Datasheet LT1950 (Analog Devices) - 9

LT1950
U OPERATIO
increases the range of MOSFETs that can be selected and compensation. A default level of slope compensation is allows applications requiring high gate drive with a large achieved with the SLOPE pin open. Increased slope com- swing in VIN voltage. When VIN2 exceeds 8V, the GATE pensation can be programmed by reducing the value of output driver is enabled. The GATE switches between 0V resistance inserted between the SLOPE pin and VREF pin. and VIN2 at a constant frequency set by a resistor from the A SYNC pin allows the LT1950 main oscillator to be ROSC pin to ground. When VIN2 reaches 11V, the internal synchronized to an external clock . To avoid loss of slope switcher at the BOOST pin is disabled to save power and compensation during synchronization, the free running only re-enabled when VIN2 drops below 10V. The internal main oscillator frequency should be programmed to ap- boost switcher runs in burst mode operation, asynchro- proximately 80% of the external clock frequency. nous to the main oscillator. If low VIN operation with high GATE drive is not required, the BOOST pin is left open and The LT1950 can be placed into shutdown mode when the the V SHDN pin drops below an accurate 1.32V threshold. This IN2 pin shorted to VIN. With VIN2 shorted to VIN the minimum operational V threshold can be used to program undervoltage lockout IN voltage will increase from 3V to 8V (required at V (UVLO) at V IN2 to enable the GATE output driver). For IN for current limited or high source resistance GATE turn on, a PWM latch is set at the start of each main supplies. SHDN pin current hysteresis also exists to allow oscillator cycle. For GATE turn off, the PWM latch is reset external programming of UVLO voltage hysteresis. When when either the current sense comparator is tripped, the VIN and VIN2 exceed internally set UVLO thresholds of 2.6V maximum duty cycle is reached, or the BLANK override and 6.8V, the VREF output becomes active. The VREF output threshold is exceeded. is a 2.5V reference supplying the majority of LT1950 control circuitry and capable of sourcing up to 2.5mA for A resistor divider from the application’s output voltage external use. generates a voltage at the FB pin that is compared to the internal 1.23V reference by the error amplifier. The error To prevent noise in the system causing premature turn off amplifier output (COMP) defines the input threshold of the external MOSFET the LT1950 has leading edge (I blanking. This means the current sense comparator out- SENSE) of the current sense comparator. Maximum ISENSE voltage is clamped to 100mV. By connecting I put is ignored during MOSFET turn on and for an extended SENSE to a sense resistor in series with the source of the external period after turn on. The extended blanking period is MOSFET, the peak switch current is controlled by COMP. adjusted by inserting a resistor from the BLANK pin to An increase in output load current causing the output ground. A short to ground defines a minimum default voltage to fall, will cause COMP to rise, increasing I blanking period. Increased resistance from the BLANK pin SENSE threshold, increasing the current delivered to the output. to ground will increase blanking duration. Fault conditions causing I This current mode technique means that the error ampli- SENSE to exceed 125mV will override blanking and reduce the I fier commands current to be delivered to the output rather SENSE to GATE delay to 60ns. than voltage. This makes frequency compensation easier For applications requiring maximum duty cycle clamping, and provides faster loop response to output load tran- the VSEC pin reduces duty cycle for increased voltage on sients. the pin. The VSEC pin provides a volt-second clamp critical in forward converter applications. The current mode architecture requires slope compensa- tion to be added to the current sensing loop to prevent Maximum duty cycle follows (105/VSEC)% for VSEC volt- subharmonic oscillations which can occur for duty cycles ages between 1.4V to 2.8V. If unused, the VSEC pin should above 50%. Unlike most current mode converters which be shorted to ground, leaving the natural maximum duty have a slope compensation ramp that is fixed internally, cycle of the part to be typically 95% for 200kHz operation. placing a constraint on inductor value and operating frequency, the LT1950 has externally adjustable slope 1950fa 9