ADP1611

Rev. 0 | Page 12 of 20

DIODE SELECTION

The output rectifier conducts the inductor current to the output

capacitor and load while the switch is off. For high efficiency,

minimize the forward voltage drop of the diode. For this reason,

Schottky rectifiers are recommended. However, for high

voltage, high temperature applications where the Schottky

rectifier reverse leakage current becomes significant and can

degrade efficiency, use an ultrafast junction diode.

Make sure that the diode is rated to handle the average output

load current. Many diode manufacturers derate the current

capability of the diode as a function of the duty cycle. Verify

that the output diode is rated to handle the average output load

current with the minimum duty cycle. The minimum duty cycle

of the ADP1611 is

OUT

MAX

IN

OUT

MIN

V

V

V

D

−

−

=

(10)

Table 6. Schottky Diode Manufacturers

Vendor

Phone No.

Web Address

On Semiconductor

602-244-6600

www.onsemi.com

Diodes, Inc.

805-446-4800

www.diodes.com

Central Semiconductor

631-435-1110

www.centralsemi.com

Sanyo

310-322-3331

www.sanyo.com

LOOP COMPENSATION

The ADP1611 uses external components to compensate the

regulator loop, allowing optimization of the loop dynamics for a

given application.

The step-up converter produces an undesirable right-half plane

zero in the regulation feedback loop. This requires compen-

sating the regulator such that the crossover frequency occurs

well below the frequency of the right-half plane zero. The right-

half plane zero is determined by the following equation:

L

R

V

V

RHP

F

LOAD

OUT

IN

Z

×

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

π

2

)

(

2

(11)

where:

divided by the load current.

To stabilize the regulator, ensure that the regulator crossover

frequency is less than or equal to one-fifth of the right-half

plane zero and less than or equal to one-fifteenth of the

switching frequency.

The regulator loop gain is

OUT

CS

COMP

MEA

OUT

IN

OUT

FB

VL

Z

G

Z

G

V

V

V

V

A

×

×

×

×

×

=

(12)

where:

GND.

current divided by the voltage at COMP), which is internally set

by the ADP1611.

To determine the crossover frequency, it is important to note

dominated by the impedance of the output capacitor. So, when

solving for the crossover frequency, the equation (by definition

of the crossover frequency) is simplified to

1

2

1

|

|

=

×

×

×

×

×

×

×

=

OUT

C

C

f

CS

G

COMP

R

MEA

G

OUT

V

IN

V

OUT

V

FB

V

VL

A

π

(13)

compensation resistor.

CS

MEA

IN

FB

OUT

OUT

OUT

C

COMP

G

G

V

V

V

V

C

f

R

×

×

×

×

×

×

×

=

π

2

(14)

IN

OUT

OUT

OUT

C

COMP

V

V

V

C

f

R

×

×

×

×

×

=

4

10

55

.

2

(15)

Once the compensation resistor is known, set the zero formed

by the compensation capacitor and resistor to one-fourth of the

crossover frequency, or

COMP

C

COMP

R

f

C

×

×

=

π

2

(16)

The capacitor, C2, is chosen to cancel the zero introduced by

output capacitance ESR.

Solving for C2,

COMP

OUT

R

C

ESR

C2

×

=

(17)