1.5A, 40V, MAXPower Step-Down

DC-DC Converters

______________________________________________________________________________________

13

Pick a value for the feedback resistor R5 in Figure 3

(values between 1k

Ω and 10kΩ are adequate).

C7 is then calculated as:

is then calculated by:

Since R3 >> R6, R3 + R6 can be approximated as R3.

R3 is then calculated as:

For larger ESR capacitors such as tantalum and alu-

loop crossover frequency. The equations that define

lower than the closed-loop crossover frequency. Figure

4 shows the error amplifier feedback as well as its gain

response for circuits that use higher-ESR output capac-

itors (tantalum or aluminum electrolytic).

Pick a value for the feedback resistor R5 in Figure 4 (val-

ues between 1k

Ω and 10kΩ are adequate).

C7 is then calculated as:

mately equal to R5/R6 (given that R6 << R3). R6 can

then be calculated as:

C6 is then calculated as:

C

C

ESR

R

6

6

=

×

OUT

R

Rf

f

LC

C

6

510

2

2

≈

××

C

1

7

20 8

5

=

××

×

π

.R

()

C

C

CR

f

8

7

27

5

1

=

××

×

−

π

P3

R

fC

3

1

26

≈

××

.

R

Cf

6

1

26

0 5

=

××

×

π

SW

C

fL

C

RG

C

6

2

5

=

××

×

×

OUT

MOD(DC)

π

()

G

G

LC

MOD(fC)

MOD(DC)

OUT

=

××

×

π

C

1

7

20 8

5

=

××

×

π

.R

GAIN

(dB)

REF

R3

COMP

R6

R5

C6

R4

FREQUENCY

CLOSED-LOOP

GAIN

EA

GAIN

C8

EA

C7

Figure 3. Error Amplifier Compensation Circuit (Closed-Loop

and Error-Amplifier Gain Plot) for Ceramic Capacitors