LM3406HVEVAL National Semiconductor, LM3406HVEVAL Datasheet - Page 13

• Manufacturer Part Number: LM3406HVEVAL
• Description: LM3406/06HV 1.5A Constant Current Buck Regulator for Driving High Power LEDs; Qty per Container: 1/
• Manufacturer: National Semiconductor

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
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
ripple which can be tolerated. This value, Δv
to the change in voltage across C
time, when C
selected with the following:
A good starting point for selection of C
voltage ripple of 5% to 10% of V
tance of 2x the C
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:
IN
IN(MIN)
supplies the load current. C
, is to select the maximum input voltage
IN(MIN)
F
while the power MOSFET is on, and
value is recommended for all
IN
IN
. A minimum input capaci-
during the converter on-
IN
is to use an input
IN(MAX)
IN(MIN)
, is equal
can be
13
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
multiplying it by I
junction-to-ambient thermal resistance, θ
used to estimate the operating die temperature of the device.
Multiplying the power dissipation (P
the temperature rise. The diode case size can then be se-
lected to maintain the Schottky diode temperature below the
operational maximum.
Transient Protection
Considerations
Considerations need to be made when external sources,
loads or connections are made to the switching converter cir-
cuit due to the possibility of Electrostatic Discharge (ESD) or
Electric Over Stress (EOS) events occurring and damaging
the integrated circuit (IC) device. All IC device pins contain
zener based clamping structures that are meant to clamp
ESD. ESD events are very low energy events, typically less
than 5µJ (microjoules). Any event that transfers more energy
than this may damage the ESD structure. Damage is typically
represented as a short from the pin to ground as the extreme
localized heat of the ESD / EOS event causes the aluminum
metal on the chip to melt, causing the short. This situation is
common to all integrated
CS PIN PROTECTION
When hot swapping in a load (e.g. test points, load boards,
LED stack), any residual charge on the load will be immedi-
ately transferred through the output capacitor to the CS pin,
which is then damaged as shown in
event due to the residual charge from the load is represented
as V
From measurements, we know that the 8V ESD structure on
the CS pin can typically withstand 25mA of direct current
(DC). Adding a 1kΩ resistor in series with the CS pin, shown
in
pass through the discrete sense resistor rather than the de-
vice. The series resistor limits the peak current that can flow
during a transient event, thus protecting the CS pin. With the
1kΩ resistor shown, a 33V, 49A transient on the LED return
connector terminal could be absorbed as calculated by:
This is an extremely high energy event, so the protection
measures previously described should be adequate to solve
this issue.
D
Figure
, from the I-V curve on the product datasheet and then
TRANSIENT
8, results in the majority of the transient energy to
.
V = 25mA * 1kΩ + 8V = 33V
D
. Diode datasheets will also provide a typical
I = 33V / 0.67Ω = 49A
I
D
= (1 – D) x I
Figure 7
D
F
= I
D
x V
JA
below. The EOS
, which can be
D
) by θ
www.national.com
JA
gives