Datasheet LTC1871-1 (Analog Devices) - 8

LTC1871-1
OPERATION Main Control Loop
which causes the current comparator C1 to trip at a higher peak inductor current value. The average inductor current The LTC1871-1 is a constant frequency, current mode will therefore rise until it equals the load current, thereby controller for DC/DC boost, SEPIC and fl yback converter maintaining output regulation. applications. The LTC1871-1 is distinguished from con- ventional current mode controllers because the current The nominal operating frequency of the LTC1871-1 is control loop can be closed by sensing the voltage drop programmed using a resistor from the FREQ pin to ground across the power MOSFET switch instead of across a and can be controlled over a 50kHz to 1000kHz range. In discrete sense resistor, as shown in Figure 2. This sensing addition, the internal oscillator can be synchronized to technique improves effi ciency, increases power density, an external clock applied to the MODE/SYNC pin and can and reduces the cost of the overall solution. be locked to a frequency between 100% and 130% of its nominal value. When the MODE/SYNC pin is left open, it L D V V IN OUT is pulled low by an internal 50k resistor and Burst Mode VIN operation is enabled. If this pin is taken above 2V or an + SENSE COUT external clock is applied, Burst Mode operation is disabled VSW and the IC operates in continuous mode. With no load (or GATE an extremely light load), the controller will skip pulses in GND order to maintain regulation and prevent excessive output GND ripple.
2a. SENSE Pin Connection for Maximum Efficiency (VSW < 36V)
The RUN pin controls whether the IC is enabled or is in a low current shutdown state. A micropower 1.248V reference L D V V IN OUT and comparator C2 allow the user to program the supply V V SW IN voltage at which the IC turns on and off (comparator C2 GATE has 100mV of hysteresis for noise immunity). With the + RUN pin below 1.248V, the chip is off and the input supply SENSE COUT current is typically only 10μA. GND RS 18711 F02 GND An overvoltage comparator OV senses when the FB pin exceeds the reference voltage by 6.5% and provides a
2b. SENSE Pin Connection for Precise
reset pulse to the main RS latch. Because this RS latch is
Control of Peak Current or for VSW > 36V
reset-dominant, the power MOSFET is actively held off for
Figure 2. Using the SENSE Pin On the LTC1871-1
the duration of an output overvoltage condition. The LTC1871-1 can be used either by sensing the voltage For circuit operation, please refer to the Block Diagram of drop across the power MOSFET or by connecting the the IC and Figure 1. In normal operation, the power MOSFET SENSE pin to a conventional shunt resistor in the source is turned on when the oscillator sets the PWM latch and of the power MOSFET, as shown in Figure 2. Sensing the is turned off when the current comparator C1 resets the voltage across the power MOSFET maximizes converter latch. The divided-down output voltage is compared to an effi ciency and minimizes the component count, but limits internal 1.230V reference by the error amplifi er EA, which the output voltage to the maximum rating for this pin (36V). outputs an error signal at the ITH pin. The voltage on the By connecting the SENSE pin to a resistor in the source ITH pin sets the current comparator C1 input threshold. of the power MOSFET, the user is able to program output When the load current increases, a fall in the FB voltage voltages signifi cantly greater than 36V. relative to the reference voltage causes the ITH pin to rise, 18711fb 8