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LTC3721-1 Datasheet(PDF) 9 Page - Linear Integrated Systems |
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LTC3721-1 Datasheet(HTML) 9 Page - Linear Integrated Systems |
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9 / 16 page  LTC3721-1 9 sn37211 37211fs OPERATIO Please refer to the detailed Block Diagram for this discus- sion. The LTC3721-1 is a PWM push-pull controller that operates with pulse-by-pulse peak current mode control. It is 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. The controller generates 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 LTC3721-1 attenuates the voltage on COMP and compares it to the current sense signal to terminate the switching cycle. If the voltage on CS exceeds 300mV, the present cycle is terminated. If the voltage on CS exceeds 600mV, all switching stops and a soft-start sequence is initiated. 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. Programming Driver Dead-Time The LTC3721-1 includes a feature to program the mini- mum time between the output signals on DRVA and DRVB commonly referred to as the driver dead-time. This func- tion will come into play if the controller 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 350µA or less. An internal 10µA 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 external 200k resistor connected from DPRG to GND will compensate for the internal 10µA current source and linearize the deadtime delay vs RDPRG characteristic. Powering the LTC3721-1 The LTC3721-1 utilizes 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 LTC3721-1 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 accurate. In addition, the LTC3721-1 exhibits very low (145µA typ) start-up current that allows the use of 1/8W to 1/4W trickle charge start-up resistors. The trickle charge resistor should be selected as follows: RSTART(MAX) = VIN(MIN) – 10.7V/250µA Adding a small safety margin and choosing standard values yields: APPLICATION VIN RANGE RSTART DC/DC 36V to 72V 100k Off-Line 85V to 270VRMS 430k PFC Preregulator 390VDC 1.4M – + TURN-ON OUTPUT 2.5V + – V 2V VREF DPRG RDPRG 37211 F01 200k OPTIONAL Figure 1. Deadtime Adjust |
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