Datasheet LTC3562 (Analog Devices) - 10

LTC3562
OPERATION VOUT Adjustable (Type-B) Regulators
the peak inductor current to the output of an error amplifi er. The output of the current comparator resets the internal Unlike the Type-A regulators, the two Type-B regulators do not require an external resistor divider network to latch which causes the main P-channel MOSFET switch to program its output voltage. Regulators R600B and R400B turn off and the N-channel MOSFET synchronous rectifi er have feedback resistor networks internal to the chip whose to turn on. The N-channel MOSFET synchronous rectifi er values can be adjusted through I2C control. These inter- turns off at the end of the 2.25MHz cycle or if the current nal feedback resistors can be confi gured such that the through the N-channel MOSFET synchronous rectifi er output voltages can be programmed directly. The output drops to zero. Using this method of operation, the error voltages can be programmed from 600mV to 3.775V in amplifi er adjusts the peak inductor current to deliver the 25mV increments. required output power. All necessary compensation is internal to the switching regulator requiring only a single Pins OUT600B and OUT400B are feedback sense pins that ceramic output capacitor for stability. At light loads in connect to the top of the internal resistor divider networks. pulse skip mode, the inductor current may reach zero These output pins should sense the output voltages of on each pulse which will turn off the N-channel MOSFET the regulators right at the output capacitor CO (after the synchronous rectifi er. In this case, the switch node (SW) inductor), as illustrated in Figure 2. goes high impedance and the switch node voltage will The maximum operating current for regulators R600B and “ring.” This is discontinuous mode operation, and is R400B are 600mA and 400mA, respectively. The Type-B normal behavior for a switching regulator. At very light regulators do not have individual run pins as do the Type-A loads in pulse skip mode, the switching regulators will regulators. Thus regulators R600B and R400B can only automatically skip pulses as needed to maintain output be enabled through control of the I2C port. When the regulation. At high duty cycle (VOUT > VIN/2) it is possible part initially powers up, the Type-B regulators default to for the inductor current to reverse at light loads, causing shutdown mode and remain disabled until programmed the step-down switching regulator to operate continuously. through I2C. When operating continuously, regulation and low noise output voltage are maintained, but input operating current
Regulator Operating Modes
will increase to a couple mA. All of the LTC3562’s switching regulators include four In forced Burst Mode operation, the switching regulators possible operating modes to meet the noise/power needs use a constant-current algorithm to control the inductor of a variety of applications. current. By controlling the inductor current directly and In pulse skip mode, an internal latch is set at the start of using a hysteretic control loop, both noise and switch- every cycle which turns on the main P-channel MOSFET ing losses are minimized. In this mode output power is switch. During each cycle, a current comparator compares limited. While operating in forced Burst Mode operation, LTC3562 L 600mV to 3.775V SWxB CO OUTxB GND 3562 F02
Figure 2. Type-B Regular Application Circuit
3562fa 10