LT3954
18
3954fa
For more information www.linear.com/3954
APPLICATIONS INFORMATION
V
IN(MAX)
= 40V, f
SW
= 400kHz, and having an average
switching current of 4A at 70% duty cycle, the maximum
IC junction temperature will be approximately:
T
J
= 85°C + [(4A)
2
• 0.04Ω • 0.7 + 40V •
(2.2mA + 400kHz • 7nC)] • 34°C/W= 107°C
The exposed pad on the bottom of the package must be
soldered to a ground plane. This ground should then be
connected to an internal copper ground plane with thermal
vias placed directly under the package to spread out the
heat dissipated by the IC.
Open LED Reporting – Constant Voltage Regulation
Status Pin
The LT3954 provides an open-drain status pin, VMODE,
that pulls low when the FB pin is within 50mV of its 1.25V
regulated voltage AND output current sensed by V
ISP-ISN
has reduced to 25mV, or 10% of the full-scale value. The
10% output current qualification (C/10) is unique for an LED
driver but fully compatible with open LED indication– the
qualification is always satisfied since for an open load zero
current flows in the load. The C/10 feature is particularly
useful in the case where VMODE is used to indicate the
end of a battery charging cycle and terminate charging or
transition to a float charge mode.
For monitoring the LED string voltage, if the open LED
clamp voltage is programmed correctly using the FB
resistor divider then the FB pin should not exceed 1.18V
when LEDs are connected. If the VMODE pulldown is as
-
serted when the PWM pin transitions low, the pulldown
will continue
to be asserted until the next rising edge of
PWM even if FB falls below the VMODE threshold. Figure9
shows the VMODE logic block diagram.
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
IN
(µF) = I
LED
(A) •
V
OUT
V
IN
• t
SW
(µs) •
µF
A • µs
Therefore, a 10μF capacitor is an appropriate selection
for a 400kHz boost regulator with 12V input, 36V output
and 1A load.
With the same V
IN
voltage ripple of 100mV, the input ca-
pacitor for a buck converter can be estimated as follows:
C
IN
(µF) = I
LED
(A) • t
SW
(µs) • 4.7 •
µF
A • µs
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
Figure 9. VMODE (CV Mode) Logic Block Diagram
+
–
1.2V
FB
PWM
VMODE
OPEN LED
COMPARATOR
C/10
COMPARATOR
1mA
ISN
25mV
ISP
1. VMODE ASSERTS WHEN V
ISP-ISN
< 25mV AND FB > 1.2V, AND IS LATCHED
2. VMODE DE-ASSERTS WHEN FB < 1.19V, AND PWM = LOGIC “1”
3. ANY FAULT CONDITION RESETS THE LATCH, SO LT3955 STARTS UP
WITH VMODE DE-ASSERTED
LED
I
LED
+
–
S Q
R
+
–
