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LT3478IFE-1#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3478/LT3478-1
16
34781f
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(5) Higher output capacitor value improves PDR
(6) Choose the schottky diode (D2, Figure 9) for minimum
reverse leakage
See Typical Performance Characteristics graph “LED Cur-
rent vs PWM Duty Cycle”.
Soft-Start
To limit inrush current and output voltage overshoot dur-
ing startup/recovery from a fault condition, the LT3478/
LT3478-1 provide a soft-start pin SS. The SS pin is used
to program switch current ramp up timing using a ca-
pacitor to ground. The LT3478/LT3478-1 monitor system
parameters for the following faults: V
IN
<2.8V,
⎯
S
⎯
H
⎯
D
⎯
N <1.4,
inductor current >6A and boosted output voltage >OVP.
On detection of any of these faults, the LT3478/LT3478-1
stop switching immediately and a soft-start latch is set
causing the SS pin to be discharged (see Timing Diagram
for the SS pin in Figure 11). When all faults no longer ex-
ist and the SS pin has been discharged to at least 0.25V,
the soft-start latch is reset and an internal 12µA supply
charges the SS pin. A gradual ramp up of SS pin voltage
is equivalent to a ramp up of switch current limit until SS
exceeds V
C
.
The ramp rate of the SS pin is given by:
ΔV
SS
/Δt = 12µA/C
SS
To limit inductor current overshoot to <0.5A when SS
charges past the V
C
level required for loop control, the C
SS
capacitor should be chosen using the following formula:
C
SS(MIN)
= C
C
(7.35 – 0.6(I
LED
• V
OUT
/V
S
))
Example: V
S
= 8V, V
OUT
= 16V, I
LED
= 1.05A, C
C
= 0.1µF,
C
SS(MIN)
= 0.1µF (7.35 – 0.6(1.05 • 16/8))
= 0.612µF (choose 0.68µF).
High Inductor Current “Inrush” Protection
The LT3478/LT3478-1 provide an integrated resistor
between the V
S
and L pins to monitor inductor current
(Figure 1). During startup or “hotplugging” of the induc-
tor supply, it is possible for inductor currents to exceed
the maximum switch current limit. When inductor current
exceeds 6A, the LT3478/LT3478-1 protect the internal
power switch by turning it off and triggering a soft-start
latch. This protection prevents the switch from repetitively
turning on during excessive inductor currents by delay-
ing switching until the fault has been removed. To defeat
inductor current sensing the inductor supply should be
connected to the L pin and the V
S
pin left open. See details
in the Applications Information section “Soft-Start”.
LED Open Circuit Protection and Maximum PWM
Dimming Ratios
The LT3478/LT3478-1 LED drivers provide optimum pro-
tection from open LED faults by clamping the converter
output to a programmable overvoltage protection level
(OVP). In addition, the programmable OVP feature draws
zero current from the output during PWM = 0 to allow
higher PWM dimming ratios. This provides an advantage
over other LED driver applications which connect a resistor
divider directly from V
OUT
.
An open LED fault occurs when the connection to the
LED(s) becomes broken or the LED(s) fails open. For an
LED driver using a step-up switching regulator, an open
circuit LED fault can cause the converter output to exceed
the voltage capabilities of the regulator’s power switch,
causing permanent damage. When V
OUT
exceeds OVP, the
Figure 11. LT3478 Fault Detection and SS Pin Timing Diagram
SW
3478 F11
SS
0.65V (ACTIVE THRESHOLD)
0.25V (RESET THRESHOLD)
0.15V
SOFT-START LATCH RESET:
FAULTS TRIGGERING
SOFT-START LATCH
WITH SW TURNED OFF
IMMEDIATELY:
V
IN
< 2.8V OR
SHDN < 1.4V OR
V
OUT
> OVP OR
I
(INDUCTOR)
> 6A
SS < 0.25V AND
V
IN
> 2.8V AND
SHDN > 1.4V AND
V
OUT
< OVP AND
I
(INDUCTOR)
< 6A
SOFT-START
LATCH SET:
LT3478/LT3478-1
17
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LT3478/LT3478-1 immediately stop switching, a soft-start
latch is set and the SS pin is discharged. The SS latch can
only be reset when V
OUT
falls below OVP and the SS pin
has been discharged below 0.25V (Figure 11). If the LED(s)
simply go open circuit and are reconnected, however, the
OVP used to protect the switch might be too high for the
reconnected LED(s). The LT3478/LT3478-1 therefore allow
OVP to be programmable to protect both the LED driver
switch and the LED(s). (The minimum allowable OVP for
normal operation for a given LED string depends on the
number of LEDs and their maximum forward voltage rat-
ings.) OVP is programmed using the OVPSET pin (front
page), given by,
OVP = (OVPSET • 41)V
where the programmable range for the OVPSET pin is 0.3V
to 1V resulting in an OVP range of 12.3V to 41V.
The OVPSET pin can be programmed with a single resistor
by tapping off of the resistor divider from V
REF
used to
program CTRL1. If both CTRL1 and CTRL2 are connected
directly to V
REF
(maximum LED current setting) then OVP-
SET requires a simple 2 resistor divider from V
REF
.
Thermal Calculations
To maximize output power capability in an application
without exceeding the LT3478/LT3478-1 125°C maximum
operational junction temperature, it is useful to be able
to calculate power dissipation within the IC. The power
dissipation within the IC comes from four main sources:
switch DC loss, switch AC loss, Inductor and LED cur-
rent sensing and input quiescent current. These formulas
assume a boost converter architecture, continuous mode
operation and no PWM dimming.
(1) Switch DC loss = P
SW(DC)
= (R
SW
• I
L(AVE)
2
• D)
R
SW
= switch resistance = 0.07Ω (at T
J
= 125°C)
I
L(AVE)
= P
OUT
/(η • V
S
)
P
OUT
= V
OUT
• I
LED
η = converter effi ciency = P
OUT
/(P
OUT
+ P
LOSS
)
V
S
= inductor supply input
D = switch duty cycle = (V
OUT
+ V
F
– V
S
)/(V
OUT
+ V
F
– V
SAT
)
V
F
= forward voltage drop of external Schottky diode
V
SAT
= I
L(AVE)
• R
SW
(2) Switch AC loss = P
SW(AC)
= t
EFF
(1/2)I
L(AVE)
(V
OUT
+ V
F
)(F
OSC
)
t
EFF
= effective switch current and switch V
CE
voltage
overlap time during turn on and turn off = 2 • (t
ISW
+
t
VSW
)
t
ISW
= I
SWITCH
rise/fall time = I
L(AVE)
• 2ns
t
VSW
= SW fall/rise time = (V
OUT
+ V
F
) • 0.7ns
f
OSC
= switching frequency
(3) Current sensing loss = P
SENSE
=
P
SENSE(IL)
+ P
SENSE(ILED)
P
SENSE(IL)
= I
L(AVE)
2
• 9.5mΩ
P
SENSE(ILED)
= I
LED
2
• 0.1Ω
(4) Input quiescent loss = P
Q
= V
IN
• I
Q
where
I
Q
= (6.2mA + (100mA • D))
Example (Using LT3478-1):
For V
IN
= V
S
= 8V, I
LED
= 700mA, V
OUT
= 24.5V (7 LEDs),
V
F
= 0.5V and f
OSC
= 0.2Mhz,
η = 0.89 (initial assumption)
I
L(AVE)
= (24.5 • 0.7)/(0.89 • 8) = 2.41A
D = (24.5 + 0.5 – 8)/(24.5 + 0.5 – 0.17) = 0.684
T
EFF
= 2 • ((2.41 • 2)ns + (24.5 + 0.5) • 0.7)ns = 45ns
Total Power Dissipation:
P
IC
= P
SW(DC)
+ P
SW(AC)
+ P
SENSE
+ P
Q
P
SW(DC)
= 0.07 • (2.41)
2
• 0.684 = 0.278W
P
SW(AC)
= 45ns • 0.5 • 2.41 • 25 • 0.2MHz = 0.271W
P
SENSE
= ((2.41)
2
• 0.0095) + ((0.7)
2
• 0.1) = 0.104W
P
Q
= 8 • (6.2mA + (100mA • 0.684)) = 0.597W
P
IC
= 0.278 + 0.271 + 0.104 + 0.597 = 1.25W
LT3478/LT3478-1
18
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Local heating from the nearby inductor and Schottky diode
will also add to the fi nal junction temperature of the IC.
Based on empirical measurements, the effect of diode and
inductor heating on the LT3478-1 junction temperature
can be approximated as:
ΔT
J
(LT3478-1) = 5°C/W • (P
DIODE
+ P
INDUCTOR
)
P
DIODE
= (1 – D) • V
F
• I
L(AVE)
1 – D = 0.316
V
F
= 0.5V
I
L(AVE)
= 2.41
P
DIODE
= 0.316 • 0.5 • 2.41 = 0.381W
P
INDUCTOR
= I
L(AVE)
2
• DCR
DCR = inductor DC resistance (assume 0.05Ω)
P
INDUCTOR
= (2.41)
2
• 0.05 = 0.29W
The LT3478/LT3478-1 use a thermally enhanced FE pack-
age. With proper soldering to the Exposed Pad on the
underside of the package combined with a full copper plane
underneath the device, thermal resistance (θ
JA
) will be
about 35°C/W. For an ambient temperature of T
A
= 70°C,
the junction temperature of the LT3478-1 for the example
application described above, can be calculated as:
T
J
(LT3478-1)
= T
A
+ θ
JA
(P
TOT
) + 5(P
DIODE
+ P
INDUCTOR
)
= 70 + 35(1.25) + 5(0.671)
= 70 + 44 + 4
= 118°C
In the above example, effi ciency was initially assumed to
be η = 0.89. A lower effi ciency (η) for the converter will
increase I
L(AVE)
and hence increase the calculated value
for T
J
. η can be calculated as:
η = P
OUT
/(P
OUT
+ P
LOSS
)
P
OUT
= V
OUT
• I
LED
= 17.15W
P
LOSS
(estimated) = P
IC
+ P
DIODE
+ P
INDUCTOR
= 1.92W
η = 17.15/(17.15 + 1.92) = 0.9
If an application is built, the inductor current can be mea-
sured and a new value for junction temperature estimated.
Ideally a thermal measurement should be made to achieve
the greatest accuracy for T
J
.
Note: The junction temperature of the IC can be reduced
if a lower V
IN
supply is available – separate from the
inductor supply V
S
. In the above example, driving V
IN
from an available 3V source (instead of V
S
= 8V) reduces
input quiescent losses in item(4) from 0.597W to 0.224W,
resulting in a reduction of T
J
from 118°C to 105°C.
Layout Considerations
As with all switching regulators, careful attention must be
given to PCB layout and component placement to achieve
optimal thermal,electrical and noise performance (Figure
12). The exposed pad of the LT3478/LT3478-1 (Pin 17)
is the only GND connection for the IC. The exposed pad
should be soldered to a continuous copper ground plane
underneath the device to reduce die temperature and
maximize the power capability of the IC. The ground path
for the R
T
resistor and V
C
capacitor should be taken from
nearby the analog ground connection to the exposed pad
(near Pin 9) separate from the power ground connection
to the exposed pad (near Pin 16). The bypass capacitor
for V
IN
should be placed as close as possible to the V
IN
pin and the analog ground connection. SW pin voltage rise
and fall times are designed to be as short as possible for
maximum effi ciency. To reduce the effects of both radiated
and conducted noise, the area of the SW trace should be
kept as small as possible. Use a ground plane under the
switching regulator to minimize interplane coupling. The
schottky diode and output capacitor should be placed as
close as possible to the SW node to minimize this high
frequency switching path. To minimize LED current sensing
errors for the LT3478, the terminals of the external sense
resistor R
SENSE
should be tracked to the V
OUT
and LED
pins separate from any high current paths.
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LT3478IFE-1#TRPBF

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Buy LT3478IFE-1#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LED Lighting Drivers 4.5A 42V, 2.25MHz Boost LED Driver w/ True Color PWM Dimming
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