MPX2003

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

MPX2003 Rev. 1.0

MonolithicPower.com

20

8/5/2022

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The primary IC does not start up again until there

is a valid brown-in detection condition. Brown-in

and brownout detection are suspended when the

HV current source turns on (to avoid the effect of

the HV voltage drop). These protections can be

detected after the HV current source turns off.

Soft Start (SS)

To reduce stress on the power circuits, a soft-

start function is implemented for the primary IC.

When the primary IC enters normal start-up

mode and starts switching on its own, the

internal soft start gradually increases the current

limitation from VIPK-MIN (typically 0.1V) to VIPK-MAX

(typically 0.4V). During this time, the switching

frequency (fSW) rises from fSW-SOFT (typically

10kHz) to fSW-MAX (typically 110kHz). The soft-

start period lasts for tSOFT (typically 9.6ms).

Peak Current Control with Internal Slope

Compensation

The primary IC employs peak current mode

control with a switching on time. This on time is

controlled by comparing the voltage across the

CS

pin’s current-sense resistor with the internal

reference voltage. The reference voltage is

regulated by fSW with a maximum value of

VIPK-MAX and a minimum value of VIPK-MIN.

The reference voltage is clamped at VIPK-MAX

when fSW exceeds fSW-H (typically 60kHz); the

reference voltage is clamped at VIPK-MIN when fSW

is below fSW-L (typically 20kHz) (see Figure 3).

When fSW is between fSW-H and fSW-L, the

reference voltage is adjusted continuously.

fSW (Load)

fSW/IPK

fSW-L

fPFM-MAX

VIPK-MIN

VIPK-MAX

fSW-H

Figure 3: Peak Current Reference vs. Switching

Frequency

A synchronized positive slope is added to the

current-sense

signal

on

CS

to

prevent

subharmonic oscillations and guarantee stable

peak current mode control across a wide range

of input voltages.

Due to parasitic capacitance, a spike usually

occurs on the current-sense resistor after the

MOSFET turns on. To prevent the peak current

limitation

comparator

from

being

falsely

triggered by this turn-on spike, a leading edge

blanking (LEB) function is implemented on the

comparator. During the blanking time, the peak

current limitation comparator is disabled, so the

MOSFET cannot turn off. Two-level LEB is

adopted in the comparator. Normal operation

has tLEB-L (typically 400ns), while tLEB-S (typically

250ns) is the blanking time during SCP.

Line Compensation for Peak Current Control

Ideally, the primary peak current should be

exactly the same as the reference because of

peak current control operation. However, the IC

has a logic propagation delay and driver delay,

so the actual peak current always exceeds the

reference value. The difference between the

actual value and reference value varies with the

input voltage. A higher input voltage results in a

higher peak current overshoot (see Figure 4).

This leads to a significant variation in the

maximum power limitation.

Turn-Off Delay (tD)

VIN1/LM

VIN2/LM

IPK x RS Variation

tONP2

tONP1

VLIMIT

Figure 4: Peak Current Difference Caused by

Turn-Off Delay

The slope compensation voltage is proportional

to the primary MOSFET’s turn-on time, which is

determined by the load and input voltage. This

means that the slope compensation value is

lower when the input voltage is higher, even if

the load is the same.

To compensate for this variation and improve

overload protection (OLP) accuracy, an offset

proportional to the input voltage is added on the