LM3402MRX National Semiconductor, LM3402MRX Datasheet - Page 14

• 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

www.national.com
Design Considerations
SWITCHING FREQUENCY
Switching frequency is selected based on the tradeoffs 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 LM3402/02HV can be programmed
for switching frequencies ranging from the 10’s of kHz to over
1 MHz. The maximum switching frequency is limited only by
the minimum on-time requirement.
LED RIPPLE CURRENT
Selection of the ripple current, Δi
analogous to the selection of output ripple voltage in a stan-
dard voltage regulator. Where the output ripple in a voltage
regulator is commonly ±1% to ±5% of the DC output voltage,
LED manufacturers generally recommend values for Δi
ranging from ±5% to ±20% of I
allows the use of smaller inductors, smaller output capacitors,
or no output capacitors at all. The advantages of higher ripple
current are reduction in the solution size and cost. Lower rip-
ple current requires more output inductance, higher switching
frequency, or additional output capacitance. The advantages
of lower ripple current are a reduction in heating in the LED
itself and greater range of the average LED current before the
current limit of the LED or the driving circuitry is reached.
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
LM3402/02HV 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
LM3402/02HV the ripple current is described by the following
expression:
A minimum ripple voltage of 25 mV is recommended at the
CS pin to provide good signal-to-noise ratio (SNR). The CS
pin ripple voltage, ΔV
BUCK CONVERTERS WITH OUTPUT CAPACITORS
A capacitor placed in parallel with the LED or array of LEDs
can be used to reduce the LED current ripple while keeping
the same average current through both the inductor and the
LED array. This technique is demonstrated in Design Exam-
ple 1. With this topology the output inductance can be low-
F
. For a controlled on-time converter such as
ΔV
SNS
SNS
, is described by the following:
L
, also apply to the LED ripple cur-
= Δi
F
F
. Higher LED ripple current
F
x R
, through the LED array is
SNS
F
14
ered, making the magnetics smaller and less expensive.
Alternatively, the circuit could be run at lower frequency but
keep the same inductor value, improving the efficiency and
expanding the range of output voltage that can be regulated.
Both the peak current limit and the OVP/OCP comparator still
monitor peak inductor current, placing a limit on how large
Δi
capacitor is also useful in applications where the inductor or
input voltage tolerance is poor. Adding a capacitor that re-
duces Δi
changes in inductance or V
peak LED ripple current too high.
Figure 4 shows the equivalent impedances presented to the
inductor current ripple when an output capacitor, C
equivalent series resistance (ESR) are placed in parallel with
the LED array. The entire inductor ripple current flows through
R
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 LM3402/02HV. EMI reduction becomes more important
as the length of the connections between the LED and the
rest of the circuit increase.
SNS
L
can be even if Δi
to provide the required 25 mV of ripple voltage for proper
F
F
FIGURE 4. LED and C
vs. I
to well below the target provides headroom for
F
curve. Note that dividing V
O
D
C
that do not significantly reduce Δi
of one device multiplied by n. Inductor
F
assumes that the shape of the inductor
is made very small. A parallel output
IN
that might otherwise push the
O
Ripple Current
F
D
when using a parallel
. LED dynamic resis-
F
20192115
by I
F
will give an
O
, and its
F
can