Electronic Components Datasheet Search 

ADP3207C Datasheet(PDF) 28 Page  ON Semiconductor 

ADP3207C Datasheet(HTML) 28 Page  ON Semiconductor 
28 / 32 page ADP3207C http://onsemi.com 28 impedance that is entirely resistive over the widest possible frequency range, including dc, and equal to the droop resistance (RO). With the resistive output impedance, the output voltage droops in proportion with the load current at any load current slew rate. This ensures the optimal positioning and minimizes the output decoupling. With the multi−mode feedback structure of the ADP3207C, users need to set the feedback compensation to make the converter output impedance work in parallel with the output decoupling. Several poles and zeros are created by the output inductor and decoupling capacitors (output filter) that need to be compensated for. A type−three compensator on the voltage feedback is adequate for proper compensation of the output filter. Equations 28 to 36 is intended to yield an optimal starting point for the design; some adjustments can be necessary to account for PCB and component parasitic effects (refer to the Tuning Procedure for ADP3207C section). The first step is to compute the time constants for all of the poles and zeros in the system: RE + n RO ) AD RDS ) RL VRT VID (eq. 28) ) 2 L (1 * n D) VRT n CX RO VVID TA + CX (RO * R ) ) LX RO RO * R RX (eq. 29) TB + (RX ) R * RO) CX (eq. 30) TC + VRT L * AD RDS 2 fSW VVID RE (eq. 31) TD + CX CZ RO 2 CX (RO * R ) ) CZ RO (eq. 32) Where: R’ is the PCB resistance from the bulk capacitors to the ceramics. RDS is the total low−side MOSFET on−resistance per phase. For this example, AD is 5, VRT = 1.5 V, R’ is approximately 0.4 m W (assuming an 8−layer motherboard), and LX is 250 pH for the four Panasonic SP capacitors. The compensation values can be solved using the following: CA + n RO TA RE RB (eq. 33) RA + TC CA (eq. 34) CB + TB RB (eq. 35) CFB + TD RA (eq. 36) The standard values for these components are subject to the tuning procedure, as introduced in the CIN Selection and Input Current DI/DT Reduction section. CIN Selection and Input Current DI/DT Reduction In continuous inductor current mode, the source current of the high−side MOSFET is approximately a square wave with a duty ratio equal to n x VOUT/VIN and an amplitude of 1−nth the maximum output current. To prevent large voltage transients, a low ESR input capacitor sized for the maximum rms current must be used. The maximum rms capacitor current happens at the lowest input voltage, and is given by: ICRMS + D IO 1 n D * 1 (eq. 37) ICRMS + 0.164 44 A 1 2 0.164 * 1 + 10.3 A In a typical notebook system, the battery rail decouplings are MLCC capacitors or a mixture of MLCC capacitors and bulk capacitors. In this example, the input capacitor bank is formed by eight pieces of 10 mF, and 25 V MLCC capacitors with a ripple current rating of about 1.5 A each. RC Snubber It is important in any buck topology to use a resistor capacitor snubber across the low side power MOSFET. The RC snubber dampens ringing on the switch node when the high side MOSFET turns on. The switch node ringing could cause EMI system failures and increased stress on the power components and controller. The RC snubber should be placed as close as possible to the low side MOSFET. Typical values for the resistor range from 1 W to 10 W. Typical values for the capacitor range from 330 pF to 4.7 nF. The exact value of the RC snubber depends on the PCB layout and MOSFET selection. Some fine tuning must be done to find the best values. The equation below is used to find the starting values for the RC subber. RSnubber + 1 2 p fRinging COSS (eq. 38) CSnubber + 1 p fRinging RSnubber (eq. 39) PSnubber + CSnubber VInput 2 fSwithing (eq. 40) Where RSnubber is the snubber resistor. CSnubber is the snubber capacitor. fRinging is the frequency of the ringing on the switch node when the high side MOSFET turns on. COSS is the low side MOSFET output capacitance at VInput. This is taken from the low side MOSFET data sheet. Vinput is the input voltage. fSwitching is the switching frequency. PSnubber is the power dissipated in RSnubber. 
