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LT3956
APPLICATIONS INFORMATION
T SS = C SS ?
T SW ? V LED ( V IN – V LED )
L BUCK =
L BUCK-BOOST =
T SW ? V IN ( V LED – V IN )
L BOOST =
OutputCapacitorSelection
The selection of the output capacitor depends on the load
and converter configuration, i.e., step-up or step-down
and the operating frequency. For LED applications, the
equivalent resistance of the LED is typically low and the
output filter capacitor should be sized to attenuate the
current ripple. Use of an X7R type ceramic capacitor is
recommended.
To achieve the same LED ripple current, the required filter
capacitor is larger in the boost and buck-boost mode ap-
plications than that in the buck mode applications. Lower
operating frequencies will require proportionately higher
capacitor values.
Soft-Start Capacitor Selection
For many applications, it is important to minimize the
inrush current at start-up. The built-in soft-start circuit
significantly reduces the start-up current spike and output
voltage overshoot. The soft-start interval is set by the soft-
start capacitor selection according to the equation:
2 V
10 μA
A typical value for the soft-start capacitor is 0.01μF. The
soft-start pin reduces the oscillator frequency and the
maximum current in the switch. The soft-start capacitor
is discharged when EN/UVLO falls below its threshold,
during an overtemperature event or during an INTV CC un-
dervoltage event. During start-up with EN/UVLO, charging
of the soft-start capacitor is enabled after the first PWM
high period.
it is important to consider diode leakage, which increases
with the temperature, from the output during the PWM
low interval. Therefore, choose the Schottky diode with
sufficiently low leakage current. Table 3 has some recom-
mended component vendors.
Table 3. Schottky Rectifier Manufacturers
VENDOR WEB SITE
On Semiconductor www.onsemi.com
Diodes, Inc. www.diodes.com
Central Semiconductor www.centralsemi.com
Inductor Selection
The inductor used with the LT3956 should have a saturation
current rating appropriate to the maximum switch current
of 4.6A. Choose an inductor value based on operating
frequency, input and output voltage to provide a current
mode signal of approximately 0.6A magnitude. The fol-
lowing equations are useful to estimate the inductor value
(T SW = 1/f OSC ):
V IN ? 0 . 6 A
T SW ? V LED ? V IN
( V LED + V IN ) ? 0 . 6 A
V LED ? 0 . 6 A
Table 4 provides some recommended inductor vendors.
Table 4. Inductor Manufacturers
VENDOR WEB SITE
Schottky Rectifier Selection
The power Schottky diode conducts current during the
interval when the switch is turned off. Select a diode rated
for the maximum SW voltage of the application and the
RMS diode current. If using the PWM feature for dimming,
Sumida
Würth Elektronik
Coiltronics
Renco
Coilcraft
www.sumida.com
www.we-online.com
www.cooperet.com
www.rencousa.com
www.coilcraft.com
3956f
  
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