MAX17498A(2012) Ver la hoja de datos (PDF) - Maxim Integrated

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Fabricante

MAX17498A
(Rev.:2012)

AC-DC and DC-DC Peak Current-Mode Converters for Flyback/Boost Applications

Maxim Integrated 

MAX17498A / MAX17498B / MAX17498C

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

for Flyback/Boost Applications

Peak/RMS-Current Calculation

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

and internal nMOSFET can be calculated as:

IPK







VOUT × DMAX

LINMIN ×

× (1− DMAX)

fSWMIN

+

(I1O−UDT)

× 1.2 for DMAX ≥ 0.5

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:

CIN

=



3.75 ×

VINMIN × fSW

IOUT

× (1−



D MAX ) 

= IPK



L

0.25 × VOUT

INMIN × fSWMIN

+

(I1O−UDT)

× 1.2

for

DMAX

≥

0.5

The input-capacitor RMS current can be calculated as:

ICIN_RMS

=

∆ILIN

2× 3

DMAX, the maximum duty cycle, is obtained by substitut-

ing the minimum input operating voltage (VINMIN) in the

where:

equation above for duty cycle. LINMIN is the minimum

vaanlduesaotfutrhaetioinnpuetffeincdtsu.ctfoSrWtMakINingisintth= oeamccinoiumnutmtolsewraitncche- ∆ILIN

VOUTL×INDMMINA×X

× (1− DMAX

fSWMIN

)





for

DMAX

< 0.5

ing frequency for the MAX17498B from the Electrical

Characteristics section.

= ∆ILIN

Output-Capacitor Selection

LIN0M.2I5N××VfSOWUMT IN







for

DMAX

≥

0.5

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,

Error-Amplifier Compensation Design

The loop-compensation values for the error amplifier can

now be calculated as:

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

of the output-voltage change. The output capacitance

can be calculated as:

RZ

=

203

×

VOUT 2 × COUT × (1−

IOUTMAX × LIN

DMAX )

COUTF

=

ISTEP

× tRESPONSE

∆VOUT

t RESPONSE

≅

(0.33

fC

+

1

fSW

)

where ISTEP is the load step, tRESPONSE is the

response time of the controller, DVOUT is the allowable

output-voltage deviation, and fC is the target closed-

loop crossover frequency. fC is chosen to be 1/10 the

switching frequency (fSW). For the boost converter, the

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:

∆VCOUT

= IOUT × DMAX

COUT × fSW

where IOUTMAX is the maximum load current:

CZ

=

VOUT × COUT

2 ×IOUTMAX × RZ

CP

=

π

×

1

fSW

×RZ

Slope-Compensation Ramp

The slope required to stabilize the converter at duty

cycles greater than 50% can be calculated as:

SE

=

0.41(VOUT −

LIN

VINMIN)

V

per

µs

where LIN is in µH.

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