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MC33260P

GreenLine™ Compact Power Factor Controller: Innovative Circuit for Cost Effective Solutions

ON Semiconductor 

MC33260

Pin Numbers are Relevant to the PDIP−8 Version

This equation shows that the maximum on−time is inversely

proportional to the squared output voltage. This property is

used for follower boost operation (refer to Follower Boost

section).

CURRENT SENSE BLOCK

The inductor current is converted into a voltage by

inserting a ground referenced resistor (Rcs) in series with the

input diodes bridge (and the input filtering capacitor).

Therefore a negative voltage proportional to the inductor

current is built:

Vcs + −ǒRcs ILǓ

where:

IL is the inductor current,

Rcs is the current sense resistor,

Vcs is the measured Rcs voltage.

Zero Current Detection

The Zero Current Detection function guarantees that the

MOSFET cannot turn on as long as the inductor current

hasn’t reached zero (discontinuous mode).

The Pin 4 voltage is simply compared to the (−60 mV)

threshold so that as long as Vcs is lower than this threshold,

the circuit gate drive signal is kept in low state.

Consequently, no power MOSFET turn on is possible until

the inductor current is measured as smaller than (60 mV/Rcs)

that is, the inductor current nearly equals zero.

D1...D4

1

ROCP

4

Rcs VOCP

Iocp (205 mA)

Output_Ctrl

−60 mV

0

+

−

LEB

S

PWM

Latch

R

Output_Ctrl

Q

R

Time

VOCP

−60 mV

Zero Current Detection

VOCP = ROCP  IOCP

An overcurrent is detected if Vpin4 crosses the threshold (−60 mV)

during the Power Switch on state

Figure 27. Current Sensing

The negative signal Vcs is applied to the current sense

through a resistor ROCP. The pin is internally protected by a

negative clamp (−0.7 V) that prevents substrate injection.

As long as the Pin 4 voltage is lower than (−60 mV), the

Current Sense comparator resets the PWM latch to force the

gate drive signal low state. In that condition, the power

MOSFET cannot be on.

During the on−time, the Pin 4 information is used for the

overcurrent limitation while it serves the zero current

detection during the off time.

To Output Buffer

(Output_Ctrl Low <=> Gate Drive in Low State)

Figure 28. Current Sense Block

Overcurrent Protection

During the power switch conduction (i.e. when the Gate

Drive Pin voltage is high), a current source is applied to the

Pin 4. A voltage drop VOCP is then generated across the

resistor ROCP that is connected between the sense resistor

and the Current Sense Pin (refer to Figure 28). So, instead of

Vcs, the sum (Vcs + VOCP) is compared to (−60 mV) and the

maximum permissible current is the solution of the

following equation:

−ǒRcs IpkmaxǓ ) VOCP + −60 mV

where:

Ipkmax is maximum allowed current,

Rcs is the sensing resistor.

The overcurrent threshold is then:

ǒ Ipkmax + ROCP

Ǔ IOCP ) 60

Rcs

10−3

where:

ROCP is the resistor connected between the pin and the

sensing resistor (Rcs),

IOCP is the current supplied by the Current Sense Pin

when the gate drive signal is high (power switch

conduction phase). IOCP equals 205 mA typically.

Practically, the VOCP offset is high compared to 60 mV

and the precedent equation can be simplified. The maximum

current is then given by the following equation:

Ipkmax

[

ROCP(kW)

Rcs(W)

0.205 (A)

Consequently, the ROCP resistor can program the OCP level

whatever the Rcs value is. This gives a high freedom in the

choice of Rcs. In particular, the inrush resistor can be utilized.

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