LM3406_09 Datasheet, PDF (13/26 Pages) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

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 LM3406/06HV are se-
lected using requirements for minimum capacitance and rms
ripple current. The input capacitors supply pulses of current
approximately equal to I_F while the power MOSFET is on, and
are charged up by the input voltage while the power MOSFET
is off. All switching regulators have a negative input
impedance due to the decrease in input current as input volt-
age increases. This inverse proportionality of input current to
input voltage can cause oscillations (sometimes 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 comparing the input
impedance to the converter’s negative resistance; 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_IN(MIN), is to select the maximum input voltage
ripple which can be tolerated. This value, Δv_IN(MAX), is equal
to the change in voltage across C_IN during the converter on-
time, when C_IN supplies the load current. C_IN(MIN) can be
selected with the following:
A good starting point for selection of C_IN is to use an input
voltage ripple of 5% to 10% of V_IN. A minimum input capaci-
tance of 2x the C_IN(MIN) value is recommended for all
LM3406/06HV circuits. To determine the rms current rating,
the following formula can be used:
Ceramic capacitors are the best choice for the input to the
LM3406/06HV 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 LM3406/06HV 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_D, calculated as:
I_D = (1 – D) x I_F
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_D, from the I-V curve on the product datasheet and then
multiplying it by I_D. Diode datasheets will also provide a typical
junction-to-ambient thermal resistance, θ_JA, which can be
used to estimate the operating die temperature of the device.
Multiplying the power dissipation (P_D = I_D x V_D) by θ_JA gives
the temperature rise. The diode case size can then be se-
lected to maintain the Schottky diode temperature below the
operational maximum.
Design Example 1
The first example circuit uses the LM3406 to create a flexible
LED driver capable of driving anywhere from one to five white
series-connected LEDs at a current of 1.5A ±5% from a reg-
ulated DC voltage input of 24V ±10%. In addition to the ±5%
tolerance specified for the average output current, the LED
ripple current must be controlled to 10%_P-P of the DC value,
or 150 mA_P-P. The typical forward voltage of each individual
LED at 1.5A is 3.9V, hence the output voltage ranges from
4.1V to 19.7V, adding in the 0.2V drop for current sensing. A
complete bill of materials can be found in Table 1 at the end
of this datasheet.
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