NSC LM3404

National Semiconductor
Application Note 1853
Chris Richardson
September 23, 2008
The constant on-time (COT) control method used by the
LM3402 and LM3404 constant-current buck regulators provides a balance between control over switching frequency
and fast transient response. Normally this "quasi-hysteretic"
control senses the input voltage and adjusts the on-time tON
of the power MOSFET as needed to keep fSW constant. Investigating a little more deeply reveals that tON is in fact
proportional to the current flowing into the RON pin. The addition of a single, general purpose PNP transistor forces tON
to be proportional to (VIN - VO) and provides two benefits that
are particularly useful to LED drivers: improved tolerance of
the average LED current, IF, and constant LED ripple current,
ΔiF.
LEDs have a relationship between their luminous flux and
forward current, IF, that is linear up to a point. Beyond that
point, increasing IF causes more heat than light. High ripple
current forces the LED to spend half of the time at a high peak
current, putting it in the lower lm/W region of the flux curve.
This reduces the light output when compared to a purely DC
drive current even though the average forward current remains the same.
Close inspection of LED datasheets also reveals that the absolute maximum ratings for peak current are close to or often
equal to the ratings for average current. High current density
in the LED junction lowers lumen maintenance, providing yet
another incentive for keeping the ripple current under control.
Benefits of Constant Ripple
Circuit Performance
The luminous flux and dominant wavelength (or color temperature for white LEDs) of LED light are controlled by average current. The constant-ripple LED driver in Figure 1 is
much better at controlling average LED current over changes
in both input voltage and changes in output voltage because
it fixes the valley of the inductor current and also fixes the
current ripple.
Controlling LED ripple current implies control over peak LED
current, which in turn affects the luminous flux of an LED. All
The circuit of Figure 1 uses the PNP-based constant ripple
concept to take an input voltage of 24VDC ±10% and drive
1A through as many LEDs in series as the maximum output
voltage will allow. For a circuit with 'n' LEDs of forward voltage
VF in series, the output voltage is:
VO = 0.2 + n x VF
COT Drivers Control LED Ripple Current
COT Drivers Control LED
Ripple Current
30064501
FIGURE 1. Constant Ripple LED Driver Using the LM3404 Buck Regulator
The maximum voltage that can be achieved is then:
VO-MAX = VIN-MIN x (1 - fSW x 300 ns)
In the above equation, the 300 ns term reflects the minimum
off -time of the LM3402 and LM3404 buck regulators.
Figure 2 and Figure 3 show the dependence of ripple current
and switching frequency against output voltage. This change
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300645
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AN-1853
Making a "Universal" Current
Source
in output voltage is effectively a change in the number of series-connected LEDs that the circuit drives.
One circuit with both average current and ripple current controlled independently of VO can now power anything from a
single infrared LED (VF-TYP of ~1.8V) to as many as five white
LEDs in series, yielding a VO of ~18V. Such a circuit would be
ideal for an LED-driving power-supply module. Many of the
existing, commercial AC-input 'brick' modules for driving
LEDs are specified to provide a constant current of 'x' mA at
a voltage up to 'y' volts. Depending on the need for galvanic
AN-1853
desired peak-to-peak inductor ripple current, ΔiL. The
required inductance is then:
isolation and/or power factor correction, the LM3402 or
LM3404 buck regulator could be paired with an existing ACDC regulator to provide the 24V, resulting in a high-quality
universal current source.
2.
Select the closest standard inductor value to L and call it
LSTD. RON can then be calculated with the following
expression:
3.
4.
Use the closest 1% resistor value for RON.
Design for the remaining components (input capacitor,
Schottky diode, etc.) remains the same, and is outlined
in the LM3402 and LM3404 datasheets.
Switching Frequency Changes
When using the LM3402 and LM3404 buck regulators in the
constant-ripple configuration, the switching frequency will
change with VIN and VO. Careful attention to PCB layout and
proper filtering must be employed will all switching converters,
and particular care is needed for systems where fSW changes.
The following steps can be used to predict the switching frequency:
1. Calculate the on-time at the minimum and maximum
values of VIN and VO using the actual 1% resistor value
of RON and the following equation:
30064502
FIGURE 2. Ripple Current vs. Output Voltage
2.
The switching frequency can then be determined using
tON and the following expression:
Conclusion
A pure DC LED drive current would be ideal for LEDs, but in
practice the majority of LED lighting is powered from the AC
mains and includes at least one switching regulator between
the wall and the LEDs. Even battery or solar-powered systems are likely to employ a switching regulator in the interest
of power efficiency. Therefore, some amount of ripple current
will be present in almost every LED driver design. Allowing
higher ripple current reduces the size and cost of the drive
circuit, but comes at the expense of light output and reliability.
Armed with the ability to control both LED ripple current and
switching frequency, the LED lighting designer can make his/
her own trade-offs between solution size, cost, and quality
based on the needs of the application.
30064503
FIGURE 3. Switching Frequency vs. Output Voltage
Design Procedure
Designing for constant ripple in a COT converter requires a
change in the selection of the on-time setting resistor RON:
1. Start with the typical input voltage, VIN-TYP, and an output
voltage that is at the center between the minimum and
maximum expected value, VO-CTR. Use the maximum
permissible switching frequency, fSW-MAX, and the
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2
ID
Part Number
Type
Size
Parameters
Qty
Vendor
U1
LM3404
LED Driver
SO-8
42V, 1.2A
1
NSC
Q1
CMPT3906
PNP
SOT23-6
40 VCE, 10 mA
1
Central Semi
L1
VLF10040T-330M2R1
Inductor
10 x 10 x 4.0 mm
33 µH, 2.1A, 80 Ω 1
TDK
D1
CMSH2-40M
Schottky Diode
SMA
40V, 2A
1
Central Semi
CF
VJ0603Y104KXXAT
Capacitor
0603
100 nF, 10%
1
Vishay
CB
VJ0603Y103KXXAT
Capacitor
0603
10 nF, 10%
1
Vishay
CIN
C4532X7R1H685M
Capacitor
1812
6.8 µF, 50V
1
TDK
RSNS
ERJ8RQFR20V
Resistor
1206
0.2Ω, 1%
1
Panasonic
RON
CRCW06035762F
Resistor
0603
57.6 kΩ, 1%
1
Vishay
3
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AN-1853
BOM
COT Drivers Control LED Ripple Current
Notes
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AN-1853
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