LTC3530

8

3530fb

Depending on the control voltage, the IC will operate in

is a level shifted voltage from the output of the error amp

transfer between operating modes is continuous, smooth

the buck/boost region is reached where the conduction

time of the four switch region is typically 150ns. Referring

to Figures 1 and 2, the various regions of operation will

now be described.

Switch D is always on and switch C is always off during

voltage V1, output A begins to switch. During the off-time of

switch A, synchronous switch B turns on for the remainder

of the time. Switches A and B will alternate similar to a

typical synchronous buck regulator. As the control volt-

age increases, the duty cycle of switch A increases until

the maximum duty cycle of the converter in buck mode

where f = operating frequency, Hz.

Beyond this point the “four switch,” or buck/boost region

is reached.

and the switch pair AC begins to phase in. As switch pair

AC phases in, switch pair BD phases out accordingly.

range, at voltage V3, the AC switch pair completely phase

out the BD pair, and the boost phase begins at duty cycle

begins is given by:

1– (150ns • f)

The point at which the four switch region ends is given

by:

Switch A is always on and switch B is always off during

voltage V3, switch pair CD will alternately switch to provide

a boosted output voltage. This operation is typical of a

synchronous boost regulator. The maximum duty cycle

of the converter is limited to 90% typical and is reached

BURST MODE OPERATION

Burst mode reduces the LTC3530’s quiescent current

consumption at light loads and improves overall conver-

sion efficiency, increasing battery life. During Burst Mode

operation the LTC3530 delivers energy to the output until

it is regulated and then goes into sleep mode where the

outputs are off and quiescent current drops to 40μA (typ).

In this mode the output ripple has a variable frequency

component that depends upon load current, and will

typically be about 2% peak-to-peak. Burst Mode opera-

tion ripple can be reduced slightly by using more output

capacitance (47μF or greater). Another method of reducing

Burst Mode operation ripple is to place a small feed-forward

divider network (as in Type III compensation).

During the period where the device is delivering energy to

the output, the peak switch current will be equal to 450mA

typical and the inductor current will terminate at zero

current for each cycle. In this mode the typical maximum

average output current is given by:

450mA

2

450mA

2

•

Since the input and output are connected together for

most of the cycle.

OPERATION