tonc =T×VIN/VOUT Equation 2
When ton<tonc, the mode is the discontinuous mode, and when ton=tonc, the mode is the continuous mode.
ꢀ OUTPUT CURRENT AND SELECTION OF EXTERNAL COMPONENTS
When P-channel Tr. of Lx is ON:
(Wherein, Ripple Current P-P value is described as IRP, ON resistance of P-channel Tr. and N-channel Tr. of Lx are
respectively described as Ronp and Ronn, and the DC resistor of the inductor is described as RL.)
VIN=VOUT+(Ronp+RL)×IOUT+L×IRP/ton
Equation 3
Equation 4
When P-channel Tr. of Lx is “OFF”(N-channel Tr. is “ON”):
L×IRP/toff = RL×IOUT + VOUT + Ronn×IOUT
Put Equation 4 to Equation 3 and solve for ON duty of P-channel transistor, ton/(toff+ton)=DON,
DON=(VOUT-Ronn×IOUT+RL×IOUT)/(VIN+ Ronn×IOUT -Ronp×IOUT) Equation 5
Ripple Current is as follows;
IRP=(VIN-VOUT-Ronp×IOUT-RL×IOUT)×DON/fosc/L
…Equation 6
wherein, peak current that flows through L, and Lx Tr. is as follows;
ILmax=IOUT+IRP/2
…Equation 7
Consider ILmax, condition of input and output and select external components.
ꢀThe above explanation is directed to the calculation in an ideal case in continuous mode.
ꢀ How to Adjust Output Voltage and about Phase Compensation
As for Adjustable Output type, feedback pin (VFB) voltage is controlled to maintain 0.8V.
Output Voltage, VOUT is as following equation;
VOUT: R1+R2=VFB: R2
VOUT=VFB×(R1+R2)/R2
Thus, with changing the value of R1 and R2, output voltage can be set in the specified range.
In the DC/DC converter, with the load current and external components such as L and C, phase might be behind 180
degree. In this case, the phase margin of the system will be less and stability will be worse. To prevent this, phase
margin should be secured with proceeding the phase. A zero is formed with R1, R2, and Cb.
Fpole 1/2π√LCOUT
Fzero 1/(2π×(R1+R2)×Cb)
Considering external components, LC, set the appropriate value of R1+R2 and Cb.
Rb is effective for reducing the noise on VFB, therefore, add Rb so that R1+Rb=30kΩ.
When the R1≥30kΩ, additional Rb is not necessary, so it should be wired to VFB directly.
ꢀ External Components
1. Inductor
Select an inductor that peak current does not exceed ILmax. If larger current than allowable current flows, magnetic
saturation occurs and make transform efficiency worse.
Supposed that the load current is at the same, the smaller value of L is used, the larger the ripple current is.
Provided that the allowable current is large in that case and DC current is small, therefore, for large output current,
efficiency is better than using an inductor with a large value of L and vice versa.
2. Capacitor
Rev. 1.12
- 10 -
RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
