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LTC3723-2 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3723-2 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC3723-1/LTC3723-2 10 372312f – + TURN-ON OUTPUT 2.5V + – V 2V 10 µA VREF DPRG RDPRG 372312 F01 200k OPTIONAL Please refer to the detailed Block Diagrams for this discus- sion. The LTC3723-1 and LTC3723-2 are synchronous PWM push-pull controllers. The LTC3723-1 operates with peak pulse-by-pulse current mode control while the LTC3723-2 offers voltage mode control operation. They are best suited for moderate to high power isolated power systems where small size and high efficiency are required. The push-pull topology delivers excellent transformer utilization and requires only two low side power MOSFET switches. Both controllers generate 180 ° out of phase 0% to < 50% duty cycle drive signals on DRVA and DRVB. The external MOSFETs are driven directly by these power- ful on-chip drivers. The external MOSFETs typically con- trol opposite primary windings of a centertapped power transformer. The centertap primary winding is connected to the input DC feed. The secondary of the transformer can be configured in different synchronous or nonsynchronous configurations depending on the application needs. The duty ratio is controlled by the voltage on COMP. A switching cycle commences with the falling edge of the internal oscillator clock pulse. The LTC3723-1 attenuates the voltage on COMP and compares it to the current sense signal to terminate the switching cycle. The LTC3723-2 compares the voltage on COMP to a timing ramp to terminate the cycle. The LTC3723-2’s CT waveform can be used for this purpose or separate R-C components can be connected to RAMP to generate the timing ramp. If the voltage on CS exceeds 300mV, the present cycle is termi- nated. If the voltage on CS exceeds 600mV, all switching stops and a soft-start sequence is initiated. The LTC3723-1 / LTC3723-2 also provide drive signals for secondary side synchronous rectifier MOSFETs. Synchro- nous rectification improves converter efficiency, espe- cially as the output voltages drop. Independent turn-off control of the synchronous rectifiers is provided via SPRG in order to optimize the benefit of the synchronous recti- fiers. A resistor from SPRG to GND sets the desired turn off delay. A host of other features including an error amplifier, system UVLO programming, adjustable leading edge blank- ing, slope compensation and programmable dead-time provide flexibility for a variety of applications. OPERATIO Programming Driver Dead-Time The LTC3723-1/LTC3723-2 controllers include a feature to program the minimum time between the output signals on DRVA and DRVB commonly referred to as the driver dead-time. This function will come into play if the control- ler is commanded for maximum duty cycle. The dead-time is set with an external resistor connected between DPRG and VREF (see Figure 1). The nominal regulated voltage on DPRG is 2V. The external resistor programs a current which flows into DPRG. The dead-time can be adjusted from 90ns to 300ns with this resistor. The dead-time can also be modulated based on an external current source that feeds current into DPRG. Care must be taken to limit the current fed into DPRG to 350 µA or less. An internal 10 µA current source sets a maximum deadtime if DPRG is floated. The internal current source causes the programmed deadtime to vary non-linearly with increasing values of RDPRG (see typical performance characteristics). An ex- ternal 200k resistor connected from DPRG to GND will compensate for the internal 10 µA current source and linearize the deadtime delay vs RDPRG characteristic. Powering the LTC3723-1/LTC3723-2 The LTC3723-1/LTC3723-2 utilize an integrated VCC shunt regulator to serve the dual purposes of limiting the voltage applied to VCC as well as signaling that the chip’s bias voltage is sufficient to begin switching operation (under voltage lockout). With its typical 10.2V turn-on voltage and 4.2V UVLO hysteresis, the LTC3723-1/LTC3723-2 is tolerant of loosely regulated input sources such as an auxiliary transformer winding. The VCC shunt is capable of sinking up to 40mA of externally applied current. The UVLO turn-on and turn-off thresholds are derived from an internally trimmed reference making them extremely ac- curate. In addition, the LTC3723-1/LTC3723-2 exhibits Figure 1. Delay Timeout Circuitry |
Similar Part No. - LTC3723-2_15 |
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Similar Description - LTC3723-2_15 |
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