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Product Number Input Voltage Output Voltage Maximun Output Current Packge Name

Step1 Output Voltage

  Vout is calculated from RFB1and RFB2.

Vo:[V] RFB2: [kΩ]  RFB1: [kΩ]


  Vout verification

RFB1 = [kΩ] ±[%] +  [Ω] ±[%]

RFB2 = [kΩ] ±[%]

Vo: Typ: [V]  Min: [V]  Max: [V]

Step2 Soft Start Time

 Capacitance(Css) is calculated from startup time(SST).

SST:[mS]  Css: [nF]


 Startup time verification.

Css:  SST: [mS]

Step3 External Inductor & Capacitor

  Lo is caluclated from Iox,freq.,Max_Vin(Ei_max) and Vout. Please put the same value as Minimun operating current into Iox. Detailed information is here


freq: [MHz]   Ei_max:[V]  Vo:[V]

Lo [µH]


  Minimum capacitance(Co) is caluculated from target ripple voltage. @Rc=0.0

Vrpl:[V]  Lo:   Co ≥ [µF]


  Vrpl,ΔIL,Iox Verification.

Co:   Rc:[Ω]

Vrpl: [V]  ΔIL: [A]  Iox: [A]

Please check that Minimum operationg current has become more than Iox.

Step1 Output voltage setting

The output voltage is set by a resistor divider from the output node to the VFB pin as follows equation.

step1-1 step1-1

This table shows recomemended resistance at typical output.


Note: RFB2 should be no more than 10KΩ(inclusive). Too high impedance may make the VFB signal noisy,and as aresult it may casuse noisy output voltage or voltage error.

Step2 Soft start Setting

The equation for soft start time is shown as below.


Css:Externalcapacitance which connected to the SS pin.
When Css is set at 10nF,soft start time is approximately 3ms

soft start time

Step3 Inductor and Capacitor Setting.

Given the desired input and output voltage,the inductor value and operating frequency determine the ripple current.



Lo and Co is determined to achive the condition that that Vout ripple satisfy the target value. To reduce the effect of ESR, we recommend higher value of Lo.

Max. Lo current:fig3-3

Max. Co ripple current:fig3-4

Iox is the critical current when CCM change to DCM. Minimun value of normal operating load current have to larger than Iox.



Circuit diagram(MCP)

Circuit diagram(MCP)

Please refer follows equeations for more understanding of output voltage.


Minimun operating current and critical current(Iox)

Fig.1 shows fundamental circuit of step down converter. This converter has two basic operating mode, first one is CCM(Continuous Conduction Mode) and 2nd one is Discontinuous Conduction Mode(DCM) as shown Fig.1 (a)(b). The Value of critical current (Iox) is defined as a current when coverter changes from CCM to DCM. Input-Output function are shown bellow and determined using assumption of some ideal conditions(the switing resistance and forward voltage of diode are negligible small).

Circuit diagram(MCP)

No need to adjust about Ton(Active time of switch Q) for fluctuation of output current(Io),but DCM require large value of Ton adjustment. Because CCM and DCM have different control operation(different transfer function), converter back and forth each mode(CCM/DCM) in the boundary condition(Fig.3) and causes it unstable. In addition, It's difficault to realize a high speed respose when large fluctuation of output current extend over CCM/DCM. We strongly recommend to satisfy a minimun value of normal operating load current adopts larger value than Iox for stablity and fast responsibility fo the system. Finaly, converter remain CCM when load current changed.

Critical current Iox

Internal Block Diagrams and External circuits   NN30321A (1-ch DC-DC converter IC with a built-in FET)


Internal Circuits

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