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

Preliminary

" Application Information

# DC/DC Waveform (3.3V, 1.2MHz)

Ics=

(VIN - DCOUT) x OnDuty / (L x Fosc)

Ics : Switching current from synchronous rectification to non-synchronous rectification

OnDuty : OnDuty ratio of P-ch driver transistor (

step down ratio : DCOUT / VIN)

L : Coil inductance value

Fosc : Oscillation Frequency

IDOUT : The DC/DC load current

XC9511 Series

1. The XC9511 series is designed for use with a ceramic output capacitor. If, however, the potential difference between dropout voltage or

output current is too large, a ceramic capacitor may fail to absorb the resulting high switching energy and oscillation could occur on the

output. If the input-output potential difference is large, connect an electrolytic capacitor in parallel to compensate for insufficient

capacitance.

2. Spike noise and ripple voltage arise in a switching regulator as with a DC/DC converter. These are greatly influenced by external

component selection, such as the coil inductance, capacitance values, and board layout of external components. Once the design has

been completed, verification with actual components should be done.

3. When the difference between VIN and VOUT is large in PWM control, very narrow pulses will be outputted, and there is the possibility

that some cycles may be skipped completely.

4. When the difference between VIN and VOUT is small, and the load current is heavy, very wide pulses will be outputted and there is the

possibility that some cycles may be skipped completely: in this case, the Lx pin may not go low at all.

5. The IC's DC/DC converter operates in synchronous mode when the coil current is in a continuous state and non-synchronous mode when

the coil current is in a discontinuous state. In order to maintain the load current value when synchronous switches to non-synchronous and

vise versa, a ripple voltage may increase because of the repetition of switching between synchronous and non-synchronous. When this

state continues, the increase in the ripple voltage stops. To reduce the ripple voltage, please increase the load capacitance value or use a

schottky diode externally. When the current used becomes close to the value of the load current when synchronous switches to non-

synchronous and vise versa, the switching current value can be changed by changing the coil inductance value. In case changes to coil

inductance are to values other than the recommended coil inductance values, verification with actual components should be done.

Semiconductor Ltd.

CH1:LX

CH2:DCOUT

Off-set Voltage:3.3V

VIN=3.7V, IDOUT=100mA

<External Components>

L: 4.7

µH (CDRH4D28C, SUMIDA)

CIN: 4.7

µF (ceramic)

CL: 10

µF (ceramic)

CH2:DCOUT

Off-set Voltage:3.3V

VIN=3.5V, IDOUT=100mA

<External Components>

L: 4.7

µH (CDRH4D28C, SUMIDA)

CIN: 4.7

µF (ceramic)

CL: 10

µF (ceramic)

CH1:LX

Data Sheet

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