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XC9281/XC9282

Series

■OPERATIONAL EXPLANATION

This IC consists of a reference voltage source, error amplifier, comparator, phase compensation, on time generation circuit,

output voltage adjustment resistors, current limiter circuit, UVLO circuit, PWM/PFM selection circuit and so on.

BLOCK DIAGRAM (XC9281/XC9282 Series Type B)

The control method is HiSAT-COT (High Speed circuit Architecture for Transient with Constant On Time), which features the

On time control method and the fast transient response with low ripple voltage.

＜Nomal operation＞

In HiSAT-COT control, ON time (tON) dependent on input voltage and output voltage is generated and Pch MOS driver Tr. Is

turned on. On time is set as follows.

IC with 4MHz :

tON = (VOUT / VIN) x 250ns

IC with 6MHz :

tON = (VOUT / VIN) x 167ns

The off time (tOFF) is controlled by comparing the output voltage and the reference voltage with the error amplifier and the

comparator. Specifically, the reference voltage and a voltage which is obtained by dividing the output voltage with R1 and R2 are

compared with using the error amplifier, apply phase compensation to the output of the error amplifier, and send it to the

comparator. In the comparator, the output of the error amplifier is compared with the reference voltage, and when it falls below

the reference voltage, the SR latch is set and it becomes the ON period again.

The XC9281 series (PWM control) operates in continuous conduction mode and operates at a stable oscillation frequency

regardless of the load. The oscillation frequency can be obtained by the following equation.

fOSC = (VOUT / VIN) x (1 / tON)

The XC9282 series (PWM/PFM automatic switching control) lowers the oscillation frequency at light load by operating in

discontinuous conduction mode at light load. By this operation, it is possible to reduce switching loss at light load and achieve

high efficiency from light load to heavy load.

In the phase compensation circuit, the frequency characteristic of the error amplifier is optimized, and ramp waves which are

similar to ripple voltages generated at the output are generated to modulate the output signal of the error amplifier. This enables

a stable feedback system to be obtained even when a low ESR capacitor such as a ceramic capacitor is used, and a fast transient

response and stabilization of the output voltage are achieved.

ILX

VLX

tON

0mA

0V

fOSC

IOUT

0mA

VLX

tON

fOSC

ILX

0V

IOUT

Continuous Conduction Mode waveform

Discontinuous Conduction Mode waveform

+

-

Vref with

Soft Start

S

R

Q

On Time

Generator

VIN

VOUT

Synch.

Buffer

Driver

Logic

Current Limit

CE Control Logic,

UVLO

CL Auto-Discharge

-

+

Phase

Compensation

Error Amp.

Comparator

CFB

R2

R1

GND

Lx

VIN

CE

PWM/PFM

Selector

VOUT