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LTC3526L Datasheet(PDF) 8 Page - Linear Technology |
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LTC3526L Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LTC3526/LTC3526B 3526bfd Error Amplifier The positive input of the transconductance error amplifier is internally connected to the 1.195V reference and the negative input is connected to FB. Clamps limit the mini- mumandmaximumerrorampoutputvoltageforimproved large-signal transient response. Power converter control loop compensation is provided internally. An external resistive voltage divider from VOUT to ground programs the output voltage via FB from 1.6V to 5.25V. V V R R OUT = + 1 195 1 2 1 . • Current Sensing Losslesscurrentsensingconvertsthepeakcurrentsignalof theN-channelMOSFETswitchintoavoltagethatissummed with the internal slope compensation. The summed signal is compared to the error amplifier output to provide a peak current control command for the PWM. Current Limit The current limit comparator shuts off the N-channel MOSFET switch once its threshold is reached. The cur- rent limit comparator delay to output is typically 60ns. Peak switch current is limited to approximately 700mA, independent of input or output voltage, unless VOUT falls below 0.7V, in which case the current limit is cut in half. Zero Current Comparator The zero current comparator monitors the inductor cur- rent to the output and shuts off the synchronous rectifier (Refer to Block Diagram) operaTion ZETEX ZC2811E 3526 F01 LTC3526/LTC3526B SHDN VIN VCNTRL 4M ±30% LTC3526/LTC3526B SHDN 4M ±30% 1M R VCNTRL 1M R > (VCNTRL/(VIN + 0.4) – 1)M Figure 1. Recommended Shutdown Circuits when Driving SHDN above VIN exceeds the input by 0.24V, the IC powers itself from VOUT instead of VIN. At this point the internal circuitry has no dependency on the VIN input voltage, eliminating the requirement for a large input capacitor. The input voltage can drop as low as 0.5V. The limiting factor for the ap- plication becomes the availability of the power source to supply sufficient energy to the output at low voltages, and maximum duty cycle, which is clamped at 90% typical. Note that at low input voltages, small voltage drops due to series resistance become critical, and greatly limit the power delivery capability of the converter. LOW NOISE FIXED FREqUENCY OPERATION Soft-Start TheLTC3526/LTC3526Bcontaininternalcircuitrytoprovide soft-start operation. The soft-start circuitry slowly ramps the peak inductor current from zero to its peak value of 700mA (typical) in approximately 0.5ms, allowing start- up into heavy loads. The soft-start circuitry is reset in the event of a shutdown command or a thermal shutdown. Oscillator An internal oscillator sets the switching frequency to 1MHz. Shutdown Shutdown is accomplished by pulling the SHDN pin below 0.3V and enabled by pulling the SHDN pin above 0.8V typical. Although SHDN can be driven above VIN or VOUT (up to the absolute maximum rating) without damage, the LTC3526/LTC3526B have a proprietary test mode that may be engaged if SHDN is held in the range of 0.5V to 1V higher than the greater of VIN or VOUT. If the test mode is engaged, normal PWM switching action is interrupted, which can cause undesirable operation in some applications. Therefore, in applications where SHDN may be driven above VIN, a resistor divider or other means must be employed to keep the SHDN voltage below (VIN + 0.4V) to prevent the possibility of the test mode being engaged. Please refer to Figure 1 for two possible implementations. |
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Similar Description - LTC3526L |
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