Synchronous Step-Down DC/DC Converter with Built-In LDO Regulator in Parallel plus Voltage Detector

CIN

: 4.7

µF (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 >

XC9509 Series

L

The XC9509 series consists of a synchronous step-down DC/DC converter, a high speed LDO voltage regulator, and a voltage detector.

FOSC

4.7

µH (CDRH4D28C, SUMIDA)

10

µH (CDRH4D28C, SUMIDA)

22

µH (CDRH4D28C, SUMIDA)

1.2MHz

600kHz

300kHz

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.

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.

MSOP-10

(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.

The PWM mode of the XC9509A to F series are controlled on a specified frequency from light loads through the 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 XC9509H to L series can switch in any timing between PWM control and PWM/PFM automatic switching control. The series can not

control only PFM mode. If needed, the operation can be set on a specified frequency; therefore, the control of the noise etc. is possible and the

high efficiency at the time of the light load during PFM control mode is possible. 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. 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.

Semiconductor Ltd.

LX

DCOUT

VROUT

MODE

AGND

PGND

CE

PVDD

AVDD

1

CL2

L

CL1

SD

2

3

4

5

6

7

8

9

10

CIN2

VDOUT

VROUT

DCOUT

CIN1

Data Sheet

7