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PDF LM3402HV Data sheet ( Hoja de datos )
| Número de pieza | LM3402HV | |
| Descripción | 0.5A Constant Current Buck Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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October 2006
LM3402/LM3402HV
0.5A Constant Current Buck Regulator for Driving High
Power LEDs
General Description
The LM3402/02HV are monolithic switching regulators de-
signed to deliver constant currents to high power LEDs. Ideal
for automotive, industrial, and general lighting applications,
they contain a high-side N-channel MOSFET switch with a
current limit of 735 mA (typical) for step-down (Buck) regu-
lators. Hysteretic control with controlled on-time coupled with
an external resistor allow the converter output voltage to
adjust as needed to deliver a constant current to series and
series - parallel connected arrays of LEDs of varying number
and type, LED dimming by pulse width modulation (PWM),
broken/open LED protection, low-power shutdown and ther-
mal shutdown complete the feature set.
Features
n Integrated 0.5A N-channel MOSFET
n VIN Range from 6V to 42V (LM3402)
n VIN Range from 6V to 75V (LM3402HV)
n 500 mA Output Current Over Temperature
n Cycle-by-Cycle Current Limit
n No Control Loop Compensation Required
n Separate PWM Dimming and Low Power Shutdown
n Supports all-ceramic output capacitors and
capacitor-less outputs
n Thermal shutdown protection
n MSOP-8 Package
Applications
n LED Driver
n Constant Current Source
n Automotive Lighting
n General Illumination
n Industrial Lighting
Typical Application
© 2006 National Semiconductor Corporation DS201921
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Electrical Characteristics VIN = 24V unless otherwise indicated. Typicals and limits appearing in plain type
apply for TA = TJ = +25˚C. (Note 4) Limits appearing in boldface type apply over full Operating Temperature Range.
Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. (Continued)
LM3402/LM3402HV (Continued)
Symbol
Parameter
Conditions
N-MOSFET AND DRIVER
RDS-ON
VDR-UVLO
Buck Switch On Resistance
BOOT Under-voltage
Lock-out Threshold
ISW = 200mA, BOOT-SW = 6.3V
BOOT–SW Increasing
VDR-HYS
BOOT Under-voltage
Lock-out Hysteresis
BOOT–SW Decreasing
THERMAL SHUTDOWN
TSD
TSD-HYS
Thermal Shutdown Threshold
Thermal Shutdown
Hysteresis
Min Typ Max Units
0.7 1.5
Ω
1.7 3 4 V
400 mV
165 ˚C
25 ˚C
THERMAL RESISTANCE
θJA Junction to Ambient
MUA Package
200 ˚C/W
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see Electrical Characteristics.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Note 3: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading.
Note 4: Typical specifications represent the most likely parametric norm at 25˚C operation.
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Application Information (Continued)
MINIMUM OUTPUT VOLTAGE
The minimum recommended on-time for the LM3402/02HV
is 300 ns. This lower limit for tON 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, CB. VCC charges CB 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 VCC voltage minus the diode
drop to drive the MOSFET when its source voltage is equal
to VIN.
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. A logic 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, fDIM, and duty cycle, DDIM, 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, fDIM 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
VSNS exceeds 300 mV. This threshold provides a hard limit
for the output current. Output current overshoot is limited to
300 mV / RSNS by this comparator during transients.
The OVP/OCP comparator can also be used to prevent the
output voltage from rising to VO(MAX) 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 VSNS to fall to zero,
commanding maximum duty cycle. Figure 2 shows a method
using a zener diode, Z1, and zener limiting resistor, RZ, to
limit output voltage to the reverse breakdown voltage of Z1
plus 200 mV. The zener diode reverse breakdown voltage,
VZ, must be greater than the maximum combined VF of all
LEDs in the array. The maximum recommended value for RZ
is 1 kΩ.
As discussed in the Maximum Output Voltage section, there
is a limit to how high VO can rise during an output open-
circuit that is always less than VIN. If no output capacitor is
used, the output stage of the LM3402/02HV is capable of
withstanding VO(MAX) indefinitely, however the voltage at the
output end of the inductor will oscillate and can go above VIN
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 VO(MAX) in-
definitely as long as CO is rated to handle VIN. 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.
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| PDF Descargar | [ Datasheet LM3402HV.PDF ] | |
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