NCP1351BPG datasheet(9/27 Pages) ONSEMI

Part #	NCP1351BPG

Description	Variable Off Time PWM Controller

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NCP1351

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The NCP1351 implements a fixed peak current mode

technique whose regulation scheme implements a variable

switching frequency. As shown on the typical application

diagram, the controller is designed to operate with a

minimum number of external components. It incorporates

the following features:

• Frequency Foldback: Since the switching period

increases when power demand decreases, the switching

frequency naturally diminishes in light load conditions.

This helps to minimize switching losses and offers

good standby power performance.

• Very Low Startup Current: The patented internal

supply block is specially designed to offer a very low

current consumption during startup. It allows the use of

a very high value external startup resistor, greatly

reducing dissipation, improving efficiency and

minimizing standby power consumption.

• Natural Frequency Dithering: The quasi-fixed tON

mode of operation improves the EMI signature since

the switching frequency varies with the natural bulk

ripple voltage.

• Peak Current Compression: As the load becomes

lighter, the frequency decreases and can enter the

audible range. To avoid exciting transformer

mechanical resonances, hence generating acoustic

noise, the NCP1351 includes a patented technique,

which reduces the peak current as power goes down. As

such, inexpensive transformer can be used without

having noise problems.

• Negative Primary Current Sensing: By sensing the

total current, this technique does not modify the

MOSFET driving voltage (VGS) while switching.

Furthermore, the programming resistor, together with

the pin capacitance, forms a residual noise filter which

blanks spurious spikes.

• Programmable Primary Current Sense: It offers a

second peak current adjustment variable, which

improves the design flexibility.

• Extended VCC Range: By accepting VCC levels up to

28 V, the device offers added flexibility in presence of

loosely coupled transformers. The gate drive is safely

clamped below 20 V to avoid stressing the driven

MOSFET.

• Easy OPP: Connecting a resistor from the CS pin to

the auxiliary winding allows easy bulk voltage

compensation.

• Secondary or Primary Regulation: The feedback

loop arrangement allows simple secondary or primary

side regulation without significant additional external

components.

• Latch Input: If voltage on Pin 7 is externally brought

above 5 V, the controller permanently latches off and

stays latched until the user cycles VCC down, below 4

V typically.

• Fault Timer: In presence of badly coupled transformer,

it can be quite difficult to detect an overload or a

short-circuit on the primary side. When the feedback

current disappears, a current source charges a capacitor

connected to Pin 8. When the voltage on this pin

reaches a certain level, all pulses are shut off and the

VCC voltage is pulled down below the VCC(min) level.

This protection is latched on the A version (the

controller must be shut down and restart to resume

normal operation), and auto-recovery on Version B (if

the fault goes away, the controller automatically

resumes operation).

• Dual Trip Point: in some applications, such as printer

power supplies, it is necessary to let the power supply

deliver more power on a transient event. If the event

lasts longer than what the fault timer authorizes, then

the NCP1351 either latches-off (C Version) or enters an

auto-recovery mode (D Version). The level at which

the timer starts is internally set to 55% of the maximum

power capability.

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