Datasheet LTC3704 (Analog Devices) - 8

LTC3704
U OPERATIO Main Control Loop
The nominal operating frequency of the LTC3704 is pro- The LTC3704 is a constant frequency, current mode grammed using a resistor from the FREQ pin to ground controller for DC/DC positive-to-negative converter appli- and can be controlled over a 50kHz to 1000kHz range. In cations. The LTC3704 is distinguished from conventional addition, the internal oscillator can be synchronized to an current mode controllers because the current control loop external clock applied to the MODE/SYNC pin and can be can be closed by sensing the voltage drop across the locked to a frequency between 100% and 130% of its power MOSFET switch instead of across a discrete sense nominal value. When the MODE/SYNC pin is left open, it is resistor, as shown in Figure 2. This sensing technique pulled low by an internal 50k resistor and Burst Mode improves efficiency, increases power density, and re- operation is enabled. If this pin is taken above 2V or an duces the cost of the overall solution. external clock is applied, Burst Mode operation is disabled and the IC operates in continuous mode. With no load (or an extremely light load), the controller will skip pulses in VIN VSW order to maintain regulation and prevent excessive output VIN ripple. SENSE The RUN pin controls whether the IC is enabled or is in a GATE GND low current shutdown state. A micropower 1.248V refer- GND ence and comparator C2 allow the user to program the supply voltage at which the IC turns on and off (compara-
2a. SENSE Pin Connection for Maximum Efficiency (V
tor C2 has 100mV of hysteresis for noise immunity). With
SW < 36V)
the RUN pin below 1.248V, the chip is off and the input V V IN SW supply current is typically only 10μA. VIN GATE The LTC3704 can be used either by sensing the voltage drop across the power MOSFET or by connecting the SENSE GND SENSE pin to a conventional shunt resistor in the source RSENSE of the power MOSFET, as shown in Figure 2. Sensing the 3704 F02 GND voltage across the power MOSFET maximizes converter
2b. SENSE Pin Connection for Precise
efficiency and minimizes the component count, but limits
Control of Peak IIN/IOUT or for VSW > 36V
the output voltage to the maximum rating for this pin
Figure 2. Using the SENSE Pin On the LTC3704
(36V). By connecting the SENSE pin to a resistor in the source of the power MOSFET, the user is able to program For circuit operation, please refer to the Block Diagram of output voltages significantly greater than the 36V maxi- the IC and Figure 1. In normal operation, the power mum input voltage rating for the IC. MOSFET is turned on when the oscillator sets the PWM
Programming the Operating Mode
latch and is turned off when the current comparator C1 resets the latch. The divided-down output voltage is com- For applications where maximizing the efficiency at very pared to an internal 1.230V reference by the error amplifier light loads (e.g., <100μA) is a high priority, Burst Mode EA, which outputs an error signal at the ITH pin. The voltage operation should be applied (i.e., the MODE/SYNC pin on the ITH pin sets the current comparator C1 input should be connected to ground). In applications where threshold. When the load current increases, a fall in the fixed frequency operation is more critical than low cur- NFB voltage relative to the reference voltage causes the ITH rent efficiency, or where the lowest output ripple is pin to rise, which causes the current comparator C1 to trip desired, pulse-skip mode operation should be used and at a higher peak inductor current value. The average the MODE/SYNC pin should be connected to the INTVCC inductor current will therefore rise until it equals the load pin. This allows discontinuous conduction mode (DCM) current, thereby maintaining output regulation. operation down to near the limit defined by the chip’s 3704fb 8