Synchronous Step-Down DC/DC Converter with built-in LDO Regulator in parallel plus Voltage Detector

Preliminary

SD : XB0ASB03A1BR (TOREX)

CIN

: 4.7

µF x 2 (ceramic, TAIYO-YUDEN)

CL1

: 10

µF (ceramic, TAIYO-YUDEN)

CL2

:

1

µF (ceramic, TAIYO-YUDEN), VROUT>1.5V

: 2.2

µF (ceramic, TAIYO-YUDEN), VROUT<1.5V

# DC/DC Converter

< Reference Voltage Source >

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

< Ramp Wave Circuit >

< Error Amplifier >

< PWM/PFM >

XC9511 Series

22

µH (CDRH6D28, SUMIDA)

1.2MHz

The DC/DC converter of the XC9511 series automatically switches between synchronous / non-synchronous. The Schottky diode is not

normally needed. However, in cases where high efficiency is required when using the DC/DC converter during light load while in non-

synchronous operation, please connect a Schottky diode externally.

The XC9511 series consists of a synchronous step-down DC/DC converter, a high speed LDO voltage regulator, and a voltage detector.

600kHz

300kHz

L

FOSC

4.7

µH (CDRH4D28C, SUMIDA)

10

µH (CDRH5D28, SUMIDA)

The ramp wave circuit determines switching frequency. The frequency is fixed internally and can be selected from 300kHz, 600 kHz and 1.2

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

circuits.

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. When a voltage 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.

SOP-8 (TOP VIEW)

The series consists of a reference voltage source, ramp wave circuit, error amplifier, PWM comparator, phase compensation circuit, output

voltage adjustment resistors, driver transistor, synchronous switch, current limiter circuit, U.V.L.O. circuit and others. The series ICs compare,

using the error amplifier, the voltage of the internal voltage reference source with the feedback voltage from the VOUT pin through split

resistors. Phase compensation is performed on the resulting error amplifier output, to input a 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.

Semiconductor Ltd.

The XC9511A to C series are PWM control, while the XC9511D to F series can be automatically switched to PWM/PFM control.

The PWM mode of the XC9511A to C series are controlled on a specified frequency from light loads through to heavy loads. Since the frequency

is specified, the composition of a noise filter etc. becomes easy. However, the efficiency at the time of the light load may become low.

The XC9511D to F series can switch to PWM/PFM automatic switching control. With the automatic PWM/PFM switching control function, the

series ICs are automatically switched from PWM control to PFM control mode under light load conditions. The series can not control only PFM

mode. If during light load conditions the coil current becomes discontinuous and on-time rate falls lower than 30%, the PFM circuit operates to

output a pulse with 30% of a fixed on-time rate from the Lx pin. During PFM operation with this fixed on-time rate, pulses are generated at

different frequencies according to conditions of the moment. This causes a reduction in the number of switching operations per unit of time,

resulting in efficiency improvement under light load conditions. However, since pulse output frequency is not constant, consideration should be

given if a noise filter or the like is needed. Necessary conditions for switching to PFM operation depend on input voltage, load current, coil value

and other factors.

1

2

3

4

8

7

5

6

AVDD

PVDD

AGND

LX

DCOUT

VROUT

VDOUT

PGND

VROUT

L

CL2

DCOUT

SD

CIN1

( )

CL1

CIN2

+

+

+

+

Data Sheet

8