SC2569 datasheet(10/13 Pages) ETC

10 / 13 page

SC2569

Safety-Chip

www.safety-chip.com

SC2569_Datasheet_V1.0_EN

SC2569 current mode PWM control. The switch

current is detected by a sense resistor into the

SENSE pin. An internal leading edge blanking

circuit chops off the sensed voltage spike at initial

internal power MOSFET on state due to snubber

diode reverse recovery and surge gate current of

power MOSFET. The current limiting comparator

is disabled and cannot turn off the internal power

MOSFET during the blanking period. The PWM

duty cycle is determined by the current sense

input voltage and the FB input voltage.

Internal Synchronized Slope Compensation

Built-in slope compensation circuit adds voltage

ramp onto the current sense input voltage for

PWM generation. This greatly improves the close

loop

stability

CCM

and

prevents

the

sub-harmonic oscillation and thus reduces the

output ripple voltage.

Over Temperature Protection

A NTC resistor in series with a regular resistor

should connect between RT and GND for

temperature sensing and protection. NTC resistor

value

becomes

lower

when

the

ambient

temperatures rise. With the fixed internal current

IRT flowing through the resistors, the voltage at RT

pin becomes lower at high temperature. The

internal OTP circuit is triggered and shutdown the

MOSFET when the sensed input voltage is lower

than VTH_OTP.

Gate Drive

SC2269 GATE is connected to the gate of an

external MOSFET for power switch control. Too

weak the gate drive strength results in higher

conduction and switch loss of MOSFET while too

strong gate drive output compromises the EMI.

A good trade-off is achieved through the built-in

totem pole gate design with right output strength

and dead time control. The low idle loss and good

EMI system design is easier to achieve with this

dedicated control scheme. An internal 18V clamp

is added for MOSFET gate protection at higher

than expected VDD input.

Protection Controls

Good power supply system reliability is achieved

with auto-recovery protection features including

Cycle-by-Cycle current limiting (OCP), Under

Voltage

Lockout

VDD

(UVLO),

Over

Temperature

Protection

(OTP),

VDD

Over

Voltage Protection (OVP).

The OCP threshold value is self adjusted lower at

higher current into VIN pin. This OCP threshold

slope adjustment helps to compensate the

increased output power limit at higher AC voltage

caused by inherent Over-Current sensing and

control delay. A constant output power limit is

achieved with recommended OCP compensation

scheme on SC2569.

At output overload condition when FB input

voltage exceeds power limit threshold value for

more than TD_PL, control circuit reacts to shut

down the power MOSFET. It restarts when VDD

voltage drops below UVLO limit.

Similarly, when an Over Temperature condition is

detected, control circuit shutdowns the power

MOSFET. SC2569 resumes the operation when

temperature drops below the hysteresis value.

VDD

supplied

with

transformer

auxiliary

winding output. It is clamped when VDD is higher

than 35V. MOSFET is shut down when VDD

drops below VDD_ON limit and device enters

power on startup sequence thereafter.

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