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FAN5026 Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN5026 Datasheet(HTML) 11 Page - Fairchild Semiconductor |
11 / 17 page 11 www.fairchildsemi.com FAN5026 Rev. 1.0.5 Setting the Current Limit A ratio of ISNS is also compared to the current established when a 0.9 V internal reference drives the ILIM pin. The threshold is determined at the point when the . Since therefore, Since the tolerance on the current limit is largely depen- dent on the ratio of the external resistors, it is fairly accu- rate if the voltage drop on the Switching Node side of RSENSE is an accurate representation of the load current. When using the MOSFET as the sensing element, the variation of RDS(ON) causes proportional variation in the ISNS. This value not only varies from device to device, but also has a typical junction temperature coefficient of about 0.4%/°C (consult the MOSFET datasheet for actual values), so the actual current limit set point will decrease proportional to increasing MOSFET die tem- perature. A factor of 1.6 in the current limit setpoint should compensate for all MOSFET RDS(ON) variations, assuming the MOSFET’s heat sinking will keep its oper- ating die temperature below 125°C. Figure 11. Improving Current Sensing Accuracy More accurate sensing can be achieved by using a resis- tor (R1) instead of the RDS(ON) of the FET as shown in Figure 11. This approach causes higher losses, but yields greater accuracy in both VDROOP and ILIMIT. R1 is a low value (e.g. 10m Ω) resistor. Current limit (ILIMIT) should be set sufficiently high as to allow inductor current to rise in response to an output load transient. Typically, a factor of 1.3 is sufficient. In addition, since ILIMIT is a peak current cut-off value, we will need to multiply ILOAD(MAX) by the inductor ripple cur- rent (we'll use 25%). For example, in Figure 5 the target for ILIMIT would be: ILIMIT > 1.2 × 1.25 × 1.6 × 6A ≈ 14A (4) Gate Driver Section The adaptive gate control logic translates the internal PWM control signal into the MOSFET gate drive signals providing necessary amplification, level shifting and shoot-through protection. Also, it has functions that help optimize the IC performance over a wide range of oper- ating conditions. Since MOSFET switching time can vary dramatically from type to type and with the input voltage, the gate control logic provides adaptive dead time by monitoring the gate-to-source voltages of both upper and lower MOSFETs. The lower MOSFET drive is not turned on until the gate-to-source voltage of the upper MOSFET has decreased to less than approximately 1 volt. Simi- larly, the upper MOSFET is not turned on until the gate- to-source voltage of the lower MOSFET has decreased to less than approximately 1 volt. This allows a wide vari- ety of upper and lower MOSFETs to be used without a concern for simultaneous conduction, or shoot-through. There must be a low-resistance, low-inductance path between the driver pin and the MOSFET gate for the adaptive dead-time circuit to work properly. Any delay along that path will subtract from the delay generated by the adaptive dead-time circuit and shoot-through may occur. Frequency Loop Compensation Due to the implemented current mode control, the modu- lator has a single pole response with -1 slope at fre- quency determined by load where RO is load resistance and CO is load capacitance. For this type of modulator, Type 2 compensation circuit is usually sufficient. To reduce the number of external com- ponents and simplify the design task, the PWM controller has an internally compensated error amplifier. Figure 12 shows a Type 2 amplifier and its response along with the responses of a current mode modulator and of the con- verter. The Type 2 amplifier, in addition to the pole at the origin, has a zero-pole pair that causes a flat gain region at frequencies between the zero and the pole. This region is also associated with phase ‘bump’ or reduced phase shift. The amount of phase shift reduction depends the width of the region of flat gain and has a maximum value of 90 degrees. To further simplify the converter compensation, the modulator gain is kept inde- pendent of the input voltage variation by providing feed- forward of VIN to the oscillator ramp. ISNS 9 --------------- ILIM 4 × 3 ---------------------- > ISNS I LOAD R DS ON () × 100 R + SENSE -------------------------------------------- = I LIMIT 0.9V R ILIM -------------- 4 3 --- × 9 100 R SENSE + () × R DS ON () ---------------------------------------------------- × = (3a) or R ILIM 10.8 I LIMIT --------------- 100 R SENSE + () R DS ON () ------------------------------------------- × = (3b) LDRV PGND ISNS R SENSE Q2 F PO 1 2 πR OCO ------------------------ = (5) F Z 1 2 πR 2C1 ---------------------- 6kHz == (6a) F P 1 2 πR 2C2 ---------------------- 600kHz == (6b) |
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