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LT3478

4.5A Monolithic LED Drivers with True Color PWM Dimming

Linear Technology 

LT3478/LT3478-1

APPLICATIO S I FOR ATIO

Shutdown and Programming Undervoltage Lockout

The LT3478/LT3478-1 have an accurate 1.4V shutdown

threshold at the ⎯S⎯H⎯D⎯N pin. This threshold can be used in

conjunction with a resistor divider from the system input

supply to define an accurate undervoltage lockout (UVLO)

threshold for the system (Figure 2). ⎯S⎯H⎯D⎯N pin current

hysteresis allows programming of hysteresis voltage for

this UVLO threshold. Just before part turn on, 10µA flows

into the ⎯S⎯H⎯D⎯N pin. After part turn on, 0µA flows from the

S⎯ H⎯ D⎯ N⎯ pin. Calculation of the on/off thresholds for a system

input supply using the LT3478/LT3478-1 ⎯S⎯H⎯D⎯N pin can

be made as follows:

VSUPPLY OFF = 1.4 [1 + R1/R2)]

VSUPPLY ON = VSUPPLY OFF + (10µA • R1)

An open drain transistor can be added to the resistor

divider network at the ⎯S⎯H⎯D⎯N pin to independently control

the turn off of the LT3478/LT3478-1.

VSUPPLY

R1

11 SHDN

–

Programming Switching Frequency

The switching frequency is programmed using an external

resistor (RT) connected between the RT pin and ground. The

internal free-running oscillator is programmable between

200kHz and 2.25MHz. Table 4 shows the typical RT values

required for a range of switching frequencies.

Selecting the optimum switching frequency depends

on several factors. Inductor size is reduced with higher

frequency but efficiency drops due to higher switching

losses. In addition, some applications require very high duty

cycles to drive a large number of LEDs from a low supply.

Low switching frequency allows a greater operational duty

cycle and hence a greater number of LEDs to be driven.

In each case the switching frequency can be tailored to

provide the optimum solution. When programming the

switching frequency the total power losses within the IC

should be considered. See “Thermal Calculations” in the

Applications Information section.

10000

TA = 25°C

R2

OFF ON

1.4V +

1000

10µA

3478 F02

Figure 2. Programming Undervoltage Lockout (UVLO)

with Hysteresis

With the ⎯S⎯H⎯D⎯N pin connected directly to the VIN pin, an

internal undervoltage lockout threshold exists for the VIN

pin (2.8V max). This prevents the converter from operat-

ing in an erratic mode when supply voltage is too low.

The LT3478/LT3478-1 provide a soft-start function when

recovering from such faults as ⎯S⎯H⎯D⎯N <1.4V and/or VIN

<2.8V. See details in the Applications Information section

“Soft-Start”.

100

1

10

100

RT (kΩ)

1000

3478 F03

Figure 3. Switching Frequency vs RT Resistor Value

Table 4. Switching Frequencies vs RT Values

SWITCHING FREQUENCY (MHz)

2.25

1

0.2

RT (kΩ)

9.09

31.6

200

34781f

11

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