LT3755 LINER [Linear Technology], LT3755 Datasheet - Page 15

• Manufacturer Part Number: LT3755
• Description: 40VIN, 75VOUT LED Controller
• Manufacturer: LINER [Linear Technology]

the required capacitor value for a boost converter can be
estimated as follows:
Therefore, a 4.7μF capacitor is an appropriate selection
for a 400kHz boost regulator with 12V input, 48V output
and 1A load.
With the same V
tor for a buck converter can be estimated as follows:
A 10μF input capacitor is an appropriate selection for a
400kHz buck mode converter with a 1A load.
In the buck mode confi guration, the input capacitor has
large pulsed currents due to the current returned through
the Schottky diode when the switch is off. In this buck
converter case it is important to place the capacitor as close
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. The RMS
input current for a buck mode LED driver is:
where D is the switch duty cycle.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER
TDK
Kemet
Murata
Taiyo Yuden
Output Capacitor Selection
The selection of the output capacitor depends on the load
and converter confi guration, 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 fi lter capacitor should be sized to attenuate the
APPLICATIONS INFORMATION
C
C
I
IN(RMS)
IN F
IN F
(
(
μ
μ
)
)
=
=
=
I
I
I
LED A
LED A
LED
IN
PHONE
516-535-2600
408-986-0424
814-237-1431
408-573-4150
( )
( )
voltage ripple of 100mV, the input capaci-
•
•
•
(
T
1
V
V
SW s
–
OUT
IN
D D
(
)
μ
•
•
)
WEB
www.tdk.com
www.kemet.com
www.murata.com
www.t-yuden.com
T
•
SW s
4 7
A
.
(
μ
•
μ
μ
)
s
F
•
A
1
•
μ
μ
F
s
current ripple. Use of X7R type ceramic capacitors is
recommended.
To achieve the same LED ripple current, the required fi lter
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
signifi cantly 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:
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 SHDN/UVLO falls below its threshold,
during an overtemperature event or during an INTV
undervoltage event. During start-up with SHDN/UVLO,
charging of the soft-start capacitor is enabled after the
fi rst PWM high period.
Power MOSFET Selection
For applications operating at high input or output voltages,
the power NMOS FET switch is typically chosen for drain
voltage V
of switch on-resistance, R
cause switching losses dominate power loss. The INTV
regulator on the LT3755 has a fi xed current limit to protect
the IC from excessive power dissipation at high V
FET should be chosen so that the product of Q
switching frequency does not exceed the INTV
limit. For driving LEDs be careful to choose a switch with
a V
in case of an open-load fault. Several MOSFET vendors
are listed in Table 3. The MOSFETs used in the application
circuits in this datasheet have been found to work well
with the LT3755. Consult factory applications for other
recommended MOSFETs.
T
DS
SS
rating that exceeds the threshold set by the FB pin
=
DS
C
SS
rating and low gate charge Q
•
10
2
μA
V
LT3755/LT3755-1
DS(ON)
, is usually secondary be-
G
. Consideration
G
CC
at 7V and
IN
15
current
, so the
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CC
CC