LT3954
19
3954fa
For more information www.linear.com/3954
APPLICATIONS INFORMATION
as possible to the Schottky diode and to the GND return
of the switch (i.e., the sense resistor). It is also important
to consider the ripple current rating of the capacitor. For
best reliability, this capacitor should have low ESR and
ESL and have an adequate ripple current rating.
Table 3. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER WEB
TDK www.tdk.com
Kemet www.kemet.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Output Capacitor Selection
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 X7R type ceramic capacitors 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. Connect a capacitor from the DIM/SS
pin to GND to use this feature. The soft-start interval is
set by the soft-start capacitor selection according to the
equation:
T
SS
= C
SS
•
12µA
= C
SS
•
nF
provided there is no additional current supplied to the
DIM/SS pin for programming the duty cycle of the PWM
dimming signal generator. A typical value for the soft-start
capacitor is 10nF which gives a 1ms start-up interval. The
soft-start pin reduces the oscillator frequency and the
maximum current in the switch.
The soft-start capacitor discharges if one of the follow
-
ing events occurs: the EN/UVLO falls below its threshold;
output overcurrent is detected at the ISP
/ISN pins; IC
overtemperature; or INTV
CC
undervoltage. During start-
up with EN/UVLO, charging of the soft-start capacitor is
enabled after the first PWM high period. In the start-up
sequence, after switching is enabled by PWM the switching
continues until V
ISP-ISN
> 25mV or DIM/SS > 1V. PWM
pin negative edges during this start-up interval are not
processed until one of these two conditions are met so
that the regulator can reach steady state operation shortly
after PWM dimming commences.
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,
it may be important to consider diode leakage, which in
-
creases with the temperature, from the output during the
P
WM low inter
val. Therefore, choose the Schottky diode
with sufficiently low leakage current. Table 4 has some
recommended component vendors. The diode current
and V
F
should be considered when selecting the diode
to be sure that power dissipation does not exceed the
rating of the diode. The power dissipated by the diode in
a converter is:
P
D
= I
D
• V
F
• (1-D
MAX
)
It is prudent to measure the diode temperature in steady
state to ensure that its absolute maximum ratings are not
exceeded.
Table 4. Schottky Rectifier Manufacturers
MANUFACTURER WEB
On Semiconductor www.onsemi.com
Central Semiconductor www.centralsemi.com
Diodes, Inc. www.diodes.com
