AC-DC and DC-DC Peak-Current-Mode Converters

for Flyback/Boost Applications

MAX17498A/MAX17498B/MAX17498C

 Maxim Integrated Products 21

Peak/RMS-Current Calculation

To set the current limit, the peak current in the inductor

and internal nMOSFET can be calculated as:

OUT

MAX

MAX

OUT

PK

INMIN

SWMIN

MAX

OUT

OUT

PK

MAX

INMIN

SWMIN

V

D

(1 D

)

I

I

L

f

(1 D)

1.2 for D

0.5

0.25 V

I

I

1.2 for D

0.5

L

f

(1 D)



×

×−

=

+



×−



×

≥



×

=

+×

≥



×

−



value of the input inductor taking into account tolerance

ing frequency for the MAX17498C from the Electrical

Characteristics section.

Output-Capacitor Selection

X7R ceramic output capacitors are preferred in industrial

applications due to their stability over temperature. The

output capacitor is usually sized to support a step load

of 50% of the maximum output current in the application,

such that the output-voltage deviation is contained to 3%

of the output-voltage change. The output capacitance

can be calculated as:

STEP

RESPONSE

OUTF

OUT

RESPONSE

C

SW

It

C

V

0.33

1

t

()

ff

×

=

∆

≅+

output capacitor supplies the load current when the main

switch is on, and therefore, the output-voltage ripple is a

function of duty cycle and load current. Use the following

equation to calculate the output-capacitor ripple:

OUT

MAX

COUT

OUT

SW

ID

V

Cf

×

∆

=

×

Input-Capacitor Selection

The input ceramic capacitor value required can be cal-

culated based on the ripple allowed on the input DC bus.

The input capacitor should be sized based on the RMS

value of the AC current handled by it. The calculations are:

OUT

IN

INMIN

SW

MAX

3.75 I

C

V

f

(1 D

)



×

= 

×

×−



The input-capacitor RMS current can be calculated as:

LIN

CIN_RMS

I

I

23

∆

=

×

where:

OUT

MAX

MAX

LIN

MAX

INMIN

SWMIN

OUT

LIN

MAX

INMIN

SWMIN

V

D

(1 D

)

I

for D

0.5

Lf

0.25 V

I

for D

0.5

Lf



×

×−

∆

=

<



×





×

∆

=

≥



×



Error-Amplifier Compensation Design

The loop-compensation values for the error amplifier can

now be calculated as:

2

OUT

OUT

MIN

Z

OUTMIN

IN

250 V

C

(1 D

)

R

IL

×

×

×−

=

×

OUT

OUT

Z

OUT

Z

P

SW

Z

VC

C

2I

R

1

C

fR

×

=

×

×

=

π×

×

Slope-Compensation Ramp

The slope required to stabilize the converter at duty

cycles greater than 50% can be calculated as:

OUT

INMIN

E

IN

0.5 (0.82 V

V

)

S

V per µs

L

×

×

−

=