BD9C301FJ-LB Datasheet, PDF (13/22 Pages) Rohm – 4.5V to 18V Input, 3.0A Integrated MOSFET Single Synchronous Buck DC/DC Converter

BD9C301FJ-LB
2. Output Voltage Setting
The output voltage value can be set by the feedback resistance ratio.
V_OUT
0.8V
ERR
V_OUT =
R₁
×
R₂
R₂
×
0.8
　
[V]
R1
FB
R2
V_OUT has restriction with V_IN by the following equation_.
V_OUTMin : V_IN × 0.125 ≥ 0.8V
V_OUTMax : V_IN × 0.7
Figure 25. Feedback Resistor Circuit
3. Phase Compensation Component
A current mode control buck DC/DC converter is a two-pole, one-zero system. Two poles are formed by an error amplifier
and load and the one zero point is added by phase compensation. The phase compensation resistor R_CMP determines
the crossover frequency F_CRS where the total loop gain of the DC/DC converter is 0 dB. A high value crossover frequency
F_CRS provides a good load transient response characteristic but inferior stability. Conversely, a low value crossover
frequency F_CRS greatly stabilizes the characteristics but the load transient response characteristic is impaired. Here,
select the constant so that the crossover frequency F_CRS will be 1/20 of the switching frequency.
(1) Selection of Phase Compensation Resistor RCMP
The Phase Compensation Resistance R_CMP can be determined by using the following equation.
R_CMP =
2π ×V_OUT × F_CRS × C_OUT
V_FB × G_MP × G_MA
　
[Ω]
where :
V_OUT is Output Voltage
F_CRS is Crossover Frequency
C_OUT is Output Capacitance
V_FB is Feedback Reference Voltage(0.8 V (Typ))
G_MP is Current Sense Gain (7.8 A/V (Typ))
G_MA is Error Amplifier Trans conductance (300 μA/V (Typ))
(2) Selection of Phase Compensation Capacitance C_CMP
The phase compensation capacitance C_CMP can be determined by using the following equation.
C_CMP =
V_OUT ×
I_OUT ×
C _OUT
R_CMP
　
[F]
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21.Feb.2014 Rev.002