Datasheet LTC3534 (Analog Devices) - 10

LTC3534
operaTion
shifted voltage from the output of the error amplifier (VC The VIN potential at which the four switch region ends is pin), see Figure 3. The four power switches are properly given by: phased so the transfer between operating modes is con- V tinuous, smooth and transparent to the user. When V IN = VOUT • (1 – D) = VOUT • (1 – 125ns • ƒ) V IN approaches VOUT the buck-boost region is entered, where where f = operating frequency in Hz, typically 1MHz. the conduction time of the four switch region is typically Hence, for the LTC3534, 125ns. Referring to Figures 1 and 2, the various regions of operation will now be described. V V OUT IN(ENTER4SW) ≅ V 0.875
Buck Region (VIN > VOUT)
Approximate VIN potential at which the four switch Switch D is always on and switch C is always off during region is entered. this mode. When the internal control voltage, VCI, is above voltage V1, output A begins to switch. During the off- VIN(4SWEXIT) @ 0.875 • VOUT V time of switch A, synchronous switch B turns on for the Approximate VIN potential at which the four switch remainder of the period. Switches A and B will alternate region is exited. similar to a typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch A
Boost Region (VIN < VOUT)
increases until the maximum duty cycle of the converter Switch A is always on and switch B is always off during in buck mode reaches DMAX_BUCK, given by: this mode. When the internal control voltage, VCI, is above DMAX_BUCK = (100 – D4SW)% voltage V3, switch pair CD will alternately switch to provide where D4 a boosted output voltage. This operation is typical to a SW = duty cycle % of the four switch range. synchronous boost regulator. The maximum duty cycle D4SW = (125ns • f) • 100% of the converter is limited to 85% typical and is reached where f = operating frequency in Hz, typically 1MHz. when VCI is above V4. Hence, D4SW = 12.5% for the LTC3534.
Burst Mode OPERATION
DMAX_BUCK = 87.5% Burst Mode operation reduces the LTC3534’s quiescent Beyond this point the “four switch”, or buck-boost region current consumption at light loads and improves overall is reached. conversion efficiency, increasing battery life. During Burst Mode operation the LTC3534 delivers energy to the output
Buck-Boost or Four Switch (V
until it is regulated and then enters a sleep state where
IN ~ VOUT)
the switches are off and the quiescent current drops to When the internal control voltage, VCI, is above voltage V2, 25µA typical. In this mode the output ripple has a variable switch pair AD remain on for duty cycle DMAX_BUCK, and frequency component that depends upon load current, the switch pair AC begins to phase in. As switch pair AC and will typically be about 2% peak-to-peak. Burst Mode phases in, switch pair BD phases out accordingly. When operation ripple can be reduced slightly by using more VCI reaches the edge of the buck-boost range, at voltage output capacitance (47µF or greater). Another method of V3, the AC switch pair completely phase out the BD pair, reducing Burst Mode operation ripple is to place a small and the boost phase begins at duty cycle D4SW. feedforward capacitor across the upper resistor in the The input voltage, VIN, where the four switch region begins VOUT feedback divider network (as in Type III compensa- is given by: tion), see Figure 6. V V OUT IN = 1−(125ns• f) V 3534fb 10 For more information www.linear.com/LTC3534 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 Related Parts