NCP1252 データシートの表示（PDF） - Unspecified

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NCP1252

Current Mode PWM Controller for Forward and Flyback Applications

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NCP1252

Application Information

Introduction

The NCP1252 hosts a high−performance current−mode

controller specifically developed to drive power supplies

designed for the ATX and the adapter market:

• Current Mode operation: implementing peak

current−mode control topology, the circuit offers

UC384X−like features to build rugged power supplies.

• Adjustable switching frequency: a resistor to ground

precisely sets the switching frequency between 50 kHz

and a maximum of 500 kHz. There is no

synchronization capability.

• Internal frequency jittering: Frequency jittering

softens the EMI signature by spreading out peak energy

within a band ±5% from the center frequency.

• Wide Vcc excursion: the controller allows operation

up to 28 V continuously and accepts transient voltage

up to 30 V during 10 ms with IVCC < 20 mA

• Gate drive clamping: a lot of power MOSFETs do not

allow their driving voltage to exceed 20 V. The

controller includes a low−loss clamping voltage which

prevents the gate from going beyond 15 V typical.

• Low startup−current: reaching a low no−load standby

power represents a difficult exercise when the

controller requires an external, lossy, resistor connected

to the bulk capacitor. The start−up current is guaranteed

to be less than 100 mA maximum, helping the designer

to reach a low standby power level.

• Short−circuit protection: by monitoring the CS pin

voltage when it exceeds 1 V (maximum peak current),

the controller detects a fault and starts an internal

digital timer. On the condition that the digital timer

elapses, the controller will permanently latch−off. This

allows accurate overload or short−circuit detection

which is not dependant on the auxiliary winding. Reset

occurs when: a) a BO reset is sensed, b) VCC is cycled

down to VCC(min) level. If the short circuit or the fault

disappear before the fault timer ends, the fault timer is

reset only if the CS pin voltage level is below 1 V at

least during 3 switching frequency periods. This delay

before resetting the fault timer prevents any false or

missing fault or over load detection.

• Adjustable soft−start: the soft−start is activated upon

a start−up sequence (VCC going−up and crossing

VCC(on)) after a minimum internal time delay of 120 ms

(SSdelay). But also when the brown−out pin is reset

without in that case timer delay. This internal time

delay gives extra time to the PFC to be sure that the

output PFC voltage is in regulation. The soft start pin is

grounded until the internal delay is ended.

• Shutdown: if an external transistor brings the BO pin

down, the controller is shut down, but all internal

biasing circuits are alive. When the pin is released, a

new soft−start sequence takes place.

• Brown−Out protection: BO pin permanently monitors

a fraction of the input voltage. When this image is

below the VBO threshold, the circuit stays off and does

not switch. As soon the voltage image comes back

within safe limits, the pulses are re−started via a

start−up sequence including soft−start. The hysteresis is

implemented via a current source connected to the BO

pin; this current source sinks a current (IBO) from the

pin to the ground. As the current source status depends

on the brown−out comparator, it can easily be used for

hysteresis purposes. A transistor pulling down the BO

pin to ground will shut−off the controller. Upon release,

a new soft−start sequence takes place.

• Internal ramp compensation: a simple resistor

connected from the CS pin to the sense resistor allows

the designer to inject ramp compensation inside his

design.

• Skip cycle feature: When the power supply loads are

decreasing to a low level, the duty cycle also decreases

to the minimum value the controller can offer. If the

output loads disappear, the converter runs at the

minimum duty cycle fixed by the propagation delay and

driving blocks. It often delivers too much energy to the

secondary side and it trips the voltage supervisor. To

avoid this problem, the FB is allowed to impose the min

tON down to ~ Vf and it further decreases down to

Vskip, zero duty cycle is imposed. This mode helps to

ensure no−load outputs conditions as requested by

recently updated ATX specifications. Please note that

the converter first goes to min tON before going to zero

duty cycle: normal operation is thus not disturbed. The

following figure illustrates the different mode of

operation versus the FB pin level.

http://onsemi.com

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