Datasheet LTC1871-7 (Analog Devices) - 8

LTC1871-7
operaTion Main Control Loop
The nominal operating frequency of the LTC1871-7 is The LTC1871-7 is a constant frequency, current mode programmed using a resistor from the FREQ pin to ground controller for DC/DC boost, SEPIC and flyback converter and can be controlled over a 50kHz to 1000kHz range. In applications. With the LTC1871-7 the current control loop addition, the internal oscillator can be synchronized to can be closed by sensing the voltage drop either across an external clock applied to the MODE/SYNC pin and can the power MOSFET switch or across a discrete sense be locked to a frequency between 100% and 130% of its resistor, as shown in Figure 2. nominal value. When the MODE/SYNC pin is left open, it is pulled low by an internal 50k resistor and Burst Mode operation is enabled. If this pin is taken above 2V or an L D external clock is applied, Burst Mode operation is disabled VIN VOUT and the IC operates in continuous mode. With no load (or VIN + SENSE C an extremely light load), the controller will skip pulses OUT VSW in order to maintain regulation and prevent excessive GATE output ripple. GND The RUN pin controls whether the IC is enabled or is in a low GND current shutdown state. A micropower 1.248V reference
2a. SENSE Pin Connection for
and comparator C2 allow the user to program the supply
Maximum Efficiency (VSW < 36V)
voltage at which the IC turns on and off (comparator C2 L D has 100mV of hysteresis for noise immunity). With the VIN VOUT RUN pin below 1.248V, the chip is off and the input supply V V SW IN current is typically only 10µA. GATE + COUT An overvoltage comparator OV senses when the FB pin SENSE exceeds the reference voltage by 6.5% and provides a GND RS reset pulse to the main RS latch. Because this RS latch is GND 18717 F02 reset-dominant, the power MOSFET is actively held off for
2b. SENSE Pin Connection for Precise
the duration of an output overvoltage condition.
Control of Peak Current or for VSW > 36V
The LTC1871-7 can be used either by sensing the volt- age drop across the power MOSFET or by connecting the
Figure 2. Using the SENSE Pin On the LTC1871-7
SENSE pin to a conventional shunt resistor in the source of the power MOSFET, as shown in Figure 2. Sensing the For circuit operation, please refer to the Block Diagram of voltage across the power MOSFET maximizes converter the IC and Figure 1. In normal operation, the power MOSFET efficiency and minimizes the component count, but limits is turned on when the oscillator sets the PWM latch and the output voltage to the maximum rating for this pin (36V). is turned off when the current comparator C1 resets the By connecting the SENSE pin to a resistor in the source latch. The divided-down output voltage is compared to an of the power MOSFET, the user is able to program output internal 1.230V reference by the error amplifier EA, which voltages significantly greater than 36V. outputs an error signal at the ITH pin. The voltage on the ITH pin sets the current comparator C1 input threshold.
Programming the Operating Mode
When the load current increases, a fall in the FB voltage relative to the reference voltage causes the ITH pin to rise, For applications where maximizing the efficiency at very which causes the current comparator C1 to trip at a higher light loads (e.g., <100µA) is a high priority, the current in peak inductor current value. The average inductor current the output divider could be decreased to a few microamps will therefore rise until it equals the load current, thereby and Burst Mode operation should be applied (i.e., the maintaining output regulation. MODE/SYNC pin should be connected to ground). 18717fd 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts