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ISL6224CAZ Datasheet(PDF) 6 Page - Renesas Technology Corp |
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ISL6224CAZ Datasheet(HTML) 6 Page - Renesas Technology Corp |
6 / 13 page ISL6224 FN9042 Rev 8.00 Page 6 of 13 June 8, 2006 grounded. Table 1. shows the configuration for different modes of operation. Figure 1 below shows plots of the ramp speed compensation. The synchronous converter light-load efficiency is enhanced by a hysteretic mode of operation which is automatically engaged at light loads when the inductor current becomes discontinuous. As the filter inductor resumes continuous current, the PWM mode of operation is automatically restored. The ISL6224 control IC employs an average current mode control scheme with input voltage feedforward ramp programming for better rejection of input voltage variations. Current Sensing and Current Limit Protection The PWM converter uses the lower MOSFET on-state resistance, rDS(ON), as the current-sensing element. This technique eliminates the need for a current sense resistor and the associated power losses. If more accurate current protection is desired, current sense resistors may be used in series with the lower MOSFET’s source. A current proportional signal is used to provide average current mode control and overcurrent protection. The gain in the current sense circuit is set by the resistor connected from ISEN (pin 12) to the PHASE node of the buck converter. The value of this resistor can be estimated by the following expression: where Iomax is the maximum inductor current. The value of RISEN should be specified for the expected maximum operating temperature. An overcurrent protection threshold is set by an external resistor connected from OCSET (pin 4) to ground. The value of this resistor can be obtained from the following expression: where Ioc is the value of overcurrent. The resulting current out of the ISEN pin through RISEN, is used for current feedback and current limit protection. This is compared with an internal current limit threshold. When a sampled value of the output current is determined to be above the current limit threshold, the PWM drive is terminated and a counter is initiated. This limits the inductor current build-up and essentially switches the converter into current-limit mode. If an overcurrent is detected between 26ms to 53ms later, an overcurrent shutdown is initiated. If during the 26ms to 53ms period, an overcurrent is not detected, the counter is reset and sampling continues as normal. This current limit scheme has proven to be very robust in applications like portable computers where fast inductor current build-up is common due to a large difference between input and output voltages and a low value of the inductor. Light-Load (Hysteretic) Operation In the light-load (hysteretic) mode the output voltage is regulated by the hysteretic comparator which regulates the output voltage by maintaining the output voltage ripple as shown in Figure 2. In hysteretic mode, the inductor current flows only when the output voltage reaches the lower limit of the hysteretic comparator and turns off at the upper limit. Hysteretic mode saves converter energy at light loads by supplying energy only at the time when the output voltage requires it. This mode conserves energy by reducing the power dissipation associated with continuous switching. During the time between inductor current pulses, both the upper and lower MOSFETs are turned off. This is referred to as ‘diode emulation mode’ because the lower MOSFET performs the function of a diode. This diode emulation mode prevents the output capacitor from discharging through the lower MOSFET when the upper MOSFET is not conducting. NOTE: the PWM only operation can intentionally be forced by tying pin 16, FCCM, to VCC. TABLE 1. CONFIGURATION FOR MODES OF OPERATION OPERATION PIN 1 CONNECTION PIN 1 POTENTIAL One-stage 300kHz Vin V1 > 4V Two-stage 300kHz 150K-GND 1V < V1 < 2V Two-stage 600kHz GND V1 < 0.5V FIGURE 1. RAMP SPEED COMPENSATION Vo = 2.5V 300kHz CLOCK 600kHz CLOCK Vo/8 Vo/4 Vin 8 ---------- t T --- Vin 4 ---------- t T --- Vin 2 ---------- t T --- Risen Iomax Rdson 75 A ------------------------------------------- 100 – = Rocset 11 Risen Ioc Rdson --------------------------------- = |
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