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LT3478/LT3478-1
APPLICATIO S I FOR ATIO
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 = 0.07 Ω (at T J = 125°C)
I L(AVE) = P OUT /( η ? V S )
P OUT = V OUT ? I LED
η = converter ef?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 ? 9.5m Ω
P SENSE(ILED) = I LED2 ? 0.1 Ω
(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
34781f
17
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