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XC9508 Datasheet(PDF) 10 Page - Torex Semiconductor

No. de Pieza. XC9508
Descripción  Synchronous Step-Down DC/DC Converter with Built-In LDO Regulator plus Voltage Detector
Descarga  36 Pages
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Fabricante  TOREX [Torex Semiconductor]
Página de inicio  http://www.torex.co.jp
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XC9508 Datasheet(HTML) 10 Page - Torex Semiconductor

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10
XC9508 Series
Data Sheet
ud200541
DC/DC Converter (Continued)
<PWM/PFM>
The XC9508A series is PWM control, while the XC9508C series can be automatically switched between PWM control
and PWM/PFM control. The PWM of the XC9508A series is 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 XC9508C series can switch in any timing between PWM control and PWM/PFM automatic switching control. The
series cannot 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.
<Synchronous / Non-synchronous>
The XC9508 series automatically switches between synchronous / non-synchronous according to the state of the DC/DC
converter. Highly efficient operations are achievable using the synchronous mode while the coil current is in a
continuous state.
The series enters non-synchronous operation when the built-in N-ch switching transistor for
synchronous operation is shutdown, which happens when the load current becomes low and the operation changes to a
discontinuous state. The IC can operate without an external schottky diode because the parasitic diode in the N-ch
switching transistor provides the circuit's step-down operation. However, since Vf of the parasitic diode is a high 0.6V,
the efficiency level during non-synchronous operation shows a slight decrease.
Please use an external schottky diode
if high efficiency is required during light load current.
Continuous Mode: Synchronous
Discontinuous Mode: Non-Synchronous
OPERATIONAL EXPLANATION (Continued)


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