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XCL219/XCL220

Series

■OPERATIONAL EXPLANATION

The XCL219/XCL220 series consists of a reference voltage source, error amplifier, comparator, phase compensation,

minimum on time generation circuit, output voltage adjustment resistors, P-channel MOS driver transistor, N-channel MOS

switching transistor for the synchronous switch, current limiter circuit, UVLO circuit, thermal shutdown circuit, short protection

circuit, PWM/PFM selection circuit and others.

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

time control method and a fast transient response that also achieves low output voltage ripple.

The on time (ton) is determined by the input voltage and output voltage, and turns on the Pch MOS driver Tr. for a fixed time.

During the off time (toff), the voltage that is fed back through R1 and R2 is compared to the reference voltage by the error amp,

and the error amp output is phase compensated and sent to the comparator. The comparator compares this signal to the

reference voltage, and if the signal is lower than the reference voltage, sets the SR latch. On time then resumes. By doing this,

PWM operation takes place with the off time controlled to the optimum duty ratio and the output voltage is stabilized. The phase

compensation circuit optimizes the frequency characteristics of the error amp, and generates a ramp wave similar to the ripple

voltage that occurs in the output to modulate the output signal of the error amp. 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.

<Minimum on time generation circuit>

Generates an on time that depends on the input voltage and output voltage (ton). The on time is set as given by the equations below.

ton (ns) = VOUT/VIN×0.333

<Switching frequency>

The switching frequency can be obtained from the on time (ton), which is determined by the input voltage and output voltage,

as given by the equation below.

fOSC (MHz) = VOUT(V) / (VIN(V)×ton(

μs))

<100% duty cycle mode>

When the load current is heavy and the voltage difference between input voltage and output voltage is small, 100% duty cycle

mode is activated and it keeps the Pch MOS driver Tr. turning on. 100% duty cycle mode attains a high output voltage stability

and a high-speed response under all load conditions, from light to heavy, even in conditions where the dropout voltage is low.

<Error amp>

The error amp monitors the output voltage. The voltage divided by the internal R1 and R2 resistors is a feedback voltage for

Error Amp. and compared to the reference voltage. The output voltage of the error amp becomes higher when the feedback

voltage is higher than the reference voltage. The frequency characteristics of the error amp are optimized internally.

<BLOCK DIAGRAM>

Lx

Inductor

PGND

L2

L1

VIN

VOUT

CE

Error

Amp.

High Side

Current Limit

Comparator

Phase

Compensation

R2

R1

Synch

Buffer

Drive

Logic

Minimum

On Time

Generator

S

R

Q

PWM/PFM

Selector

VIN

VOUT

Vref with

Soft Start

CE Control Logic,

UVLO

Thermal Shutdown

Short

Protection

CFB

AGND