BD9E302EFJ Datasheet, PDF (33/44 Pages) Rohm – Integrated MOSFET Single Synchronous Buck DC/DC Converter

BD9E302EFJ
Datasheet
6. Phase Compensation
A current mode control buck DC/DC converter is a one-pole, one-zero system. The poles are formed by an error amplifier
and the one load and the one zero point is added by the phase compensation. The phase compensation resistor R3
determines the crossover frequency FCRS(20kHz (Typ)) where the total loop gain of the DC/DC converter is 0 dB. The high
value of this crossover frequency FCRS provides a good load transient response characteristic but inferior stability.
Conversely, specifying a low value for the crossover frequency FCRS greatly stabilizes the characteristics but the load
transient response characteristic is impaired.
(1) Selection of Phase Compensation Resistor R3
The phase compensation resistance R3 can be determined by using the following equation.
R₃
2π
V _OUT
F_CRS
C _OUT
V _FB
G _MP
G _MA
[Ω]
where :
V_OUT is the output voltage
F_CRS is the crossover frequency
C_OUT is the output capacitance
V_FB is the feedback reference voltage (0.8 V (Typ))
G _MP is the current sense gain (20A/V (Typ))
G _MA is the error amplifier transconductance (140 μA/V (Typ))
＊The actual FCRS may different from the value in equation due to DC bias characteristics of COUT .
Please set R3 base on the actual evaluation.
(2) Selection of phase compensation capacitance C2
For stable operation of the DC/DC converter, inserting a zero point under 1/6 of the zero crossover frequency cancels
the phase delay due to the pole formed by the load often, thus, providing favorable characteristics.
Please use capacitors for C2 such as ceramic type.
The phase compensation capacitance C2 can be determined by using the following equation.
C ₂  _2π
1
R₃
F_Z
[F]
where
F_Z is the Zero point inserted
* In case C2 calculated result exceeds 0.015μF, set the value of compensation capacitance C2 0.015μF. Setting too
large C2 value maybe cause startup failure etc.
(3) Selection of Phase Compensation Capacitance C1
Adding zero point at 20 kHz is recommended to get a better transient load response characteristic for DC/DC
converter. Please use capacitors for C1 such as ceramic type, and set value below 1000pF.
C1 can be determined by the following equation.
C₁
2π
1
R₁
20kHz ^{F}
(4) Loop stability
In order to ensure stability of DC/DC converter, confirm there is enough phase margin on actual equipment.
Under the worst condition, it is recommended to ensure phase margin is 45° or more. In fact, the characteristics may
variable due to PCB layout, routing of wiring, types of used components and operating environments (temperature
etc.). Use gain-phase analyzer or FRA to confirm frequency characteristics on actual equipment. Contact the
manufacturer of each measuring equipment to check its measuring method, etc.
7. Bootstrap capacitor
Bootstrap capacitor CBOOT shall be 0.1μF. Connect a bootstrap capacitor between SW pin and BOOT pin.
For capacitance of Bootstrap capacitor, take temperature characteristics, DC bias characteristics, etc. into consideration.
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TSZ02201-0J3J0AJ00890-1-2
27.Apr.2016 Rev.002