DP2525C datasheet(8/11 Pages) ETC

8 / 11 page

DP2525

Low Cost Primary Side Regulation(PSR) PWM Power Switch

8/11

Rev.2.0

(

)

Fig.2

PSR

Constant

Current

Modulation

(PSR-CCM)

Timing information at the FB pin and current

information at the CS pin allow accurate regulation

of the secondary average current. The control law

dictates that as power is increased in CV regulation

and approaching CC regulation the primary peak

current is at Ipp(max), as shown in Fig.3.

(

)

Tsw

Tdem

max

Ipp

CC_OUT

Fig.3

Referring to Fig.3 above, the primary peak current,

transformer turns ratio, secondary demagnetization

time

(Tdem),

and

switching

period

(Tsw)

determines the secondary average output current

Iout. Ignoring leakage inductance effects, the

equation for average output current is shown in

Fig.3. When the average output current Iout

reaches the regulation reference in the Primary

Side Constant Current Modulator (PSR-CCM) block,

the IC operates in pulse frequency modulation

(PFM) mode to control the output current at any

output voltage at or below the voltage regulation

target as long as the auxiliary winding can keep

VDD above the UVLO turn-off threshold.

In DP2525, the ratio between Tdem and Tsw in CC

mode is 1/2. Therefore, the average output current

can be expressed as:

(

)

( )

Ω

≅

Rcs

500mV

CC_OUT

In the equation above,

N----The turn ratio of primary side winding to

secondary side winding.

Rcs--- the sensing resistor connected between the

power BJT emitter to GND. I

Multi Mode Control in CV Mode

To meet the tight requirement of averaged system

efficiency and no load power consumption, a hybrid

frequency modulation

(FM)

and

amplitude

modulation (AM) is adopted in DP2525 which is

shown in the Fig 4.

Around the full load, the system operates in FM

mode. When normal to light load conditions, the IC

operates in FM+AM mode to achieve excellent

regulation and high efficiency. When the system is

near zero loading, the IC operates in FM again for

standby power reduction. In this way, the no-load

consumption can be less than 70mW.

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