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SP6652 Datasheet(PDF) 5 Page - Sipex Corporation |
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SP6652 Datasheet(HTML) 5 Page - Sipex Corporation |
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5 / 16 page  5 May25-07 RevH SP6652 1A, High Efficiency, Current Mode PWM Buck Regulator © 2007 Sipex Corporation Current Mode Control and Slope Compensation The SP6652 is designed to use low value ceramic capacitors and low value inductors to reduce the converter’s volume and cost in portable devices. Current mode PWM control was, therefore, chosen for the ease of compensation when using ceramic output capacitors and better transient line rejec- tion, which is important in battery powered applications. Current mode control spreads the two poles of the output power train filter far apart so that the modulator gain crosses over at -20dB/decade instead of the usual -40dB/decade. The external compensation network is, simply, a series RC circuit con- nected between ground and the output of the internal transconductance error amplifier. It is well known that an unconditional insta- bility exists for any fixed frequency current- mode converter operating above 50% duty cycle. A simple, constant-slope compensa- tionischosentoachievestabilityunderthese conditions. The most common high duty cycle application is a Li-Ion battery powered regulator with a 3.3V output (D ≥ 90%). Since thecurrentloopiscriticallydampedwhenthe compensation slope (denoted MCV) equals thenegativedischargeslope(denotedM2V), the amount of slope compensation chosen is, therefore: M2 = dIL/dTOFF =-VOUT/L = -3.3V/4.7µH = -702mA/µs MV = M•RPMOS MCV = -MV = 702mA/µs•0.2Ω = 140mV/µs, for RPMOS = 0.20Ω The inductor current is sensed as a voltage across the PMOS charging switch and the NMOS synchronous rectifier (see BLOCK DIAGRAM). Duringinductorcurrentcharge, V(PVIN)-V(LX) represents the charging cur- rent ramp times the resistance of the PMOS chargingswitch.Tokeeptheeffectivecurrent slopecompensationconstant(remembering current is being compensated, not voltage) the voltage slope must be proportional to RPMOS. To account for this, the slope com- pensation voltage is internally generated with a bias current that is also proportional to RPMOS. Over Current Protection In steady state closed loop operation the voltage at the COMP pin controls the duty cycle.Duetothecurrentmodecontrolandthe slope compensation, this voltage will be: V(COMP)• {ILPK•RPMOS + MCV•TON + VBE(Q1)} The COMP node will be clamped when its voltage tries to exceed V(BLIM) + VBE(Q1). The VBE(Q1) term is cancelled by VBE(Q2) at the output of the translator. The correct value of clamp voltage is, therefore: V(BLIM) = IL(MAX)• RPMOS + MCV •TON The IL(MAX) term is generated with a bias current that is proportional to RPMOS, to keep the value of current limit approximately constant over process and temperature variations, while the MCV •TON is generated by a peak-holding circuit that senses the amplitude of the slope compensation ramp at the end of TON. There is minimum on-time (TON) generated even if the COMP node is at zeroV, since the peak current comparator is reset at the end of a charge cycle and is held low during a blanking time after the start of the next charge cycle. This is necessary to swamp the transients in the inductor current ramp around switching times. The minimum TON (100ns, nominally) is not sufficient for the COMP node to keep control of the current DetaileD Description |
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