LM3406_09 Datasheet, PDF (12/26 Pages) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

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 and V_O
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_IN or V_O. The maximum switch-
ing frequency is limited only by the minimum on-time and
minimum off-time requirements.
LED RIPPLE CURRENT
Selection of the ripple current, Δi_F, through the LED array is
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_F ranging
from ±5% to ±20% of I_F. Higher LED ripple current allows the
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_L, also apply to the LED ripple cur-
rent, Δi_F. For a controlled on-time converter such as
LM3406/06HV the ripple current is described by the following
expression:
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_L can be
even if Δi_F is made very small. Adding a capacitor that re-
duces Δi_F to well below the target provides headroom for
changes in inductance or V_IN that might otherwise push the
peak LED ripple current too high.
Figure 6 shows the equivalent impedances presented to the
inductor current ripple when an output capacitor, C_O, and its
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_SNS to provide the required 25 mV of ripple
voltage for proper operation of the CS comparator.
30020314
FIGURE 6. LED and C_O Ripple Current
To calculate the respective ripple currents the LED array is
represented as a dynamic resistance, r_D. LED dynamic resis-
tance is not always specified on the manufacturer’s
datasheet, but it can be calculated as the inverse slope of the
LED’s V_F vs. I_F curve. Note that dividing V_F by I_F will give an
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_D of one device multiplied by n. Inductor
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_F when using a parallel
capacitor:
The triangle-wave inductor current ripple flows through R_SNS
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_CS, should be at least 25 mV_P-P. Δv_CS is de-
scribed by the following:
Δv_CS = Δi_F x R_SNS
The calculation for Z_C assumes that the shape of the inductor
ripple current is approximately sinusoidal.
Small values of C_O that do not significantly reduce Δi_F can
also be used to control EMI generated by the switching action
of the LM3406/06HV. EMI reduction becomes more important
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