Datasheet LTC3407-4 (Analog Devices) - 7

LTC3407-4
OPERATION
decrease causes the error amplifi er to increase the ITH For lower ripple noise at low currents, the pulse-skipping voltage until the average inductor current matches the mode can be used. In this mode, the LTC3407-4 continues new load current. to switch at a constant frequency down to very low cur- rents, where it will begin skipping pulses. The effi ciency in The main control loop is shut down by pulling the RUN pulse-skipping mode can be improved slightly by connect- pin to ground. ing the SW node to the MODE/SYNC input which reduces
Low Current Operation
the clock frequency by approximately 30%. Two modes are available to control the operation of the
Dropout Operation
LTC3407-4 at low currents. Both modes automatically When the input supply voltage decreases toward the switch from continuous operation to the selected mode output voltage, the duty cycle increases to 100% which when the load current is low. is the dropout condition. In dropout, the PMOS switch is To optimize effi ciency, the Burst Mode operation can be turned on continuously with the output voltage being equal selected. When the load is relatively light, the LTC3407-4 to the input voltage minus the voltage drops across the automatically switches into Burst Mode operation, in which internal P-channel MOSFET and the inductor. the PMOS switch operates intermittently based on load An important design consideration is that the R demand with a fi xed peak inductor current. By running DS(ON) of the P-channel switch increases with decreasing input cycles periodically, the switching losses which are domi- supply voltage (See Typical Performance Characteristics). nated by the gate charge losses of the power MOSFETs Therefore, the user should calculate the power dissipation are minimized. The main control loop is interrupted when when the LTC3407-4 is used at 100% duty cycle with low the output voltage reaches the desired regulated value. input voltage (See Thermal Considerations in the Applica- A hysteretic voltage comparator trips when ITH is below tions Information Section). 0.35V, shutting off the switch and reducing the power. The output capacitor and the inductor supply the power to the
Low Supply Operation
load until ITH exceeds 0.65V, turning on the switch and the main control loop which starts another cycle. To prevent unstable operation, the LTC3407-4 incorporates an Under-Voltage Lockout circuit which shuts down the part when the input voltage drops below about 1.65V.
APPLICATIONS INFORMATION
A general LTC3407-4 application circuit is shown in Accepting larger values of ΔIL allows the use of low Figure 2. External component selection is driven by the inductances, but results in higher output voltage ripple, load requirement, and begins with the selection of the greater core losses, and lower output current capability. inductor L. Once the inductor is chosen, CIN and COUT A reasonable starting point for setting ripple current is can be selected. ΔIL = 0.3 • ILIM, where ILIM is the peak switch current limit. The largest ripple current ΔI
Inductor Selection
L occurs at the maximum input voltage. To guarantee that the ripple current stays below a Although the inductor does not infl uence the operat- specifi ed maximum, the inductor value should be chosen ing frequency, the inductor value has a direct effect on according to the following equation: ripple current. The inductor ripple current ΔIL decreases with higher inductance and increases with higher V IN or V V L OUT • 1– OUT VOUT: f O • IL V IN(MAX) V V I OUT OUT L = • 1– f V The inductor value will also have an effect on Burst Mode O • L IN operation. The transition from low current operation 34074fa 7