LM3404MA/NOPB National Semiconductor, LM3404MA/NOPB Datasheet - Page 14

• Manufacturer Part Number: LM3404MA/NOPB
• Description: IC LED DRVR HP CONST CURR 8-SOIC
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3404MA/NOPB

• Constant Current: Yes
• Topology: PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: Automotive
• Type - Secondary: High Brightness LED (HBLED), White LED
• Frequency: 1MHz
• Voltage - Supply: 6 V ~ 42 V
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Operating Temperature: -40°C ~ 125°C
• Current - Output / Channel: 1A
• Internal Switch(s): Yes
• Efficiency: 96%
• Current, Input Bias: 0.1 μA
• Current, Output: 1.2 A
• Current, Supply: 625 μA
• Package Type: SOIC
• Regulator Type: Buck (Step-Down), Switching
• Temperature, Operating, Range: -40 to +125 °C
• Time, Fall: 20 ns
• Time, Rise: 20 ns
• Voltage, Input: 6 to 42 V
• Voltage, Output: 7 V
• For Use With: 551600000-001A/NOPB - BOARD WEBENCH SO8/SOP LM3404/2LM3404EVAL - BOARD EVALUATION LM3404
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Lead Free Status / Rohs Status: RoHS Compliant part Electrostatic Device
• Other names: *LM3404MA; *LM3404MA/NOPB; LM3404MA; LM3404MATR; LM3404MATR

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Small values of C
also be used to control EMI generated by the switching action
of the LM3404/04HV. EMI reduction becomes more important
as the length of the connections between the LED and the
rest of the circuit increase.
INPUT CAPACITORS
Input capacitors at the VIN pin of the LM3404/04HV are se-
lected using requirements for minimum capacitance and rms
ripple current. The input capacitors supply pulses of current
approximately equal to I
are charged up by the input voltage while the power MOSFET
is off. Switching converters such as the LM3404/04HV have
a negative input impedance due to the decrease in input cur-
rent as input voltage increases. This inverse proportionality of
input current to input voltage can cause oscillations (some-
times called ‘power supply interaction’) if the magnitude of the
negative input impedance is greater the the input filter
impedance. Minimum capacitance can be selected by com-
paring the input impedance to the converter’s negative resis-
tance; however this requires accurate calculation of the input
voltage source inductance and resistance, quantities which
can be difficult to determine. An alternative method to select
the minimum input capacitance, C
imum input voltage ripple which can be tolerated. This value,
Δv
the converter on-time, when C
C
A good starting point for selection of C
voltage ripple of 5% to 10% of V
tance of 2x the C
LM3404/04HV circuits. To determine the rms current rating,
the following formula can be used:
Ceramic capacitors are the best choice for the input to the
LM3404/04HV due to their high ripple current rating, low ESR,
low cost, and small size compared to other types. When se-
lecting a ceramic capacitor, special attention must be paid to
the operating conditions of the application. Ceramic capaci-
tors can lose one-half or more of their capacitance at their
rated DC voltage bias and also lose capacitance with ex-
tremes in temperature. A DC voltage rating equal to twice the
expected maximum input voltage is recommended. In addi-
tion, the minimum quality dielectric which is suitable for
switching power supply inputs is X5R, while X7R or better is
preferred.
RECIRCULATING DIODE
The LM3404/04HV is a non-synchronous buck regulator that
requires a recirculating diode D1 (see the Typical Application
circuit) to carrying the inductor current during the MOSFET
off-time. The most efficient choice for D1 is a Schottky diode
due to low forward drop and near-zero reverse recovery time.
D1 must be rated to handle the maximum input voltage plus
any switching node ringing when the MOSFET is on. In prac-
tice all switching converters have some ringing at the switch-
ing node due to the diode parasitic capacitance and the lead
inductance. D1 must also be rated to handle the average cur-
rent, I
IN(MIN)
IN(MAX)
D
, calculated as:
can be selected with the following:
, is equal to the change in voltage across C
O
that do not significantly reduce Δi
IN(MIN)
F
while the power MOSFET is on, and
value is recommended for all
IN
IN
IN(MIN)
supplies the load current.
. A minimum input capaci-
, is to select the max-
IN
is to use an input
IN
during
F
can
14
This calculation should be done at the maximum expected
input voltage. The overall converter efficiency becomes more
dependent on the selection of D1 at low duty cycles, where
the recirculating diode carries the load current for an increas-
ing percentage of the time. This power dissipation can be
calculating by checking the typical diode forward voltage,
V
multiplying it by I
junction-to-ambient thermal resistance, θ
used to estimate the operating die temperature of the device.
Multiplying the power dissipation (P
the temperature rise. The diode case size can then be se-
lected to maintain the Schottky diode temperature below the
operational maximum.
LED CURRENT DURING DIM MODE
The LM3402 contains high speed MOSFET gate drive cir-
cuitry that switches the main internal power MOSFET be-
tween “on” and “off” states. This circuitry uses current derived
from the VCC regulator to charge the MOSFET during turn-
on, then dumps current from the MOSFET gate to the source
(the SW pin) during turn-off. As shown in the block diagram,
the MOSFET drive circuitry contains a gate drive under-volt-
age lockout (UVLO) circuit that ensures the MOSFET remains
off when there is inadequate VCC voltage for proper operation
of the driver. This watchdog circuitry is always running in-
cluding during DIM and shutdown modes, and supplies a
small amount of current from VCC to SW. Because the SW
pin is connected directly to the LEDs through the buck induc-
tor, this current returns to ground through the LEDs. The
amount of current sourced is a function of the SW voltage, as
shown in .
Though most power LEDs are designed to run at several
hundred milliamps, some can be seen to glow with a faint light
at extremely low current levels, as low as a couple microamps
in some instances. In lab testing, the forward voltage was
found to be approximately 2V for LEDs that exhibited visible
light at these low current levels. For LEDs that did not show
light emission at very low current levels, the forward voltage
was found to be around 900mV. It is important to remember
that the forward voltage is also temperature dependent, de-
D
, from the I-V curve on the product datasheet and then
FIGURE 5. LED Current From SW Pin
D
. Diode datasheets will also provide a typical
I
D
= (1 – D) x I
D
F
= I
D
x V
JA
, which can be
D
) by θ
20205457
JA
gives