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LT3954IUHE

40VIN LED Converter with Internal PWM Generator

Linear Technology 

LT3954

APPLICATIONS INFORMATION

VIN(MAX) = 40V, fSW = 400kHz, and having an average ILED

switching current of 4A at 70% duty cycle, the maximum

IC junction temperature will be approximately:

RLED

TJ = 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.

ISP

–

25mV

ISN

+

C/10

COMPARATOR

1.2V –

FB

PWM

+

OPEN LED

COMPARATOR

VMODE

SQ

1mA

R

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 VISP-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. Figure 9

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

1. VMODE ASSERTS WHEN VISP-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

3954 F09

Figure 9. VMODE (CV Mode) Logic Block Diagram

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:

CIN(µF) = ILED(A) •

VOUT

VIN

•

tSW

(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 VIN voltage ripple of 100mV, the input ca-

pacitor for a buck converter can be estimated as follows:

CIN(µF)

=

IL E

D(A)

•

t

S

W

(µs)

•

4

.

7

•





A

µ

•

F

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

3954fa

18

For more information www.linear.com/3954

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