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XCM520

Series

●DC/DC BLOCK

The DC/DC block of the XCM520 series consists of a reference voltage source, ramp wave circuit, error amplifier, PWM

comparator, phase compensation circuit, output voltage adjustment resistors, P-channel MOSFET driver transistor, N-channel

MOSFET switching transistor for the synchronous switch, current limiter circuit, UVLO circuit and others. (See the block

diagram above.)

By using the error amplifier, the voltage of the internal voltage reference source is compared with the feedback voltage from the

signal to the PWM comparator to determine the turn-on time during PWM operation. The PWM comparator compares, in terms

of voltage level, the signal from the error amplifier with the ramp wave from the ramp wave circuit, and delivers the resulting

output to the buffer driver circuit to cause the Lx pin to output a switching duty cycle. This process is continuously performed to

ensure stable output voltage.

The current feedback circuit monitors the P-channel MOS driver transistor current for each switching operation, and modulates

the error amplifier output signal to provide multiple feedback signals. This enables a stable feedback loop even when a low

ESR capacitor such as a ceramic capacitor is used ensuring stable output voltage.

<Reference Voltage Source>

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

<Ramp Wave Circuit>

The ramp wave circuit determines switching frequency. The frequency is fixed internally and can be selected from 1.2MHz or

3.0MHz. Clock pulses generated in this circuit are used to produce ramp waveforms needed for PWM operation, and to

synchronize all the internal circuits.

<Error Amplifier>

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

voltage divided by the internal split resistors, R1 and R2. When a voltage is lower than the reference voltage is fed back, the

output voltage of the error amplifier increases. The gain and frequency characteristics of the error amplifier output are fixed

internally to deliver an optimized signal to the mixer.

■TYPICAL APPLICATION CIRCUIT

VIN

CL1

CIN1

CIN2

CL3

L

7

8

9

10

3

4

5

6

Lx

VIN2

EN2

VOUT2

11

12

2

1

VOUT3

EN1

VSS

VOUT1

VIN1

CL2

PGND

AGND

EN3/MODE

:

1μF

(Ceramic)

:

1μF

(Ceramic)

:

1μF

(Ceramic)

L

:

1.5μH

(NR3015 TAIIYO YUDEN)

:

4.7μF

(Ceramic)

:

10μF

(Ceramic)

:

1μF

(Ceramic)

:

1μF

(Ceramic)

:

1μF

(Ceramic)

L

:

4.7μH

(NR4018 TAIIYO YUDEN)

:

4.7μF

(Ceramic)

:

10μF

(Ceramic)

■OPERATIONAL EXPLANATION