LM3406HVMHX/NOPB National Semiconductor, LM3406HVMHX/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM3406HVMHX/NOPB
• Description: IC LED DRVR HP CONS CURR 14TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3406HVMHX/NOPB

• Constant Current: Yes
• Topology: PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: Automotive
• Type - Secondary: White LED
• Frequency: 1MHz
• Voltage - Supply: 6 V ~ 75 V
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP Exposed Pad, 14-eTSSOP 14-HTSSOP
• Operating Temperature: -40°C ~ 125°C
• Current - Output / Channel: 1.5A
• Internal Switch(s): Yes
• Efficiency: 96%
• For Use With: 876-1003 - LM3406 LED DRIVER EVAL BOARDLM3406MHEVAL - BOARD EVAL FOR LM3406MH
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: LM3406HVMHX

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Design Considerations
SWITCHING FREQUENCY
Switching frequency is selected based on the trade-offs 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 LM3406/06HV can be programmed
for switching frequencies ranging from the 10’s of kHz to over
1 MHz. This on-time varies in proportion to both V
in order to maintain first-order control over switching frequen-
cy, however in practice the switching frequency will shift in
response to large swings in V
ing frequency is limited only by the minimum on-time and
minimum off-time requirements.
LED RIPPLE CURRENT
Selection of the ripple current, Δi
similar to the selection of output ripple voltage in a standard
voltage regulator. Where the output ripple in a voltage regu-
lator is commonly ±1% to ±5% of the DC output voltage, LED
manufacturers generally recommend values for Δi
from ±5% to ±20% of I
use of smaller inductors, smaller output capacitors, or no out-
put capacitors at all. Lower ripple current requires more output
inductance, higher switching frequency, or additional output
capacitance, and may be necessary for applications that are
not intended for human eyes, such as machine vision or in-
dustrial inspection.
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
LM3406/06HV 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
rent, Δi
LM3406/06HV the ripple current is described by the following
expression:
The triangle-wave inductor current ripple flows through R
and produces a triangle-wave voltage at the CS pin. To pro-
vide good signal to noise ratio (SNR) the amplitude of CS pin
ripple voltage, Δv
scribed by the following:
F
. For a controlled on-time converter such as
CS
, should be at least 25 mV
Δv
F
. Higher LED ripple current allows the
CS
L
, also apply to the LED ripple cur-
= Δi
IN
F
or V
x R
F
, through the LED array is
O
SNS
. The maximum switch-
P-P
. Δv
IN
F
CS
ranging
and V
is de-
SNS
O
12
BUCK CONVERTERS WITH OUTPUT CAPACITORS
A capacitor placed in parallel with the LED(s) can be used to
reduce the LED current ripple while keeping the same aver-
age current through both the inductor and the LED array. With
an output capacitor the output inductance can be lowered,
making the magnetics smaller and less expensive. Alterna-
tively, the circuit could be run at lower frequency but keep the
same inductor value, improving the power efficiency. Both the
peak current limit and the OVP/OCP comparator still monitor
peak inductor current, placing a limit on how large Δi
even if Δi
duces Δi
changes in inductance or V
peak LED ripple current too high.
Figure 6
inductor current ripple when an output capacitor, C
equivalent series resistance (ESR) are placed in parallel with
the LED array. Note that ceramic capacitors have so little ESR
that it can be ignored. The entire inductor ripple current still
flows through R
voltage for proper operation of the CS comparator.
To calculate the respective ripple currents the LED array is
represented as a dynamic resistance, r
tance is not always specified on the manufacturer’s
datasheet, but it can be calculated as the inverse slope of the
LED’s V
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
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
capacitor:
The calculation for Z
ripple current is approximately sinusoidal.
Small values of C
also be used to control EMI generated by the switching action
of the LM3406/06HV. EMI reduction becomes more important
F
F
shows the equivalent impedances presented to the
FIGURE 6. LED and C
F
vs. I
to well below the target provides headroom for
is made very small. Adding a capacitor that re-
F
curve. Note that dividing V
SNS
O
D
C
to provide the required 25 mV of ripple
that do not significantly reduce Δi
of one device multiplied by n. Inductor
assumes that the shape of the inductor
IN
that might otherwise push the
O
Ripple Current
F
D
when using a parallel
. LED dynamic resis-
F
30020314
by I
F
will give an
O
L
, and its
can be
F
can