UCC2974

UCC3974

SLUS485A – JULY 2001 – REVISED NOVEMBER 2001

7

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1 OUT1

16

OUT2

2 VIN

15

GND

3 OUT1

14

OUT2

4 COMP1

13

CBP

5 FB1

12

COMP2

6RT1

11

FB2

7RT2

10

MODE

8 DIM

9

LFDSYNC

C8 1

µF

C7

C6

1

µF

R9

10

Ω

UCC3974

100

µF

35 V

R10

10

Ω

VIN

C5

10 nF

100

µF

35 V

D2

Q2

Q1

VIN

R2

750

Ω

VIN

C3

C2

33 pF

3 kV

D1

BRIGHT

SYNC

100

µF

35 V

L2

100

µH

D3

Q3

Q4

VIN

R6

750

Ω

VIN

C8

C9

33 pF

3 kV

R7

750

Ω

D4

T2

T1

Q5

Q6

R3

C4

R8 200 k

Ω

R4 20 k

Ω

R5

R12

10 k

Ω

R11

10 k

Ω

UDG–01020

R1

750

Ω

Figure 1. Typical Dual-Channel Application

low-frequency dimming (LFD)

The low-frequency dimming section of the device is implemented as either a low frequency pulse width

modulator (PWM) or as a direct digital input. In either case, the DIM pin is the controlling input. The type of DIM

input is determined at startup. As the MODE pin transitions through the LFD_ENABLE threshold, the LFDSYNC

pin is observed. When this pin is high, the DIM pin is a 2.5-V compatible logic input. When the DIM pin is high,

the output is enabled. When it is low the output is disabled. The user is required to provide the correct frequency

and duty ratio to the DIM pin in this mode. To change the mode of operation without power cycling the device,

the MODE pin must be brought below the LFD_ENABLE threshold and then brought above it with the LFDSYNC

pin held in the desired state.

To use DIM as an analog input, the LFDSYNC pin must be low when mode crosses the LFD_ENABLE threshold.

In this mode, DIM becomes an analog input that varies the amount of time that the lamp is on during the period

of the LFD oscillator. From 0.5 V to 3 V applied to the DIM pin varies the lamp on duty cycle from the programmed

minimum to 100%.

NOTE: The analog dimming signal is senitive to coupled noise from the lamps. Noise on this line

will be seen as lamp flicker. It is highly reccommended that precautions be taken to prevent noise

coupling to this signal for optimum results.

Applying a pulse train to the LFDSYNC pin will synchronize the LFD oscillator to that pulse train. The frequency

of the applied pulse train must be higher than the free running frequency of the oscillator.