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參數(shù)資料
| 型號： | LM3402MMX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.5A Constant Current Buck Regulator for Driving High Power LEDs |
| 中文描述： | 0.94 A SWITCHING REGULATOR, PDSO8 |
| 封裝： | PLASTIC, MSOP-8 |
| 文件頁數(shù)： | 11/22頁 |
| 文件大?。?/td> | 840K |
| 代理商： | LM3402MMX |
Application Information
(Continued)
MINIMUM OUTPUT VOLTAGE
The minimum recommended on-time for the LM3402/02HV
is 300 ns. This lower limit for t
determines the minimum
duty cycle and output voltage that can be regulated based on
input voltage and switching frequency. The relationship is
determined by the following equation:
HIGH VOLTAGE BIAS REGULATOR
The LM3402/02HV contains an internal linear regulator with
a 7V output, connected between the VIN and the VCC pins.
The VCC pin should be bypassed to the GND pin with a 0.1
μF ceramic capacitor connected as close as possible to the
pins of the IC. VCC tracks VIN until VIN reaches 8.8V
(typical) and then regulates at 7V as VIN increases. Opera-
tion begins when VCC crosses 5.25V.
INTERNAL MOSFET AND DRIVER
The LM3402/02HV features an internal power MOSFET as
well as a floating driver connected from the SW pin to the
BOOT pin. Both rise time and fall time are 20 ns each
(typical) and the approximate gate charge is 3 nC. The
high-side rail for the driver circuitry uses a bootstrap circuit
consisting of an internal high-voltage diode and an external
10 nF capacitor, C
B
. V
CC
charges C
B
through the internal
diode while the power MOSFET is off. When the MOSFET
turns on, the internal diode reverse biases. This creates a
floating supply equal to the V
CC
voltage minus the diode
drop to drive the MOSFET when its source voltage is equal
to V
IN
.
FAST SHUTDOWN FOR PWM DIMMING
The DIM pin of the LM3402/02HV is a TTL logic compatible
input for low frequency PWM dimming of the LED.Alogic low
(below 0.8V) at DIM will disable the internal MOSFET and
shut off the current flow to the LED array. While the DIM pin
is in a logic low state the support circuitry (driver, bandgap,
VCC) remains active in order to minimize the time needed to
turn the LED array back on when the DIM pin sees a logic
high (above 2.2V). A 75 μA (typical) pull-up current ensures
that the LM3402/02HV is on when DIM pin is open circuited,
eliminating the need for a pull-up resistor. Dimming fre-
quency, f
, and duty cycle, D
, are limited by the LED
current rise time and fall time and the delay from activation of
the DIM pin to the response of the internal power MOSFET.
In general, f
should be at least one order of magnitude
lower than the steady state switching frequency in order to
prevent aliasing.
PEAK CURRENT LIMIT
The current limit comparator of the LM3402/02HV will en-
gage whenever the power MOSFET current (equal to the
inductor current while the MOSFET is on) exceeds 735 mA
(typical). The power MOSFET is disabled for a cool-down
time that is 10x the steady-state on-time.At the conclusion of
this cool-down time the system re-starts. If the current limit
condition persists the cycle of cool-down time and restarting
will continue, creating a low-power hiccup mode, minimizing
thermal stress on the LM3402/02HV and the external circuit
components.
OVER-VOLTAGE/OVER-CURRENT COMPARATOR
The CS pin includes an output over-voltage/over-current
comparator that will disable the power MOSFET whenever
V
SNS
exceeds 300 mV. This threshold provides a hard limit
for the output current. Output current overshoot is limited to
300 mV / R
SNS
by this comparator during transients.
The OVP/OCP comparator can also be used to prevent the
output voltage from rising to V
in the event of an
output open-circuit. This is the most common failure mode
for LEDs, due to breaking of the bond wires. In a current
regulator an output open circuit causes V
to fall to zero,
commanding maximum duty cycle. Figure 2 shows a method
using a zener diode, Z1, and zener limiting resistor, R
, to
limit output voltage to the reverse breakdown voltage of Z1
plus 200 mV. The zener diode reverse breakdown voltage,
V
, must be greater than the maximum combined V
of all
LEDs in the array. The maximum recommended value for R
Z
is 1 k
.
As discussed in the Maximum Output Voltage section, there
is a limit to how high V
can rise during an output open-
circuit that is always less than V
. If no output capacitor is
used, the output stage of the LM3402/02HV is capable of
withstanding V
indefinitely, however the voltage at the
output end of the inductor will oscillate and can go above V
IN
or below 0V. A small (typically 10 nF) capacitor across the
LED array dampens this oscillation. For circuits that use an
output capacitor, the system can still withstand V
O(MAX)
in-
definitely as long as C
O
is rated to handle V
IN
. The high
current paths are blocked in output open-circuit and the risk
of thermal stress is minimal, hence the user may opt to allow
the output voltage to rise in the case of an open-circuit LED
failure.
L
www.national.com
11
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|---|---|
| LM3402MMX/NOPB | 功能描述:開關(guān)變換器、穩(wěn)壓器與控制器 RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3402MR | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 6V to 42V 8-Pin PSOP EP Rail |
| LM3402MR/NOPB | 功能描述:開關(guān)變換器、穩(wěn)壓器與控制器 RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3402MRX | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 6V to 42V 8-Pin HSOP EP T/R |
| LM3402MRX/NOPB | 功能描述:開關(guān)變換器、穩(wěn)壓器與控制器 RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
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