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LT3954IUHE

40VIN LED Converter with Internal PWM Generator

Linear Technology 

LT3954

APPLICATIONS INFORMATION

100

CPWM = 47nF

80

60

40

20

0

0

2

4

6

8

DIM VOLTAGE (V)

3954 F04

Figure 4. PWMOUT Duty Ratio vs DIM Voltage for RDIM = 124k

using the internal signal generator is about 4% if the DIM/

SS pin is used to adjust the dimming ratio. Consult the

factory for techniques for and limitations of generating a

duty ratio less than 4% using the internal generator. For

always on operation, the PWM pin should be connected

as shown in the application Boost LED Driver with Output

Short-Circuit Protection.

Internal PWM Oscillator Operation

The PWM oscillator operation is similar to a 555 timer (bi-

stable multi-vibrator). However, the currents that charge

and discharge the capacitor are not directly proportional

to the controlling current.

IPULL-UP = F1(IDIM/SS) = 7.2μA • exp(0.056 • IDIM/SS)

IPULL-DOWN = F2(IDIM/SS) = 84μA • exp(–0.056 • IDIM/SS)

The negative sign in the exponential makes IPULL-DOWN

decrease when IDIM/SS increases.

Voltage on the external cap ramps up at dV/dt = IPULL-UP

/CPWM. When the PWM pin reaches the high threshold

(0.8V + F3(IDIM/SS)), the flip flop SETs and IPULL-UP goes

to zero and current IPULL-DOWN goes to F2(IDIM/SS).

Duty Cycle = T1

T1+ T2

T1=

dV

IPUCLLP−WDMOWN







T2

=

dV

IPCUPLWL−MUP







After simplification, one can obtain the formula for duty

cycle of PWMOUT as a function of IDIM/SS:

CPWM

F1 (IDIM/SS)

PWM

F2 (IDIM/SS)

0.8V + F3 (IDIM/SS) –

+

–

0.8V +

S

RQ

PWMINT

FAULT

1.5mA

VPWM

VPWMINT

VTH1 = 0.8 + F3 (IDIM/SS)

dV = F3 (IDIM/SS)

VTH2 = 0.8V

t1

t2

dV/dt = IPULL-UP/ CPWM

dV/dt = IPULL-DOWN / CPWM

3954 F05

Figure 5. Internal PWM Oscillator Logic and Waveform

For more information www.linear.com/3954

3954fa

15

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