MPX2003

– UP TO 140kHz ALL-IN-ONE FLYBACK CONTROLLER

MPX2003 Rev. 1.0

MonolithicPower.com

27

8/5/2022

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2022 MPS. All Rights Reserved.

The peak value for the primary current can be

calculated with Equation (7):

AV

PEAK

P

I

I

K

(1

) D

2

=

−

(7)

The ripple value for the primary current can be

estimated with Equation (8):

RIPPLE

P

PEAK

I

K

I

=

(8)

The valley value for the primary current can be

calculated with Equation (9):

VALLEY

P

PEAK

I

(1 K ) I

= −



(9)

The magnetizing inductance can be estimated

with Equation (10):



= IN(MIN)

ON

M

RIPPLE

Vt

L

I

(10)

Selecting the Current-Sense Resistor

Figure 15 shows the peak current comparator

logic.

-

+

VLIMIT

CS

Slope

Compensation

R

S

Q

PDRV

LEB

Over-Power

Compensation

Figure 15: Peak Current Comparator Circuit

Logic

Figure 16 shows the peak current comparator

waveform.

VPEAK

SRAMP x tON

IPEAK x RSENSE

IHVCS x RHVCS

Figure 16: Peak Current Comparator Waveform

When the sum of the voltage on the current-

sense resistor, the voltage on the over-power

compensation

resistor,

and

the

voltage

generated by the slope compensation block

reach VPEAK, the comparator goes high to reset

the RS flip-flop, and DRV is pulled down to turn

off the MOSFET (see Figure 15). The maximum

current limit is VIPK-MAX. The slope compensator

is SRAMP, and at low line the over-power

compensation is zero. Given a certain margin,

VPEAK is equal to 0.95 x VIPK-MAX under full loads.

The voltage on the current-sense resistor can be

calculated with Equation (11):

SENSE

IPK MAX

RAMP

ON

V

95% V

S

t

−

=



−



(11)

The current-sense resistor can be estimated with

Equation (12):

= SENSE

SENSE

PEAK

V

R

I

(12)

Select

a

current-sense

resistor

with

an

appropriate power rating. The current-sense

resistor’s power loss can be calculated with

Equation (13):

(

)



+



=

+

−

 











2

2

PEAK

VALLEY

SENSE

PEAK

VALLEY

SENSE

II

1

P

I

I

D R

2

12

(13)

Over-Power Compensation and Selecting

the Low-Pass Filter on CS

The MPX2003 has an over-power compensation

(OPC) function that draws current from CS. The

purpose of OPC is to minimize the OLP

difference caused by different input voltages.

The offset current is proportional to the input

peak voltage sensed by HV.

Assume the resistor in the current-sense loop is

RHVCS

and

the

bus

voltage

is

VHV.

The

compensation voltage on the CS pin can be

estimated with Equation (14):

OPC

HVCS

HVCS

V

R

I

=

(14)

The compensation criteria makes the COMP pin

voltage under high-line, full-load conditions

similar to its voltage under low-line, full-load

conditions. Generally, RHVCS is recommended to

be between hundreds of Ohms to one thousand

Ohms.