Datasheet LTC3562 (Analog Devices) - 9

LTC3562
OPERATION Introduction
Through I2C control, VFBxA can be programmed from The LTC3562 is a highly integrated power management 800mV (full scale) down to 425mV in 25mV increments. IC that contains four I2C controllable, monolithic, high ef- When the RUN pins (RUN600A and RUN400A) are used fi ciency step-down regulators. Two regulators provide up to activate these regulators, the default feedback servo to 600mA of output current and the other two regulators voltage is set to 800mV. produce up to 400mA. All four regulators are 2.25MHz, constant-frequency, current mode switching regulators that LTC3562 L can be independently controlled through I2C. All regula- SWxA tors are internally compensated eliminating the need for CFB R1 CO external compensation components. FBxA 425mV to 800mV R2 The LTC3562 offers two different types of adjustable GND step-down regulators. The two Type-A regulators (R600A, 3562 F01 R400A) can have the feedback voltages adjusted through I2C from 425mV to 800mV in 25mV increments. The two
Figure 1. Type-A Regulator Application Circuit
Type-B regulators (R600B, R400B) can have the output voltages adjusted through I2C control from 600mV to Typical values for R2 are in the range of 40k to 1MΩ. The 3.775V in 25mV increments. capacitor CFB cancels the pole created by the feedback All four converters support 100% duty cycle operation resistors and the input capacitance of the FB pin and also (low dropout mode) when their input voltage drops very helps to improve transient response for output voltages close to their output voltage. To suit a variety of applica- much greater than 0.8V. A variety of capacitor sizes can be tions, four selectable mode functions are available on used for CFB but a value of 10pF is recommended for most the LTC3562’s step-down regulators to trade-off noise applications. Experimentation with capacitor sizes between for effi ciency. 2pF and 22pF may yield improved transient response. At moderate to heavy loads, the constant-frequency pulse Regulators R600A and R400A have individual RUN pins skip mode provides the lowest output switching noise solu- that can enable the regulators without accessing the I2C tion. At lighter loads, either Burst Mode operation, forced port. The RUN600A and RUN400A pins are OR’ed with the Burst Mode operation or LDO mode may be selected to enable signals coming from the I2C port (refer to the Block optimize effi ciency. The switching regulators also include Diagram) such that regulators R600A and R400A can be soft-start to limit inrush current when powering on, short- enabled if the I2C port is unavailable. The RUN600A pin is circuit current protection, and switch node slew limiting active low and the RUN400A pin is active high. circuitry to reduce radiated EMI. No external compensation When the RUN pins are activated, the Type-A regulators components are required. are enabled in a default setting. The default mode for the
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regulators is pulse skip mode and the default feedback
FB Adjustable (Type-A) Regulators
servo voltage setting is 800mV. Once enabled with these The two Type-A step-down regulators (R600A and R400A) default settings, the settings can always be changed on have individual programmable feedback servo voltages via the fl y through I2C once the I2C terminal is available. I2C control. Given a particular feedback servo voltage, the output voltage is programmed using a resistor divider from The maximum operating output current of regulators R600A the switching regulator output connected to the feedback and R400A are 600mA and 400mA, respectively. pins (Figure 1). The output voltage is related to the feedback servo voltage by the following equation: R1 VOUTxA = VFBxA + 1 R2 3562fa 9