SC2446

Dual-Phase Single or Two Output Synchronous Step-Down Controllers POWER MANAGEMENT Description The SC2446 is a high-frequency dual synchronous stepdown switching power supply controller. It provides outof-phase high-current output gate drives to all N-channel MOSFET power stages. The SC2446 operates in synchronous continuous-conduction mode. Both phases are capable of maintaining regulation with sourcing or sinking load currents, making the SC2446 suitable for generating both VDDQ and the tracking VTT for DDR applications. The SC2446 employs fixed frequency peak current-mode control for the ease of frequency compensation and fast transient response. The dual-phase step-down controllers of the SC2446 can be configured to provide two individually controlled and regulated outputs or a single output with shared current in each phase. The Step-down controllers operate from an input of at least 4.7V and are capable of regulating outputs as low as 0.5V The step-down controllers in the SC2446 have the provision to sense a synthesized MOSFET RDS(ON) for current-mode control. This sensing scheme (U.S. patent 6,441,597) eliminates the need of the current-sense resistor and is more noise-immune than direct sensing of the high-side or the low-side MOSFET voltage. Precise current-sensing with sense resistor is optional. Individual soft-start and overload shutdown timer is included in each step-down controller. The SC2446 implements hiccup overload protection. In two-phase singleoutput configuration, the master timer controls the softstart and overload shutdown functions of both controllers. SC2446 Features 2-Phase synchronous continuous conduction mode for high efficiency step-down converters Out of phase operation for low input current ripples Output source and sink currents Fixed frequency peak current-mode control 75mV/-110mV maximum current sense voltage Synthesized MOSFET RDS(ON) current-sensing for low-cost applications Optional resistor current-sensing for precise currentlimit Dual outputs or 2-phase single output operation Excellent current sharing between individual phases Wide input voltage range: 4.7V to 16V Individual soft-start, overload shutdown and enable Duty cycle up to 88% 0.5V feedback voltage for low-voltage outputs External reference input for DDR applications Buffered VDDQ/2 output Programmable frequency up to 1 MHz per phase External synchronization Industrial temperature range 28-lead TSSOP - EDP package Applications Telecommunication power supplies DDR memory power supplies Graphic power supplies Servers and base stations Typical Application Circuit VIN C92 D11 PVCC D12 Q21 VO2 C99 + R79 C96 C95 R73 C93 BST2 GDH2 BST1 GDH1 R74 C94 Q22 L12 Q24 C98 + R76 C97 R80 L11 Q23 VO1 C100 CFILTER R75 RFILTER R77 GDL2 GDL1 PGND VPN2 CS2+ VPN1 CS1+ CS1IN1COMP1 REF AGND Rosc AVCC REFOUT R78 CFILTER RFILTER RCS+ RCS- RCS+ RCS- R81 C101 R83 C103 CS2IN2COMP2 R82 C102 REF VIN SYNC REFIN VIN2 SYNC SS1/EN1 SS2/EN2 C104 C105 R84 R85 VIN C106 C107 Figure 1 Revision: September 9, 2004 SC2446 Dual Independant Outputs U1 C108 C109 1 U.S. Patent No. 6,441,597, www.semtech.com SC2446 POWER MANAGEMENT Absolute Maximum Rating Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not implied. Parameter Supply Voltage For Step-D own C ontrollers Input Voltage For the Second C onverter Hi gh-Si de D ri ver Supply Voltages Symbol AVC C , PVC C VIN2 VBST1,VBST2 Maximum R atings -0.3 to 20 -0.3 to 20 -0.3 to 32 (steady state) -0.3 to 40 (for 10 . 2πfsR esr the ripple-voltage due to ESL is ∆v ESL = L esl fs In many applications, several low ESR ceramic capacitors are added in parallel with the aluminum capacitors in order to further reduce ESR and improve high frequency decoupling. Because the values of capacitance and ESR are usually different in ceramic and aluminum capacitors, the following remarks are made to clarify some practical issues. Remark 1: High frequency ceramic capacitors may not carry most of the ripple current. It also depends on the capacitor value. Only when the capacitor value is set properly, the effect of ceramic capacitor low ESR starts to be significant. For example, if a 10 µ F, 4m Ω c eramic capacitor is connected in parallel with 2x1500µF, 90mΩ electrolytic capacitors, the ripple current in the ceramic capacitor is only about 42% of the current in the electrolytic capacitors at the ripple frequency. If a 100µF, 2mΩ ceramic capacitor is used, the ripple current in the ceramic capacitor will be about 4.2 times of that in the electrolytic capacitors. When two 100µF, 2mΩ ceramic capacitors are used, the current ratio increases to 8.3. In this case most of the ripple current flows in the ceramic decoupling capacitor. The ESR of the ceramic capacitors will then determine the output ripple-voltage. Remark 2: The total equivalent capacitance of the filter bank is not simply the sum of all the paralleled capacitors. The total equivalent ESR is not simply the parallel combination of all the individual ESR’s either. Instead they should be calculated using the following formulae. C eq (ω) := (R1a + R1b )2 ω2C1a C1b + (C1a + C1b )2 (R1a C1a + R1b C1b )ω2 C1a C1b + (C1a + C1b ) R1aR1b (R1a + R1b )ω2C1a C1b + (R1b C1b + R1a C1a ) (R1a + R1b )2 ω2 C1a C1b + (C1a + C1b )2 2 2 2 2 2 2 2 2 2 2 and the ESR ripple-voltage is ∆v ESR = R esr δIo . Aluminum capacitors (e.g. electrolytic, solid OS-CON, POSCAP, tantalum) have high capacitances and low ESL’s. The ESR has the dominant effect on the output ripple voltage. It is therefore very important to minimize the ESR. When determining the ESR value, both the steady state ripple-voltage and the dynamic load transient need to be considered. To keep the steady state output ripple-voltage < ∆Vo, the ESR should satisfy R esr1 < ∆Vo . δIo To limit the dynamic output voltage overshoot/undershoot within α (say 3%) of the steady state output voltage) from no load to full load, the ESR value should satisfy R esr 2 < αVo . Io Then, the required ESR value of the output capacitors should be Resr = min{Resr1,Resr2 }. The voltage rating of aluminum capacitors should be at least 1.5Vo. The RMS current ripple rating should also be greater than δIo 23 . R eq (ω) := Usually it is necessary to have several capacitors of the same type in parallel to satisfy the ESR requirement. The voltage ripple cause by the capacitor charge/discharge  2004 Semtech Corp. where R 1a a nd C 1a a re the ESR and capacitance of electrolytic capacitors, and R1b and C1b are the ESR and capacitance of the ceramic capacitors respectively. (Figure 7) 13 www.semtech.com SC2446 POWER MANAGEMENT Application Information (Cont.) C1a C1b Ceq R1a R1b Req Figure 7. Equivalent RC branch. Req and Ceq are both functions of frequency. For rigorous design, the equivalent ESR should be evaluated at the ripple frequency for voltage ripple calculation when both ceramic and electrolytic capacitors are used. If R1a = R1b = R1 and C1a = C1b = C1, then Req and Ceq will be frequencyindependent and Req = 1/2 R1 and Ceq = 2C1. Input Capacitor (Cin) The input supply to the converter usually comes from a pre-regulator. Since the input supply is not ideal, input capacitors are needed to filter the current pulses at the switching frequency. A simple buck converter is shown in Figure 8. Figure 9. Typical waveforms at converter input. It can be seen that the current in the input capacitor pulses with high di/dt. Capacitors with low ESL should be used. It is also important to place the input capacitor close to the MOSFET’s on the PC board to reduce trace inductances around the pulse current loop. The RMS value of the capacitor current is approximately ICin = Io D[(1 + δ2 D D )(1 − )2 + 2 (1 − D) ]. 12 η η The power dissipated in the input capacitors is then PCin = ICin2Resr. For reliable operation, the maximum power dissipation in the capacitors should not result in more than 10oC of temperature rise. Many manufacturers specify the maximum allowable ripple current (ARMS) rating of the capacitor at a given ripple frequency and ambient temperature. The input capacitance should be high enough to handle the ripple current. For higher power applications, multiple capacitors are placed in parallel to increase the ripple current handling capability. Figure 8. A simple model for the converter input In Figure 8 the DC input voltage source has an internal impedance Rin and the input capacitor Cin has an ESR of Resr. MOSFET and input capacitor current waveforms, ESR voltage ripple and input voltage ripple are shown in Figure 9.  2004 Semtech Corp. 14 www.semtech.com SC2446 POWER MANAGEMENT Application Information (Cont.) Sometimes meeting tight input voltage ripple specifications may require the use of larger input capacitance. At full load, the peak-to-peak input voltage ripple due to the ESR is δ ∆v ESR = R esr (1 + )Io . 2 ICin ≈ 0.5Io1 + D 2 (Io1 + Io 2 )2 + (D1 − D 2 − 0.5)Io 2 . 2 2 If D1>0.5 and D2 > 0.5, then ICin ≈ (D1 + D 2 − 1)(Io1 + Io 2 )2 + (1 − D 2 )Io1 + (1 − D1 )Io2 . 2 2 The peak-to-peak input voltage ripple due to the capacitor is ∆v C ≈ DIo , Cin fs Choosing Power MOSFET’s Main considerations in selecting the MOSFET’s are power dissipation, cost and packaging. Switching losses and conduction losses of the MOSFET’s are directly related to the total gate charge (Cg) and channel on-resistance (Rds(on)). In order to judge the performance of MOSFET’s, the product of the total gate charge and on-resistance is used as a figure of merit (FOM). Transistors with the same FOM follow the same curve in Figure 10. From these two expressions, CIN can be found to meet the input voltage ripple specification. In a multi-phase converter, channel interleaving can be used to reduce ripple. The two step-down channels of the SC2446 operate at 180 degrees from each other. If both step-down channels in the SC2446 are connected in parallel, both the input and the output RMS currents will be reduced. Ripple cancellation effect of interleaving allows the use of smaller input capacitors. When converter outputs are connected in parallel and interleaved, smaller inductors and capacitors can be used for each channel. The total output ripple-voltage remains unchanged. Smaller inductors speeds up output load transient. When two channels with a common input are interleaved, the total DC input current is simply the sum of the individual DC input currents. The combined input current waveform depends on duty ratio and the output current waveform. Assuming that the output current ripple is small, the following formula can be used to estimate the RMS value of the ripple current in the input capacitor. Let the duty ratio and output current of Channel 1 and Channel 2 be D1, D2 and Io1, Io2, respectively. If D1