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ISL6539CA Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL6539CA Datasheet(HTML) 10 Page - Intersil Corporation |
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10 / 20 page  10 FN9144.4 June 6, 2005 Current Sensing The current on the lower MOSFET is sensed by measuring its voltage drop within its on-time. In order to activate the current sampling circuitry, two conditions need to be met. (1) the LGATE is high and (2) the phase pin sees a negative voltage for regular buck operation, which means the current is freewheeling through lower MOSFET. For the second channel of the DDR application, the phase pin voltage needs to be higher than 0.1V to activate the current sensing circuit for bidirectional current sensing. The current sampling finishes at about 400ns after the lower MOSFET has turned on. This current information is held for current mode control and overcurrent protection. The current sensing pin can source up to 260µA. The current sense resistor and OCSET resistor can be adjusted simultaneously for the same overcurrent protection level; however, the current sensing gain will be changed only according to the current sense resistor value, which will affect the current feedback loop gain. The middle point of the ISEN current can be at 75µA, but it can be tuned up and down to fit application needs. If another channel is switching at the moment the current sample is finishing, it could cause current sensing error and phase voltage jitter. In the design stage, the duty cycles and synchronization have to be analyzed for all the input voltage and load conditions to reduce the chance of current sensing error. The relationship between the sampled current and MOSFET current is given by: Which means the current sensing pin will source current to make the voltage drop on the MOSFET equal to the voltage generated on the sensing resistor, plus the internal resistor, along the ISEN pin current flowing path. Feedback Loop Compensation Both channel PWM controllers have internally compensated error amplifiers. To make internal compensation possible several design measures were taken. • The ramp signal applied to the PWM comparator has been made proportional to the input voltage by the VIN pin. This keeps the product of the modulator gain and the input voltage constant even when the input voltage varies. • The load current proportional signal is derived from the voltage drop across the lower MOSFET during the PWM off time interval, and is subtracted from the error amplifier output signal before the PWM comparator input. This effectively creates an internal current control loop. The resistor connected to the ISEN pin sets the gain in the current sensing. The following expression estimates the required value of the current sense resistor, depending on the maximum continuous load current, and the value of the MOSFETs rDSON, assuming the ISEN pin sources 75µA current. Because the current sensing circuit is a sample-and-hold type, the information obtained at the last moment of the sampling is being used. This current sensing circuit samples the inductor current very close to its peak value. The current feedback essentially injects a resistor Ri in series with the original LC filter as shown in Figure 5, where the sample- and-hold effect of the current loop has been ignored. Vc and Vo are small signal components extracted from its DC operation points. The value of the injected resistor can be estimated by: Ri is in kΩ, and RDS and RCS are in Ω. Vin divided by Vramp, is defined as Gm, which is a constant 8 or 18dB for both channels in dual switcher applications, when Vin is above 3V. Refer to Tables 1 and 2 for the ramp amplitude in different Vin pin connections. The feed-forward effect of the Vin is reflected in Gm. Vc is defined as the error amplifier output voltage. R2 R1 UGAT E LGAT E ISL6539 L1 Q1 Q2 C1 VOUT VSEN Vin R CS ISEN OCSET R OC Cz FIGURE 4. THE INTERNAL COMPENSATOR ISEN RCS 140 + () rDS ON () ID = RCS IMAX RDS ON () • 75 µA --------------------------------------------- = 140 Ω – FIGURE 5. THE EQUIVALENT CIRCUIT OF THE POWER STAGE WITH CURRENT LOOP INCLUDED + - Gm*Vc Ri Lo DCR Co Ro Vo ESR + - Ri VIN Vramp ----------------- rDS ON () RCS 140 + ---------------------------- 4.4k Ω • = ISL6539 |
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