Rev 7/25/01

3

FIGURE 3. SP6121 Demo Board Schematic.

Demo Board Component Selection

The input capacitor, C6, can be either a ceramic or tantalum capacitor. Its choice

depends on the user’s system input voltage transient. If the highest possible efficiency

is required, a ceramic 47uF capacitor is recommended.

The demo board circuit includes a Schottky Diode, D1, across the low side switch. It

prevents Q2’s internal body diode from turning on and dissipating power during the

nonoverlap time when both Q1 and Q2 are off. At a 500kHz switching frequency, these

losses would decrease overall efficiency by 0.5 – 0.6%.

The output voltage is set by the resistor divider: R3/R4. The output voltage is calculated

using the following:

REF

OUT

V

R

R

V

÷

ø

ö

ç

è

æ

+

=

1

4

3

,

K

R

R

10

4

3

=

=

The considerations, tradeoffs and calculations required to select the power MOSFETs

L1 1.6uH

C1

2.2uF

10

POWER OK

U1

SP6121

1

2

7

6

5

4

3

8

Vcc

GND

NDRV

ISET

ISENSE

COMP

VFB

PDRV

GND

C7

0.1uF

U2

SP708

4

6

7

5

1

2

8

3

VCC

PFI

PFO

GND

RESET

Reset

NC

MR

Vin

9

C11

56pF

1,2,3,4

C9

470uF

C8

470uF

R4

10K

TRIM

C6

47uF

D1

R2

2.3K

R3

10K

V out

CERAMIC

7,8

R1

10K

C2

56pF

Q2

FDS6690A

R5

10K

C3

3.9nF

R6

5.0

Q1

FDS6375

C10

0.1uF

5,6

C5

1.0uF

+3.0V-7.0 V

2.5V