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■OPERATIONAL EXPLANATION

The XC9268 series consists internally of a reference voltage supply with soft-start function, error amp, PWM comparator, ramp

wave circuit, oscillator circuit, Current limiting PFM circuit, phase compensation (Current feedback) circuit, current limiting circuit,

High-side driver Tr., Low-side driver Tr., buffer drive circuit, internal power supply (Local Reg) circuit, under-voltage lockout

(UVLO) circuit, gate clamp (CLAMP) circuit, thermal shutdown (TSD) circuit, power good comparator, PWM/PFM control block

and other elements.

The voltage feedback from the FB pin is compared to the internal reference voltage by the error amp, the output from the error

amp is phase compensated, and the signal is input to the PWM comparator to determine the ON time of switching during PWM

operation. The output signal from the error amp is compared to the ramp wave by the PWM comparator, and the output is sent

to the buffer drive circuit and output from the LX pin as the duty width of switching. This operation is performed continuously to

stabilize the output voltage.

The driver transistor current is monitored at each switching by the phase compensation (Current feedback) circuit, and the

output signal from the error amp is modulated as a multi-feedback signal. This allows a stable feedback system to be obtained

even when a low ESR capacitor such as a ceramic capacitor is used, and this stabilizes the output voltage.

XC9268 Series

* Diodes inside the circuits are ESD protection diodes and parasitic diodes.

<Reference voltage source>

The reference voltage source provides the reference voltage to ensure stable output voltage of the DC/DC converter.

<Oscillator circuit>

The oscillator circuit determines switching frequency.1.2MHz or 2.2MHz is available for the switching frequency.

Clock pulses generated in this circuit are used to produce ramp waveforms needed for PWM operation.

<Error amplifier>

The error amplifier is designed to monitor output voltage. The amplifier compares the reference voltage with the feedback

voltage is fed back, the output voltage of the error amplifier increases. The error amplifier output is fixed internally to deliver an

optimized signal to the mixer.

PWM/PFM

ControlLOGIC

FB

+

-

+

-

Low

Side

Buffer

Lx

EN/SS

Chip

Enable

Vref

Soft Start

Under

Voltage

Lock Out

Current

Limit

Err Amp

GND

PG

(USP-6C Package Only)

+

-

Thermal

Shutdown

Comparator

PWM

LocalReg

Current

SENSE

Gate

CLAMP

PowerGood

Comparator

each

circuit

Ramp

Wave

OSC

High

Side

Buffer

Current

Limit

PFM

Operation

Enable

each

circuit

Current

feed

back