LM3402MRX National Semiconductor, LM3402MRX Datasheet - Page 12

• Manufacturer Part Number: LM3402MRX
• Description: LM3402/LM3402HV 0.5A Constant Current Buck Regulator for Driving High Power LEDs; Package: PSOP; No of Pins: 8; Qty per Container: 2500; Container: Reel
• Manufacturer: National Semiconductor

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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. Operation 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
MOSFET is off. When the MOSFET turns on, the internal
diode reverse biases. This creates a floating supply equal to
the V
when its source voltage is equal to V
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 frequen-
cy, f
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
steady state switching frequency in order to prevent aliasing.
PEAK CURRENT LIMIT
The current limit comparator of the LM3402/02HV will engage
whenever the power MOSFET current (equal to the inductor
DIM
B
. V
DIM
should be at least one order of magnitude lower than the
CC
CC
, and duty cycle, D
charges C
voltage minus the diode drop to drive the MOSFET
B
through the internal diode while the power
DIM
, are limited by the LED current
IN
.
FIGURE 2. Output Open Circuit Protection
12
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 con-
tinue, creating a low-power hiccup mode, minimizing thermal
stress on the LM3402/02HV and the external circuit compo-
nents.
OVER-VOLTAGE/OVER-CURRENT COMPARATOR
The CS pin includes an output over-voltage/over-current
comparator that will disable the power MOSFET whenever
V
for the output current. Output current overshoot is limited to
300 mV / R
The OVP/OCP comparator can also be used to prevent the
output voltage from rising to V
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
maximum duty cycle. Figure 2 shows a method using a zener
diode, Z1, and zener limiting resistor, R
age to the reverse breakdown voltage of Z1 plus 200 mV. The
zener diode reverse breakdown voltage, V
than the maximum combined V
maximum recommended value for R
As discussed in the Maximum Output Voltage section, there
is a limit to how high V
that is always less than V
output stage of the LM3402/02HV is capable of withstanding
V
the inductor will oscillate and can go above V
A small (typically 10 nF) capacitor across the LED array
dampens this oscillation. For circuits that use an output ca-
pacitor, the system can still withstand V
long as C
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.
SNS
O(MAX)
exceeds 300 mV. This threshold provides a hard limit
indefinitely, however the voltage at the output end of
O
SNS
is rated to handle V
by this comparator during transients.
O
can rise during an output open-circuit
IN
. If no output capacitor is used, the
SNS
O(MAX)
F
IN
of all LEDs in the array. The
. The high current paths are
to fall to zero, commanding
20192112
in the event of an output
Z
is 1 kΩ.
Z
O(MAX)
, to limit output volt-
Z
, must be greater
IN
indefinitely as
or below 0V.