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LT3954IUHE

40VIN LED Converter with Internal PWM Generator

Linear Technology 

LT3954

APPLICATIONS INFORMATION

Duty Cycle =

1

1+ 11.6 • exp(−0.112 •IDIM/SS )

To calculate the duty cycle of the internal PWM generator

given a voltage of the DIM signal, determine first the current

into the DIM/SS pin by the equation (referring to Figure 6):

IDIM/SS

= VDIM − 1.17V

RDIM + 2.5kΩ

in µA

RDIM

DIM

LT3954

PWMOUT

DIM/SS

PWM

GND

10nF

47nF

300Hz

3954 F06

Figure 6. Configuration of Dimming Resistor, RDIM

Knowing the IDIM/SS in μA , the duty cycle of the PWMOUT

pin can be calculated for the range –10μA < IDIM/SS < 55μA:

Duty (in%) =

100%

1+ 11.6 • exp(−0.112 •IDIM/SS )

These equations can be worked in reverse starting with a

desired duty cycle using 20%, for example, and solving for

a resistor value, RDIM, placed between VREF and DIM/SS:

IDIM/SS

=

8.93

•



ln11.6

•

Duty

(1− Duty )





=

8.93

•

ln11.6

•

0.2

0.8





=

9.51µA

RDIM

=

−2.5kΩ +

VREF − 1.17

IDIM/SS

= −2.5kΩ + 2.015 − 1.17 = 86.4kΩ

0.00951

For some applications, a duty cycle lower than 3% is

desired. It is possible to achieve a discrete value of duty

cycle that is lower than range attainable using DIM/SS

current. A resistor, RPD, and switch driven by PWMOUT

can be added as shown in Figure 7.

RDIM

DIM

LT3954

PWMOUT

DIM/SS

PWM

GND

10nF

RPD

47nF

300Hz

3954 F07

Figure 7. Configuration for Sub 4% PWM Dimming

The addition of this resistor increases the pull-down

current on PWM, thus decreasing the duration of the on-

phase of the switching regulator. Since PWM frequency

at low duty cycle is primarily determined by the pull-up

current, the additional pull-down current from RPD has

little effect on the PWM period, so frequency calculation

remains the same.

An example solving for RPD given a 1% duty cycle is

provided below. For this example, the IDIM/SS current

flowing in RDIM is assumed zero, which normally provides

an ~8% duty cycle. The average voltage on the PWM pin

is approximately 1.05V at this IDIM/SS setting.

Duty =

IPULL−UP

IPULL−UP +IPULL−DOWN +IRPD

=

7.2

= 0.01

7.2 + 84 +IRPD

IRPD

=

629µA

=

1.05V

RPD

Therefore, RPD ~ 1.65kΩ

Programming the Switching Frequency

The RT frequency adjust pin allows the user to program

the switching frequency (fSW) from 100kHz to 1MHz to

optimize efficiency/performance or external component

size. Higher frequency operation yields smaller compo-

nent size but increases switching losses and gate driving

current, and may not allow sufficiently high or low duty

3954fa

16

For more information www.linear.com/3954

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