4-166

TELCOM SEMICONDUCTOR, INC.

TC96C555

3A OUTPUT PROGRAMMABLE

POWER OSCILLATOR

APPLICATIONS (Cont.)

Figure 5. +5V Buck Regulator Power Supply with 82% Efficiency at 200mA Output

900

700

500

400

300

200

100

20

60 80

10

C in pF

Maximum Frequency

RCHG = 13K, Rdis = 6.8K

40

100

100K

10K

1K

100

10

1

0.1

100pF

.01

µF.1µF

10pF

C in pF

.001

µF

1

µF

Minimum Frequency

RCHG = 200K, Rdis = 100K

30

50

This circuit will convert a 6 to 15V input to a 5V output of

200 milliamps. Normal operating frequency is 50kHz. Peak

to peak ripple is 50 millivolts. A change from 100mA to

ery in 200

µs.

The TC96C555 is used here as a duty cycle modulator

in a buck output circuit. The source current is modulated to

control the duty cycle. Sink current is fixed at 100

µA with a

resistor (R4) of 10K. Transistor Q1, (2N2907A), is used for

current modulation into TC96C555 Pin 4, the charge current

program input. Shunt regulator TL431 is used for voltage

sense and regulation feedback. The TL431 has an internal

reference of 2.495V. Terminal R is compared with this

reference to control conduction of cathode C to anode A. R2

and R3 are selected to give proper bias current to the TL431.

C2 and R7 are for loop compensation and are optimized for

a recovery time of 200

µs. The TC96C555 outputs, Pin 7 and

6, are tied together so that when output is HIGH, current

inductor, L1. When output is OFF or LOW, energy stored in

L1 will continue to conduct through TC96C555 Pin 6 to the

lower internal MOSFET and out to Pin 5, the ground return.

This circuit does not have current limiting. A fuse is recom-

mended for protection. Figure 6 shows the duty cycle as a

function of the source current. Figure 8 shows the frequency

vs control voltage.

Figure 4. Typical Maximum and Minimum

Operating Frequency vs. Capacitor

VR1

VREF

VR2

VIN

1

3

5

TC96C555

OUT

OUT

VDD

GND

C1

470pF

2

4

C

R

A

TL431

100K

R7

C2

.1

µF

10K

R6

6

C3

100

µF

7

10K

R5

VOUT

L1, 3mH

R3, 390

Ω

+12V

C5

100

µF

C4

.1

µF

R2

560

Ω

R1

10K

Q1

2N2907A

8

R4

10K

C6

4.7

µF