MC34167 MC33167

9

MOTOROLA ANALOG IC DEVICE DATA

Figure 20. Step–Up/Down Converter

4

2

1

5

3

+

CF

RF

R1

1.5 k

CO

2200

VO

28 V/0.9 A

R2

EA

Reference

Thermal

Oscillator

S

R

Q

PWM

UVLO

ILIMIT

Vin

12 V

Cin

330

+

6.8 k

4.7 k

0.47

Q1

D1

1N5825

+

*Gate resistor RG, zener diode D3, and diode D4 are required only when Vin is greater than 20 V.

L

190

µH

*RG

620

D3

1N967A

D2

1N5822

Q2

MTP3055EL

D4

1N4148

+

+

Test

Conditions

Results

Line Regulation

Vin = 10 V to 24 V, IO = 0.9 A

10 mV =

± 0.017%

Load Regulation

Vin = 12 V, IO = 0.1 A to 0.9 A

30 mV =

± 0.053%

Output Ripple

Vin = 12 V, IO = 0.9 A

140 mVpp

Short Circuit Current

Vin = 12 V, RL = 0.1 Ω

6.0 A

Efficiency

Vin = 12 V, IO = 0.9 A

Vin = 24 V, IO = 0.9 A

80.1%

87.8%

L = Coilcraft M1496–A or General Magnetics Technology GMT–0223, 42 turns of #16 AWG on

Magnetics Inc. 58350–A2 core.

Heatsink = AAVID Engineering Inc.

MC34167: 5903B, or 5930B

MTP3055EL: 5925B

Figure 20 shows that the MC34167 can be configured as a step–up/down converter with the addition of an external power MOSFET. Energy is stored in the

inductor during the ON time of transistors Q1 and Q2. During the OFF time, the energy is transferred, with respect to ground, to the output filter capacitor and load.

This circuit configuration has two significant advantages over the basic step–up converter circuit. The first advantage is that output short circuit protection is

provided by the MC34167, since Q1 is directly in series with Vin and the load. Second, the output voltage can be programmed to be less than Vin. Notice that during

the OFF time, the inductor forward biases diodes D1 and D2, transferring its energy with respect to ground rather than with respect to Vin. When operating with Vin

greater than 20 V, a gate protection network is required for the MOSFET. The network consists of components RG, D3, and D4.

(Top View)

Vin

VO

CF

RF

R1

+–

+

–

+

+

D3

D2

RG

(Bottom View)

Figure 21. Step–Up/Down Converter Printed Circuit Board and Component Layout

3.45

″