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LT1158I Datasheet(PDF) 11 Page - Linear Technology |
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LT1158I Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT1158 11 OUTPUT CURRENT (A) 0 80 90 4.0 LT1158 F04 70 60 1.0 2.0 3.0 100 0.5 1.5 2.5 3.5 FIGURE 12 CIRCUIT VIN = 12V efficiency vs. output current for the Figure 12 regulator with VIN = 12V. Current Limit in Switching Regulator Applications Current is sensed by the LT1158 by measuring the voltage across a current shunt (low valued resistor). Normally, this shunt is placed in the source lead of the top MOSFET (see Short-Circuit Protection in Bridge Applications). However, in step-down switching regulator applications, the remote current sensing capability of the LT1158 allows the actual inductor current to be sensed. This is done by placing the shunt in the output lead of the inductor as shown in Figure 3. Routing of the sense+ and sense– PC traces is critical to prevent stray pickup. These traces must be routed together at minimum spacing and use a Kelvin connection at the shunt. When the voltage across RSENSE exceeds 110mV, the LT1158 fault pin begins to conduct. By feeding the fault signal back to a control input of the PWM, the LT1158 will assume control of the duty cycle forming a true current mode loop to limit the output current: IOUT = 110mV R in current limit SENSE In LT3525 based circuits, connecting the fault pin to the LT3525 soft-start pin accomplishes this function. In cir- cuits where the LT1158 input is being driven with a ramp or sawtooth, the fault pin is used to pull down the DC level of the input. The constant off-time circuits shown in Figures 10 and 12 are unique in that they also use the current sense during normal operation. The LT1431 output reduces the normal LT1158 110mV fault conduction threshold such that the fault pin conducts at the required load current, thus discharging the input ramp capacitor. In current limit the LT1431 output turns off, allowing the fault conduction threshold to reach its normal value. The resistor RGS shown in Figure 3 is necessary to prevent output voltage overshoot due to charge coupled into the gate of the top MOSFET by a large start-up dv/dt on VIN. If DC operation of the top MOSFET is required, RGS must be 330k or greater to prevent loading the charge pump. One fundamental difference in the operation of a step- down regulator with synchronous switching is that it never becomes discontinuous at light loads. The inductor cur- rent doesn’t stop ramping down when it reaches zero, but actually reverses polarity resulting in a constant ripple current independent of load. This does not cause any efficiency loss as might be expected, since the negative inductor current is returned to VIN when the switch turns back on. The LT1158 performs the synchronous MOSFET drive and current sense functions in a step-down switching regula- tor. A reference and PWM are required to complete the regulator. Any voltage-mode PWM controller may be used, but the LT3525 is particularly well suited to high power, high efficiency applications such as the 10A circuit shown in Figure 13. In higher current regulators a small Schottky diode across the bottom MOSFET helps to re- duce reverse-recovery switching losses. The LT1158 input pin can also be driven directly with a ramp or sawtooth. In this case, the DC level of the input waveform relative to the 1.4V threshold sets the LT1158 duty cycle. In the 5V to 3.3V converter circuit shown in Figure 11, an LT1431 controls the DC level of a triangle wave generated by a CMOS 555. The Figure 10 and 12 circuits use an RC network to ramp the LT1158 input back up to its 1.4V threshold following each switch cycle, setting a constant off time. Figure 4 shows the Figure 4. Typical Efficiency Curve for Step-Down Regulator with Synchronous Switch APPLICATIONS INFORMATION |
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