AB-28

APPLICATION BULLETIN

4

7/18/02

FAN5236 Dual Synchronous Buck

Converter

The FAN5236 PWM controller (Fig. 9) provides high

efficiency and regulation for two output voltages adjustable

in the range from 0.9V to 5.5V. Synchronous rectification

and hysteretic operation at light loads contribute to a high

efficiency over a wide range of loads. The hysteretic mode

of operation can be disabled separately on each PWM

converter if PWM mode is desired for all load levels. Again

high efficiency is obtained by using MOSFET’s RDSON for

current sensing. Out-of-phase operation with 180-degree

phase shift reduces input current ripple.

Fig. 9. FAN5236 Block Diagram

RC1585/7 Linear Regulators

In some cases if the input to output voltage difference is sen-

sibly less than the output voltage it make sense to use linear

regulators. In Fig.6 Fairchild’s RC1587, 3A and RC1585, 5A

linear regulators are showcased.

KA5H0365 Offline Converter

The high level of simplicity of the AC/DC converter in Fig. 6

is possible thanks the multi-chip approach to integration of

the this controller family (Fig. 10). The TO220 package

houses two dies, a controller die and a high voltage MOS-

FET die on board. Here again power hungry discrete current

sense resistors are avoided, in this case by means of a ratioed

sense-fet technique on board of the discrete element.

For more details and a complete bill of materials please refer

to the FAN5092, FAN5236, RC1585, RC1587 and FOD2712

data sheets available on the Fairchild web site.

For KA5H0365 please refer to the data sheet as well as to

FPS Application Notes for SMPS design available on the

Fairchild web site.

Conclusion

The merging of data, voice and video blurs the line between

computing and communications. The smart loads of either

application draw from the same advanced, high-density sub-

micron low voltage CMOS technologies and require similar

solutions for distributed power conversion. Fairchild exper-

tise in power conversion for computing and communications

offers proven solutions to the merging converged data com-

munications market.

FAN5236

5 to 24V

VCC

+5

ILIM1

DDR

ILIM2/

REF2

VOUT1

= 2.5V

VOUT2

= 1.8V

Q1

Q2

Q3

Q4

PWM1

PWM2

Fig. 10. KA5H0365 converter architecture

FB

DRAIN

GND

VCC

32V

ck

5µA

1mA

2.5R

1R

7.5V

OSC

UVLO

Volt

Ref

Voffset

Thermal

???

????

sense

S

S

R

R

+

–

+

–

Q

Q

sense

Sense

FET

Control IC

POR

POR