LT3956
3956f
applicaTions inForMaTion
will be enabled after the device temperature drops 10°C.
This function is intended to protect the device during
momentary overload conditions.
The major contributors to internal power dissipation are
the current in the linear regulator to drive the switch, and
the ohmic losses in the switch. The linear regulator power
is proportional to V
IN
and switching frequency, so at high
V
IN
the switching frequency should be chosen carefully
to ensure that the IC does not exceed a safe junction
temperature. The internal junction temperature of the IC
can be estimated by:
T
J
= T
A
+ [V
IN
• (I
Q
+ f
SW
• 7nC) + I
SW
2
• 0.14Ω • D
SW
]
•
θ
JA
where T
A
is the ambient temperature, I
Q
is the quiescent
current of the part (maximum 1.7mA) and
θ
JA
is the
package thermal impedance (43°C/W for the 5mm
×
6mm
QFN package). For example, an application with T
A(MAX)
=
85°C, V
IN(MAX)
= 60V, f
SW
= 400kHz, and having an average
switching current of 2.5A at 70% duty cycle, the maximum
IC junction temperature will be approximately:
T
J
= 85°C + [(2.5A)
2
• 0.14Ω • 0.7 + 60V •
(1.7mA + 400kHz • 7nC)] • 43°C/W= 123°C
The Exposed Pads on the bottom of the package must be
soldered to a plane. These should then be connected to inter-
nal copper planes with thermal vias placed directly under
the package to spread out the heat dissipated by the IC.
Open LED Detection
The LT3956 provides an open-drain status pin,
VMODE
,
that pulls low when the FB pin is within ~50mV of its 1.25V
regulated voltage. If the open LED clamp voltage is pro-
grammed correctly using the FB pin, then the FB pin should
never exceed 1.1V when LEDs are connected, therefore, the
only way for the FB pin to be within 50mV of the regulation
voltage is for an open LED event to have occurred.
Input Capacitor Selection
The input capacitor supplies the transient input current for
the power inductor of the converter and must be placed
and sized according to the transient current requirements.
The switching frequency, output current and tolerable input
voltage ripple are key inputs to estimating the capacitor
value. An X7R type ceramic capacitor is usually the best
choice since it has the least variation with temperature and
DC bias. Typically, boost and SEPIC converters require a
lower value capacitor than a buck mode converter. As-
suming that a 100mV input voltage ripple is acceptable,
the required capacitor value for a boost converter can be
estimated as follows:
C
I
V
V
T
µF
A µs
IN µF
LED A
OUT
IN
SW µs
( )
( )
( )
•
•
•
•
=
1
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
IN
voltage ripple of 100mV, the input capaci-
tor for a buck converter can be estimated as follows:
C
I
T
µF
A µs
IN µF
LED A
SW µs
( )
( )
( )
•
•
.
•
=
4 7
A 10µF input capacitor is an appropriate selection for a
400kHz buck mode converter with a 1A load.
In the buck mode configuration, 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 PGND
return of the switch. 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:
I
IN(RMS)
=
( )
I
D D
LED
•
–
•
1
where D is the switch duty cycle.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER
WEB SITE
TDK
www.tdk.com
Kemet
www.kemet.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com