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参数资料
| 型号： | LM3402_07 |
| 厂商： | National Semiconductor Corporation |
| 英文描述： | 0.5A Constant Current Buck Regulator for Driving High Power LEDs |
| 中文描述： | 0.5A的恒流降压稳压器用于驱动高功率LED |
| 文件页数： | 6/24页 |
| 文件大小： | 431K |
| 代理商： | LM3402_07 |
Design Considerations
SWITCHING FREQUENCY
Switching frequency is selected based on the tradeoffs be-
tween efficiency (better at low frequency), solution size/cost
(smaller at high frequency), and the range of output voltage
that can be regulated (wider at lower frequency.) Many appli-
cations place limits on switching frequency due to EMI sen-
sitivity. The on-time of the LM3402/02HV can be programmed
for switching frequencies ranging from the 10’s of kHz to over
1 MHz. The maximum switching frequency is limited only by
the minimum on-time requirement.
LED RIPPLE CURRENT
Selection of the ripple current,
Δi
F, through the LED array is
analogous to the selection of output ripple voltage in a stan-
dard voltage regulator. Where the output ripple in a voltage
regulator is commonly ±1% to ±5% of the DC output voltage,
LED manufacturers generally recommend values for
Δi
F
ranging from ±5% to ±20% of I
F. Higher LED ripple current
allows the use of smaller inductors, smaller output capacitors,
or no output capacitors at all. The advantages of higher ripple
current are reduction in the solution size and cost. Lower rip-
ple current requires more output inductance, higher switching
frequency, or additional output capacitance. The advantages
of lower ripple current are a reduction in heating in the LED
itself and greater range of the average LED current before the
current limit of the LED or the driving circuitry is reached.
BUCK CONVERTERS WITHOUT OUTPUT CAPACITORS
The buck converter is unique among non-isolated topologies
because of the direct connection of the inductor to the load
during the entire switching cycle. By definition an inductor will
control the rate of change of current that flows through it, and
this control over current ripple forms the basis for component
selection in both voltage regulators and current regulators. A
current regulator such as the LED driver for which the
LM3402/02HV was designed focuses on the control of the
current through the load, not the voltage across it. A constant
current regulator is free of load current transients, and has no
need of output capacitance to supply the load and maintain
output voltage. Referring to the Typical Application circuit on
the front page of this datasheet, the inductor and LED can
form a single series chain, sharing the same current. When
no output capacitor is used, the same equations that govern
inductor ripple current,
Δi
L, also apply to the LED ripple cur-
rent,
Δi
F.
For a controlled on-time converter such as
LM3402/02HV the ripple current is described by the following
expression:
A minimum ripple voltage of 25 mV is recommended at the
CS pin to provide good signal-to-noise ratio (SNR). The CS
pin ripple voltage,
ΔV
SNS, is described by the following:
ΔV
SNS = ΔiF x RSNS
BUCK CONVERTERS WITH OUTPUT CAPACITORS
A capacitor placed in parallel with the LED or array of LEDs
can be used to reduce the LED current ripple while keeping
the same average current through both the inductor and the
LED array. This technique is demonstrated in Design Exam-
ple 1. With this topology the output inductance can be low-
ered, making the magnetics smaller and less expensive.
Alternatively, the circuit could be run at lower frequency but
keep the same inductor value, improving the efficiency and
expanding the range of output voltage that can be regulated.
Both the peak current limit and the OVP/OCP comparator still
monitor peak inductor current, placing a limit on how large
Δi
L can be even if ΔiF is made very small. A parallel output
capacitor is also useful in applications where the inductor or
input voltage tolerance is poor. Adding a capacitor that re-
duces
Δi
F to well below the target provides headroom for
changes in inductance or V
IN that might otherwise push the
peak LED ripple current too high.
Figure 4 shows the equivalent impedances presented to the
inductor current ripple when an output capacitor, C
O, and its
equivalent series resistance (ESR) are placed in parallel with
the LED array. The entire inductor ripple current flows through
R
SNS to provide the required 25 mV of ripple voltage for proper
operation of the CS comparator.
20192115
FIGURE 4. LED and C
O Ripple Current
To calculate the respective ripple currents the LED array is
represented as a dynamic resistance, r
D. LED dynamic resis-
tance is not always specified on the manufacturer’s
datasheet, but it can be calculated as the inverse slope of the
LED’s V
F vs. IF curve. Note that dividing VF by IF will give an
incorrect value that is 5x to 10x too high. Total dynamic re-
sistance for a string of n LEDs connected in series can be
calculated as the r
D of one device multiplied by n. Inductor
ripple current is still calculated with the expression from Buck
Regulators without Output Capacitors. The following equa-
tions can then be used to estimate
Δi
F when using a parallel
capacitor:
The calculation for Z
C assumes that the shape of the inductor
ripple current is approximately sinusoidal.
Small values of C
O that do not significantly reduce ΔiF can
also be used to control EMI generated by the switching action
of the LM3402/02HV. EMI reduction becomes more important
as the length of the connections between the LED and the
rest of the circuit increase.
www.national.com
14
LM3402/LM3402HV
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM3402EVAL | 功能描述:BOARD EVALUATION FOR LM3402 RoHS:是 类别:编程器，开发系统 >> 评估板 - LED 驱动器 系列:PowerWise® 标准包装:1 系列:PowerWise® 电流 - 输出 / 通道:20mA 输出及类型:1，非隔离 输出电压:17V 特点:可调光 输入电压:2.7 ~ 5.5 V 已供物品:板 已用 IC / 零件:LM3508 相关产品:LM3508TLX-ND - IC LED DRVR WHT BCKLGT 9USMDLM3508TLDKR-ND - IC LED DRVR WHT BCKLGT 9MICROSMDLM3508TLCT-ND - IC LED DRVR WHT BCKLGT 9MICROSMDLM3508TLTR-ND - IC LED DRVR WHT BCKLGT 9MICROSMD |
| LM3402EVAL | 制造商:Texas Instruments 功能描述:EVAL BOARD |
| LM3402EVAL/NOPB | 功能描述:电源管理IC开发工具 LM3402 EVAL BOARD RoHS:否 制造商:Maxim Integrated 产品:Evaluation Kits 类型:Battery Management 工具用于评估:MAX17710GB 输入电压: 输出电压:1.8 V |
| LM3402HV | 制造商:Texas Instruments 功能描述:Buck Regulator ,0.5A ,LED Drivers,MSOP8 |
| LM3402HVEVAL | 功能描述:BOARD EVALUATION FOR LM3402HV RoHS:是 类别:编程器，开发系统 >> 评估板 - LED 驱动器 系列:PowerSmart® 标准包装:1 系列:PowerWise® 电流 - 输出 / 通道:20mA 输出及类型:1，非隔离 输出电压:17V 特点:可调光 输入电压:2.7 ~ 5.5 V 已供物品:板 已用 IC / 零件:LM3508 相关产品:LM3508TLX-ND - IC LED DRVR WHT BCKLGT 9USMDLM3508TLDKR-ND - IC LED DRVR WHT BCKLGT 9MICROSMDLM3508TLCT-ND - IC LED DRVR WHT BCKLGT 9MICROSMDLM3508TLTR-ND - IC LED DRVR WHT BCKLGT 9MICROSMD |
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