LT3478-1 データシートの表示（PDF） - Linear Technology

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LT3478-1

4.5A Monolithic LED Drivers with True Color PWM Dimming

Linear Technology 

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 VOUT 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 VREF used to

program CTRL1. If both CTRL1 and CTRL2 are connected

directly to VREF (maximum LED current setting) then OVP-

SET requires a simple 2 resistor divider from VREF.

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 = PSW(DC)

= (RSW • IL(AVE)2 • D)

RSW = switch resistance = 0.07Ω (at TJ = 125°C)

IL(AVE) = POUT/(η • VS)

POUT = VOUT • ILED

η = converter efficiency = POUT/(POUT + PLOSS)

VS = inductor supply input

D = switch duty cycle = (VOUT + VF – VS)/(VOUT + VF – VSAT)

VF = forward voltage drop of external Schottky diode

VSAT = IL(AVE) • RSW

(2) Switch AC loss = PSW(AC)

= tEFF(1/2)IL(AVE)(VOUT + VF)(FOSC)

tEFF = effective switch current and switch VCE voltage

overlap time during turn on and turn off = 2 • (tISW +

tVSW)

tISW = ISWITCH rise/fall time = IL(AVE) • 2ns

tVSW = SW fall/rise time = (VOUT + VF) • 0.7ns

fOSC = switching frequency

(3) Current sensing loss = PSENSE =

PSENSE(IL) + PSENSE(ILED)

PSENSE(IL) = IL(AVE)2 • 9.5mΩ

PSENSE(ILED) = ILED2 • 0.1Ω

(4) Input quiescent loss = PQ = VIN • IQ where

IQ = (6.2mA + (100mA • D))

Example (Using LT3478-1):

For VIN = VS = 8V, ILED = 700mA, VOUT = 24.5V (7 LEDs),

VF = 0.5V and fOSC = 0.2Mhz,

η = 0.89 (initial assumption)

IL(AVE) = (24.5 • 0.7)/(0.89 • 8) = 2.41A

D = (24.5 + 0.5 – 8)/(24.5 + 0.5 – 0.17) = 0.684

TEFF = 2 • ((2.41 • 2)ns + (24.5 + 0.5) • 0.7)ns = 45ns

Total Power Dissipation:

PIC = PSW(DC) + PSW(AC) + PSENSE + PQ

PSW(DC) = 0.07 • (2.41)2 • 0.684 = 0.278W

PSW(AC) = 45ns • 0.5 • 2.41 • 25 • 0.2MHz = 0.271W

PSENSE = ((2.41)2 • 0.0095) + ((0.7)2 • 0.1) = 0.104W

PQ = 8 • (6.2mA + (100mA • 0.684)) = 0.597W

PIC = 0.278 + 0.271 + 0.104 + 0.597 = 1.25W

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