimming ratio during PWM dimming are en-
sured.
Accepts both Digital and Analog dimming control
LED open/short fault indication (For LM3432B, no open
fault indication)
Over-Temperature Indication
■
■
■
Internal Thermal Shutdown with Hysteresis
Low profile, thermally enhanced LLP-24 (5x4x0.8mm) and
eTSSOP-28 (9.7x6.4x1.1mm) packages (The LM3432B is
available in the LLP-24 only)
The LM3432/LM3432B contain LED open/short circuit and
over-temperature fault signaling to the system micro-con-
troller (the LM3432B does not include open circuit fault sig-
naling). These devices are available in a low profile, thermally
enhanced 24 lead LLP package. The LM3432 is also avail-
able in the 28 lead eTSSOP.
Applications
LCD display backlight applications
■
■
General lighting solutions
© 2008 National Semiconductor Corporation
300064
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Typical Application Circuit
30006401
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2
Connection Diagrams
30006431
Top View
28 Lead Plastic eTSSOP-28
NS Package Number MXA28A
30006402
Top View
24 Lead Plastic LLP-24
NS Package Number SQA24B
Ordering Information
Part Number
For LM3432
Package Type
NS Package Drawing
Supplied As
LM3432SQ
LM3432SQX
LM3432SQE
LM3432MH
LM3432MHX
1000 Units on Tape and Reel
4500 Units on Tape and Reel
250 Units on Tape and Reel
Rail of 48 Units
LLP-24
SQA24B
MXA28A
eTSSOP-28
2500 Units on Tape and Reel
For LM3432B
LM3432BSQ
LM3432BSQX
LM3432BSQE
1000 Units on Tape and Reel
4500 Units on Tape and Reel
250 Units on Tape and Reel
LLP-24
SQA24B
3
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Pin Descriptions
Pin Number
Name
Description
LLP-24
eTSSOP-28
1
5
VCC
IREF
Internal linear regulator output, needs 680nF minimum
for stability.
2
6
IOUT current setting pin. An external resistor is used to
program the string current.
3
4
7
8
AGND
CDHC
Analog ground
An external capacitor to ground programs the Dynamic
Headroom Control (DHC) response time constant.
5
9
VDHC
DHC voltage output. Connecting this output through a
gain setting resistor to the DHC pin of National
Semiconductor's LM3430 enables the DHC function.
6
7
10
11
EN
Device Enable, active HIGH.
MODE
Dimming mode select pin. Short to ground for Digital
PWM dimming or connect to an external capacitor to
ground for analog dimming.
8
9
12
13
14
DIM
Digital PWM or Analog voltage input for IOUT duty cycle.
Open drain active LOW output for output fault.
FAULTb
OTMb
10
Open drain active LOW over temperature warning
output.
12,13,15,17,19,20 15, 17, 19, 21, 23, 25
IOUT1-6
Constant current sink outputs, adjustable 15mA to
40mA, voltage across this pin can be up to 80V max.
22
24
27, 28
1
PGND
VIN
Power Ground.
Supply voltage input, from 6V to 40V.
No connection and should be left open.
11,14,16,18,21,23 2, 3, 4, 16, 18, 20, 22,
24, 26
NC
EP
EP
EP
Thermal connection pad, connect directly to GND.
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ꢀ
ꢀ
Absolute Maximum Ratings (Notes 1, 6)
ESD Susceptibility (Note 2)
Human Body Model
Lead Temperature
Vapor Phase (60 sec.)
Infra-red (15 sec.)
Maximum Junction Temp.
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
2.0kV
215°C
220°C
150°C
VIN Voltage
-0.3V to 42V
-0.3V to 7V
-0.3V to 82V
-0.3V to 7V
-0.3V to 7V
-0.3V to 7V
-0.3V to 7V
-0.3V to 7V
-0.3V to 7V
-0.3V to 7V
ꢀ
IREF Voltage
IOUT1 through IOUT6 Voltage
VCC Voltage
EN Voltage
FAULTb, OTMb Voltage
MODE Voltage
PWM Voltage
VDHC Voltage
CDHC Voltage
ꢀ
Operating Ranges (Note 1)
Supply Voltage, VIN
IOUT1 through IOUT6 Voltage
Operating Junction Temp.
Storage Temperature
Thermal Resistance, θJA (Note 3)
LLP-24
6 to 40V
0 to 80V
-40°C to +125°C
-65°C to +150°C
33.2°C/W
29°C/W
eTSSOP-28
ꢀ
ꢀ
Electrical Characteristics Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the
junction temperature (TJ) range of -40°C to +125°C. Minimum and Maximum limits are guaranteed through test, design, or statistical
correlation. VIN = 18V, RIREF = 54.7 kΩ, VEN = 5V, VMODE = 0V, VDIM = 5V and VOUT1-6 = 1.2V unless otherwise indicated (Note
4). Parameter limits apply to both the LM3432 and LM3432B unless otherwise indicated.
Symbol
Input Characteristics
ISHDN Shutdown Input Supply Current
IQ
Parameter
Conditions
Min
Typ
Max Units
EN = 0V, DIM = 0V
40
90
µA
mA
V
Quiescent Current from VIN
Enable Threshold Voltage
Enable Threshold Hysteresis
Enable pin Pull-up Current
2.25 2.65
VEN
VEN rising
1.48 1.75
2
VEN_HYST
IEN
0.4
0.5
5
V
EN = 0V
EN = 2V
µA
VCC Regulator
VCCreg
VCCreg_1
VCCreg_2
IVCC_SC
VCCUVLO
VCC Regulated Output
4.7
4.7
5
5
5.25
5.25
V
V
VCC Regulated Output at Max. VIN
VCC Regulated Output at Min. VIN
VCC Short-Circuit Current
VIN = 40V
VIN = 6V, IVCC = 2mA
VIN = 6V, VCC = 0V
VCC rising
4.8
9
V
mA
V
VCC UVLO Upper Threshold
3.9
4.15
0.375
4.4
VCCUVLO_HYST VCC UVLO Hysteresis
V
Analog PWM control
IMODE
MODE pin Output Current
MODE pin Peak Voltage
MODE pin Valley Voltage
MODE = 2V
34
3.1
1.0
µA
V
VMODE_PK
VMODE_VA
CMODE = 5.6nF
CMODE = 5.6nF
V
Digital PWM control
VPWM_HIGH PWM Voltage HIGH
VPWM_LOW PWM Voltage LOW
Dynamic Headroom Control Output
MODE = GND
MODE = GND
1.7
V
V
1
VDHC_MAX
VDHC pin Max. Output Voltage
IVDHC = 2mA
2
5
2.5
9
V
mA
V
IVDHC_MAX VDHC pin Max. Output Current
VDHC_REG_20 VDHC Regulation at 20mA
VDHC_REG_40 VDHC Regulation at 40mA
VDHC = 1.2V
RIREF = 54.7k, CDHC = 100nF
RIREF = 27.5k, CDHC = 100nF
0.625
1.25
V
5
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Symbol
Switching Characteristics
tpd_H IOUT Rising Edge Phase Delay
Tr IOUT Rise Time
Parameter
Conditions
Min
Typ
Max Units
MODE = GND, Pulsing PWM 10µs
500
25
ns
ns
MODE = GND, rising edge from 10%
to 90% of IOUT
Output Current
VIREF
IREF pin Voltage
1.215 1.245 1.27
V
6V ≤ VIN ≤ 40V
IOUT20
Constant Current Sink of 20mA
RIREF = 54.7k, VIOUT1-6 = 1.1V
18.85
20
21.05
mA
18.5
21.4
IOUT40
Constant Current Sink of 40mA
RIREF = 27.5k, VIOUT1-6 = 1.6V
37.9
40
41.55
mA
37.3
42.15
IOUT20_match Output Current Matching of IOUT20 (Note 5)
IOUT40_match Output Current Matching of IOUT40 (Note 5)
2.25
2
%
%
6V ≤ VIN ≤ 40V
6V ≤ VIN ≤ 40V
IOUT_max
Maximum Output Current, IREF shorted to
GND.
VIREF = 0V, VIOUT = 4.5V
50
80
110
mA
VDROP_IOUT20 Dropout Voltage of IOUT20 (Note 7)
VDROP_IOUT40 Dropout Voltage of IOUT40 (Note 7)
RIREF = 54.7k
0.3 0.525
0.6 1.125
V
V
RIREF = 27.5k
IOUT_OFF
Output Current when EN is LOW
EN = 0, VOUT = 80V
0.025
3
µA
Fault Detection
VSHORTFAULT VIOUT Short Fault Threshold
FAULTb goes LOW during VIOUT
rising, other VIOUTs = 1.0V
7.3
7.9
8.8
V
tD_SHORTFAULT Short Fault Delay
VIOUT set to 10V, FAULTb goes LOW,
other VIOUTs = 1.0V
150
µs
tD_OPENFAULT Open Fault Delay (LM3432 only)
VFAULT_LOW FAULTb and OTMb LOW
ILEAK_FAULT FAULTb and OTMb Open Leakage
Thermal Protection
VIOUTX set open, FAULTb goes LOW
5mA into FAULTb
50
µs
V
0.7
1
VFAULTb = VOTMb = 5V
0.005
µA
OTM
OTMHYST
TSD
Over Temperature Monitor Threshold
Over Temperature Monitor Hysteresis
Thermal Shutdown Threshold
125
20
°C
°C
°C
°C
165
20
TSDHYST
Thermal Shutdown Hysteresis
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. The Recommended Operating Limits define the conditions within
which the device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 3: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ_MAX, the junction-to-ambient thermal resistance, θJA
and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated using: PD_MAX = (TJ_MAX - TA)/θJA. Exceeding
the maximum allowable power dissipation will cause excessive die temperature.
Note 4: All limits are guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate
Average Outgoing Quality Level (AOQL). Typical values represent the most likely parametric norm at TJ = 25°C and are provided for reference purposes only.
Note 5: IOUT_match is the greatest percentage delta between output string currents with respect to the median.
Note 6: Rating limits apply to both the LM3432 and LM3432B.
Note 7: Dropout voltage is defined as the IOUT pin to GND voltage at which the output current sink drops 10% from the nominal value.
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Typical Performance Characteristics Unless otherwise specified, the following conditions apply: VIN
=
18V, RIREF = 54.7 kΩ, VEN = 5V, VMODE = 0V, VDIM = 5V and VOUT1-6 = 1.2V. Typical performance characteristics are valid for both
the LM3432 and LM3432B unless otherwise indicated.
Quiescent Current vs Supply Voltage
Shutdown Current vs Supply Voltage
30006403
30006404
VCC Regulator Output vs Supply Voltage
Current Setting Reference Voltage vs Supply Voltage
30006405
30006406
Average String Current vs Current Setting Resistance
Output Current Matching vs Temperature
30006408
30006407
7
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PWM Dimming Frequency vs CMODE
PWM Digital Dimming Operation
(Channel 1 Waveform)
30006410
30006409
PWM Dimming Characteristic, Rising Edge
(Channel 1 Waveform)
PWM Dimming Characteristic, Falling Edge
(Channel 1 Waveform)
30006411
30006412
Analog Dimming Operation
30006413
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Simplified Functional Block Diagram
30006415
9
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ANALOG DIMMING OF LED STRINGS
Functional Description
Dimming of LED brightness is achieved by Pulse Width Mod-
ulation (PWM) control of the string currents. The LM3432/
LM3432B accepts both analog voltage and PWM digital dim-
OVERVIEW
The LM3432/LM3432B are 6-channel high voltage current
regulators for LED backlight applications which incorporates
individual channel current regulators to give accurate current
sinking for each LED string. String to string tolerance is kept
within ±2.0% at 40 mA and ±2.25% at 20 mA. The 6-channels
current sinks can be adjusted from 15 mA to 40 mA by an
external resistor. Channel outputs can withstand up to 80V.
Channel 5 or 6 output can be disabled by shorting the selected
output pin to ground prior to power-up. Both the digital PWM
dimming signal and analog voltage signal can be used to
control the duty cycle of all the six channels. The LM3432/
LM3432B also provide fault indications to the system MCU for
an LED open (included in the LM3432 only) or short circuit or
an over-temperature condition.
ming input signals for this feature. With a capacitor (CMODE
)
connected across the MODE pin and ground, the device will
monitor the voltage level at the DIM pin and generate the re-
quired PWM control signal internally. The internal implemen-
tation of the LM3432/LM3432B’s dimming function is illustrat-
ed in Figure 1.
INTERNAL 5V LINEAR REGULATOR
An internal 5V linear regulator with an Under-Voltage Lock-
Out (UVLO) function is integrated within the LM3432/
LM3432B. This regulated 5V is used for internal circuitry and
can support a small amount of external loading, not to exceed
2 mA when VIN = 6V. The supply input pin (VIN) can be con-
nected directly to an input voltage up to 40V, with transient
capability up to 42V. The VCC output regulates at 5V and is
current limited to 9 mA. To ensure stable operation, the ex-
ternal capacitor CVCC must be at least 680 nF with 1 µF
recommended. If the voltage at the VCC pin drops below the
UVLO threshold of 3.8V, the device will shut down the output
channels and other functional blocks. Normal operation will
be resumed once the VCC voltage is allowed to rise above the
UVLO rising threshold of 4.15V.
30006417
FIGURE 1. Analog Dimming of LEDs
An internal current source of 34 µA (typical) will charge the
external capacitor (CMODE) linearly until it reaches 3V. The
comparator (Comp1) then forces CMODE to be discharged to
1V very quickly. By repeating the cycle, a saw-tooth waveform
as shown in Figure 1 is generated. Comparator (Comp2)
compares this ramp waveform with the external dc voltage at
the DIM pin generating the desired PWM control signal.
BANDGAP VOLTAGE REFERENCE
A precision reference voltage is required for accurate control
of the output currents. The LM3432/LM3432B contain a
bandgap voltage reference block that provides a high preci-
sion reference voltage for internal operation. The bandgap
reference voltage is typically trimmed to 1.245V.
When VDIM ≤ 1V, ON duty factor, DON = 0% and when VDIM
≥ 3V, DON = 100%. The frequency of the PWM control signal
can be calculated as shown below.
OUTPUT CURRENT REGULATOR (IOUT1 to IOUT6)
The LM3432/LM3432B contains six individual integrated cur-
rent regulators to give accurate current sinking for each LED
string. String to string tolerance is kept within ±2.0% at 40 mA
and ±2.25% at 20 mA. The sink current level is adjusted by
an external resistor, RIREF in the range of 15 mA to 40 mA.
The IOUT pins can withstand up to 80V. The ability to with-
stand high voltage enables the user to add more LEDs in a
single string. The calculation of IOUT with respect to RIREF is
shown below.
Or
PWM DIGITAL DIMMING of LED STRINGS
Alternatively, the dimming control can be implemented by di-
rect application of a digital signal to the device. With the
MODE pin connected to ground, an externally applied PWM
dimming signal is applied to the DIM pin. The peak amplitude
of the externally applied PWM signal should be greater than
1.5V to guarantee clean PWM switching.
Channels 5 and 6 are designed to be user disabled without
activating the fault detection circuitry. With this feature, the
user can readily configure the device to a 4, 5 or 6 channel
driver. In order to disable a channel, the IOUT5 and/or IOUT6
pin(s) must be tied to ground before powering up the device.
During power-up, channels 5 and 6 will be checked and any
grounded channel(s) will be automatically disabled. The dis-
abled status will remain until either power is recycled or the
enable (EN) pin is toggled.
During PWM dimming, channels are not switched simultane-
ously in an effort to minimize large surge currents from being
drawn from the LED supply rail. Each channel will have 0.5
µs phase delay with respect to the preceding channel. As a
consequence of the phase delay, for a control pulse width less
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than 0.5 µs, the duty cycle will be rounded down to 0% and
for a control pulse width less than 0.5 µs off time, the duty
cycle will be rounded up to 100%. Therefore, 0.5µs becomes
the finest pulse width resolution that can be realized. The
PWM switching timing for all six channels is shown in Figure
2.
30006420
FIGURE 2. PWM Dimming Switching Timing
LED SHORT FAULT DETECT
With DHC
detect feature will be active for all channels. In the event an
LED short fault condition occurs and causes the voltage be-
tween any of the IOUT pin and ground to exceed the typical
short fault threshold of 7.9V for more than 150 μs, the affected
channel(s) will be latched off and the FAULTb pin will be
pulled to ground. The affected channel(s) will remain latched
off until recycling power or toggling the EN pin. All of the other
channels that are not affected will continue to function nor-
mally. If the voltages between all IOUT pins and ground are
greater than 2V typical, the Short Fault Detect feature will be
disabled to prevent false triggering of the short fault detect
function.
If the Dynamic Headroom Control (DHC) feature is used, the
lowest voltage between the IOUT pins and ground among the
strings will be regulated to 0.625V typical (ILED = 20 mA) by
lowering the LED supply rail voltage, VLED. If an LED short
fault condition occurs and causes the voltage between any of
the IOUT pins and ground to exceed the typical short fault
threshold of 7.9V for more than 150 µs, the affected channel
(s) will be latched off and the FAULTb pin will be pulled to
ground. The affected channel(s) will remain latched off until
recycling power or toggling the EN pin. All of the other chan-
nels that are not affected will continue to function normally.
LED OPEN FAULT DETECT (LM3432 ONLY)
For the LM3432, if an open fault should occur and the affected
channel sinks no current for greater than 50 µs, it will be
latched off (until recycling power or toggling EN pin). The
FAULTb pin’s output will be pulled to ground while the other
channels keep functioning normally. The open fault detection
and FAULTb indication features are inactive in the LM3432B.
Without DHC
In applications where the DHC function is not used, the IOUT
pin voltage on each string will be the voltage difference be-
tween the LED supply rail voltage, VLED, and the voltage drop
across the entire LED string. If the voltage between any of the
IOUT pins and ground is less than 2V typical, the short fault
11
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30006429
FIGURE 3. Fault Detect Functional Block Diagram
OVER-TEMPERATURE MONITOR
DYNAMIC HEADROOM CONTROL
If the LM3432/LM3432B junction temperature exceeds ap-
proximately 125°C, the OTMb pin will be pulled to ground but
the part will continue to function. Action must be taken to lower
the temperature at this point. The PWM duty factor may be
lowered as an example to reduce the amount of heat gener-
ated, which will in turn lower the die temperature. If the
junction temperature is allowed to rise beyond approximately
165°C, the part will shut down. When the device is cooled
down to about 145°C, device operation will resume.
To use the DHC function, connect a gain setting resistor from
the VDHC pin of the LM3432/LM3432B to the VDHC pin of
the LM3430 (National Semiconductor’s Boost Controller for
LED Backlighting) that is supplying power to the LED rails.
The LM3432/LM3432B’s DHC function will regulate the volt-
age between the IOUT pins and ground to a minimum of
0.625V typical (IOUT = 20 mA) to optimize overall efficiency.
A large DHC time constant needs to be set in order not to
interfere with the loop response of the DC-DC converter. This
can be implemented by connecting a capacitor from the
LM3432/LM3432B’s CDHC pin to ground. If the DHC function
is not needed, leave the VDHC pin floating. When operated
in this manner, if the lowest voltage between any of the IOUT
pins and ground is greater than 2V, the LED short fault de-
tection function will be disabled. For the LM3432B, if any open
condition occurs on any channels, the DHC function will be
inactive and the LED supply rail voltage from the LM3430 will
stay at its preset level. If the excess headroom voltage is
greater than the short fault detect threshold, 7.3V(min), the
device may latch off the LED string(s) due to short fault pro-
tection. In order to insure the proper operation of good LED
strings, designers must design for sufficient excess head-
room voltage below the short fault detect threshold.
Note that this thermal shutdown protection is only intended as
a fault mode protection feature. Device operation above rated
maximum operating junction temperature is neither recom-
mended nor guaranteed.
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an LED panel of up to about 120 white LEDs can be illumi-
nated. A typical application configuration driving six strings
with twelve white LEDs per string is shown in Figure 4.
Applications Information
The LM3432/LM3432B provide a simple and handy solution
for LED driving. With only a few external passive components,
30006421
FIGURE 4. Typical Application Schematic to Drive 72 White LEDs (@20 mA) (PWM Digital Dimming)
DETERMINATION OF EXTERNAL COMPONENTS
For the example shown in Figure 4, the string current is 20
mA. Applying to the equation:
The typical application only requires three external compo-
nents. The selection of those components is described in
detail below.
Programming of String Current, IOUT1 to IOUT6
The string current can be programmed by an external resistor,
RIREF. The equation to calculate the resistance of this resistor
is shown below.
Selecting the VCC Output Capacitor, CVCC
For proper operation, a VCC output capacitor (CVCC) of at
least 680 nF is required for stability reason. The recommend-
ed CVCC capacitance is 1 µF.
In order to ensure good current regulation over the full oper-
ating temperature range, a high quality resistor with ±1%
tolerance and a low temperature coefficient is recommended.
13
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Selecting the VIN Capacitor, CVIN
Driving High Current LEDs
The purpose of this capacitor is to supply transient current to
the device and suppress VIN noise in order to ensure proper
operation. A low ESR ceramic capacitor with good high fre-
quency performance is recommended. The capacitance can
range from 0.1 µF to 1 µF.
The LM3432/LM3432B can support string currents from 15
mA to 40 mA. If the application needs to drive high current
LEDs that require more than 40 mA per string, the LM3432/
LM3432B provides the alternative of connecting several IOUT
ports together to achieve higher output current per string. With
this approach, there is the obvious trade off between higher
output current and number of strings driven. Two possible
configurations are illustrated in Figure 5.
Analog Dimming Control
If analog dimming control is required, a capacitor, CMODE
should be connected from the MODE pin to ground instead of
shorting the MODE pin to ground. The relationship between
the PWM dimming frequency and the capacitance of CMODE
is illustrated below.
When VPWM ≤ 1V, DON = 0% and when VPWM ≥ 3V, DON
=
100%
The LED dimming ratio is calculated from the ratio of minimum
ON duty factor to the maximum ON duty factor. With the
LM3432/LM3432B, the LEDs can be fully turned on up to
100% ON duty factor and the minimum ON duty factor is lim-
ited by the phase delay time, 0.5 µs. The dimming ratio can
be estimated as below.
As an example, if the PWM dimming frequency is set to 500
Hz, the best achievable dimming ratio is:
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30006427
FIGURE 5. Achieving Higher LED String Current by Grouping IOUT Ports
Dynamic Headroom Control, DHC with LM3430
must be set to a large enough value by adding a capacitor
from CDHC pin to ground.
When the LM3432/LM3432B are powered with National
Semiconductor's Boost Controller for LED Backlighting,
LM3430, the Dynamic Headroom Control (DHC) feature
helps to provide the optimal system efficiency. By connecting
VDHC through a gain setting resistor to the DHC pin of the
LM3430 that is supplying the LED power rail, the LM3432/
LM3432B’s DHC function will regulate the minimum of the ON
voltage of the six channels to 0.625V typical (IOUT = 20 mA).
This is the minimum voltage headroom required at outputs to
keep the current regulator in its linear operating range. In or-
der not to interfere with the feedback loop response of the
upstream DC-DC converter, the DHC response time constant
Figure 6 is an application schematic of a LED panel driver
using both the LM3430 and the LM3432/LM3432B with the
Dynamic Headroom Control function enabled. The LM3430
boosts a voltage from VIN to 50V nominal to supply the LED
strings. With the DHC function disabled, the LM3430 will keep
the LED string supply regulated at 50V. When the DHC is
enabled, this voltage will dynamically be regulated down to a
voltage that can just keep all LED string currents in regulation.
That is about 40V in the application shown in Figure 6. The
reduction in this rail voltage can significantly improve the
overall system efficiency.
15
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30006428
FIGURE 6. LM3430 + LM3432/LM3432B Application Schematic with Dynamic Headroom Control Enabled
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16
Physical Dimensions inches (millimeters) unless otherwise noted
24-Lead LLP Package
NS Package Number SQA24B
28-Lead eTSSOP Package
NS Package Number MXA28A
17
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