Datasheet LTC3721-1 (Analog Devices) - 9

LTC3721-1
U OPERATIO
Please refer to the detailed Block Diagram for this discus- sets a maximum deadtime if DPRG is floated. The internal sion. The LTC3721-1 is a PWM push-pull controller that current source causes the programmed deadtime to vary operates with pulse-by-pulse peak current mode control. non-linearly with increasing values of RDPRG (see Typical It is best suited for moderate to high power isolated power Performance Characteristics). An external 200k resistor systems where small size and high efficiency are required. connected from DPRG to GND will compensate for the The push-pull topology delivers excellent transformer internal 10µA current source and linearize the deadtime utilization and requires only two low side power MOSFET delay vs RDPRG characteristic. switches. The controller generates 180° out of phase 0% to < 50% duty cycle drive signals on DRVA and DRVB. VREF The external MOSFETs are driven directly by these power- ful on-chip drivers. The external MOSFETs typically con- RDPRG DPRG trol opposite primary windings of a centertapped power OPTIONAL transformer. The centertap primary winding is connected + + TURN-ON 200k to the input DC feed. The secondary of the transformer can V 2V OUTPUT 2.5V – be configured in different synchronous or nonsynchronous – 37211 F01 configurations depending on the application needs.
Figure 1. Deadtime Adjust
The duty ratio is controlled by the voltage on COMP. A switching cycle commences with the falling edge of the
Powering the LTC3721-1
internal oscillator clock pulse. The LTC3721-1 attenuates the voltage on COMP and compares it to the current sense The LTC3721-1 utilizes an integrated VCC shunt regulator signal to terminate the switching cycle. If the voltage on CS to serve the dual purposes of limiting the voltage applied exceeds 300mV, the present cycle is terminated. If the to VCC as well as signaling that the chip’s bias voltage is voltage on CS exceeds 600mV, all switching stops and a sufficient to begin switching operation (under voltage soft-start sequence is initiated. lockout). With its typical 10.2V turn-on voltage and 4.2V UVLO hysteresis, the LTC3721-1 is tolerant of loosely A host of other features including an error amplifier, regulated input sources such as an auxiliary transformer system UVLO programming, adjustable leading edge blank- winding. The VCC shunt is capable of sinking up to 40mA ing, slope compensation and programmable dead-time of externally applied current. The UVLO turn-on and turn- provide flexibility for a variety of applications. off thresholds are derived from an internally trimmed reference making them extremely accurate. In addition,
Programming Driver Dead-Time
the LTC3721-1 exhibits very low (145µA typ) start-up The LTC3721-1 includes a feature to program the mini- current that allows the use of 1/8W to 1/4W trickle charge start-up resistors. mum time between the output signals on DRVA and DRVB commonly referred to as the driver dead-time. This func- The trickle charge resistor should be selected as follows: tion will come into play if the controller is commanded for R maximum duty cycle. The dead-time is set with an START(MAX) = VIN(MIN) – 10.7V/250µA external resistor connected between DPRG and V Adding a small safety margin and choosing standard REF (see Figure 1). The nominal regulated voltage on DPRG is 2V. values yields: The external resistor programs a current which flows into
APPLICATION VIN RANGE RSTART
DPRG. The dead-time can be adjusted from 90ns to 300ns DC/DC 36V to 72V 100k with this resistor. The dead-time can also be modulated Off-Line 85V to 270V based on an external current source that feeds current into RMS 430k PFC Preregulator 390V DPRG. Care must be taken to limit the current fed into DC 1.4M DPRG to 350µA or less. An internal 10µA current source sn37211 37211fs 9